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Is the car "original"


The things most likely to have been altered, or changed, from "original" are the Paint, Engine components, Transmission, Rear axle, Tires and wheels, Headlights, Instruments, Glass, Air Conditioning, and Steering Wheel.

Because of the cost of the various air suspension components, it is possible that someone has changed the air valves to the later style, or even completely removed the system and installed coil springs. Note that all finback 300SE sedans had air suspension. All 300SE coupes and convertibles had air suspension. The only 300SE model that did not have air suspension is the square back, 108 chassis 300SE built from August 1965 through 1967.

Again, because of cost, fuel injection pumps are frequently replaced with units from defunct cars. This, by itself, is ok so long as the replacement unit is of exactly the right type. In the case of the pre-1964 cars, the pumps are different even between cars with automatic transmissions versus manual transmissions.

Because of the virtually identical appearance, and ignorance on the part of the owner, early 300SE engines with the two-plunger injection pump have had pumps from 220SEs installed. If this has been done, the car will never run like it should.

It's "restored"! (uh huh)

What's this "matching numbers" and "frame-off" stuff?

There are times when it's just impossible to maintain 100% originality. When the paint is completely dried out, there is nothing you can do to bring it back to it's original appearance. However, it's generally accepted that a car can have non-original paint and still be considered a top car, as long as the repaint is a top quality job.

If the engine has been replaced with a unit from another car, well, that's a different story. Serious collectors don't like cars that have had an engine swap, and it's very hard (read as nearly impossible) to conceal an engine swap from a knowledgeable person.

Even though they aren't original, some component swaps are very popular. For example, many people like the appearance of the Euro-style headlights, and frequently install them on cars that originally had the four sealed beam lamps. They look great, and if done carefully, the swap can made to be reversible without too much trouble.

You need to give serious consideration to how important originality is to you. Some people don't consider a car truly theirs until they've customized it to their own tastes. Others won't touch a car that isn't 100% original. If you do decide to make some changes to your 300SE, limit the changes to things that can be reversed easily. If you absolutely have to change out the engine, be sure to save the original engine so that the next owner has a good starting point if he/she wants to rebuild and reinstall it, thereby bringing the vehicle back to original.

Keep in mind that original cars are always worth more than non-original cars. If you have to have a finback Mercedes and  want to make serious changes to it, a 300SE is probably not the car you should buy.


Look at the paint very carefully. If the car has had a "good" repaint, it can be difficult to detect. If the car has had an excellent repaint, it can be nearly impossible to detect, even when its a completely different color.

If it's not readily apparent that the car has been repainted, check the paint code number on the plate on the driver's side of the top of the radiator core support, to make sure that the paint is at least the right color. If the color looks right, start looking at the places that are the hardest to paint. Look under the rubber gaskets around the windshield, behind the interior door panels, around the perimeter of all the external trim pieces. It's very common to use a trim piece as a "break" line when repainting only part of the car.

Overspray in the wheel wells is a dead giveaway, but keep in mind that the wheel wells and undercarriage where painted in the outer body color by the factory. Just because there is paint in the wheel wells doesn't mean it's not original.

One of the best places to look at is the sheet metal behind the jack ports. Shine a flashlight through the jack port tube and look at the paint back in there. (This should be done anyway, because this is also a common location for rust.)

Engine components

Because engine components are so very expensive, 300SEs have had more engine swaps than probably any other Mercedes. All of the engines are fundamentally the same, but there are some  conspicuous differences making most engine swaps conspicuous as well.

It's also possible that a previous owner stepped up to the plate and bought a new short block from the factory. Factory replacement short blocks are not stamped with an engine number. It's the absence of a number that makes this swap obvious.

What you have to do is educate yourself as to which engine type came in each of the different models. The engine type is identified by the engine number. If you found an engine number of 189.988-12-001234 but the car is a 300SE SWB sedan, you would know that the engine was not original. (189.988 engines where used in post-August 1965 and later 109 chassis 300SELs.) If you found an engine number of 189.987-12-001234 in a 300SE convertible, but the engine had the two-plunger injection pump, you could assume that the engine block was not original, because all 189.987 engines where used with the six-plunger injection pump. (We can assume that it was the block that was replaced because nobody is going to replace a six-plunger injection system with a two-plunger system.)

If somebody replaced an engine in a late sedan with the engine from another late sedan, then there isn't anything obvious to indicate an engine swap. In this case, the only reliable method of verifying the engine is to compare the engine number from the engine with the engine number on the Build Card. Hopefully, the Build Card is readily available in the glove compartment of the car. However, this is rare. In most cases the Build Card disappeared a long time ago.

Duplicate Build Cards are available from the factory, but you aren't going to get one very quickly, so, patience is something both you and the seller get to exercise.


Automatic transmissions: Most all of the Mercedes automatic transmissions of the period look almost identical. The only way to tell for sure that it's the right type for a 300SE is to check the factory part number and, the transmission will have a metal tag attached to the lower left side that actually says "300SE". Valid part numbers are 112-270-01-01 for early LHD cars, 112-270-04-01 and 112-270-10-01 for early RHD cars, 112-270-09-01 for middle LHD and RHD cars, and 112-270-16-01 for late LHD and RHD cars. To only way to confirm that it's actually the original unit is compare the transmission serial number to that on the Build Card.

Manual transmissions: First, it should be stated that 300SEs,  initially, were not even available with a manual transmission. It's not clear exactly when the manual transmission became available, but it's safe to say that it wasn't until at least early 1963 and it wasn't until at least VIN# 003400 (and more likely 003500). If you find a car with a manual transmission in it and a VIN number less than 003400, it's not original. Another quick check is to make sure that the transmission type corresponds to the body type indicated by the VIN and Engine numbers (see Vehicle Identification and Engine numbers). As always, the definitive test is to compare the transmission serial number with that on the Build Card.

Rear axle

The rear axle in the '61-'65 300SEs are unique. These cars where the first Mercedes model to have rear disc brakes, and this made these rear ends different. One thing that is unique is that these rear axles accommodate the 13" wheels. So long as the replacement unit is of the same vintage as the original, the swap isn't really a big issue.

As is generally the case with any completely new design, there were minor changes during production. The initial design used lug nuts to attach the wheels to the axle, the later design used lug bolts. See Evolution of the Rear Axle Assembly for specifics.

In August 1965, at the same time that the finback 300SE sedans were discontinued, the factory started using a newer rear axle design. (See End of Life/Break in Production.) For lack of a better name, I will refer to the newer rear axle design as the 1965 design. The primary difference in the 1965 design was the way the brake rotors are attached to the axle. At the same time, the brake size was increased. Because of the larger brakes, the 13" wheels do not fit the 1965 rear axle.

It was, and remains, very common to change out a worn rear axle with one of the newer 1965 design. The original style rear axles are hard to come by, and the 1965 style is readily available. For all intents and purposes, the two are interchangeable, except for the issue with the 13" wheels not fitting the 1965 unit. If the car has 13" wheels on it, then the car has a true 300SE style rear axle. If the car has 14" wheels on it, get suspicious. (The 14" wheels will fit on the original style rear axle.)

If the Build Card is available, check the original rear axle ratio against the ratio stamped on the rear of the differential case.

Tires and wheels

The original tire size for all 300SEs with the 13" wheels was 7.50x13, and the steel wheel was 5.5x13. This steel wheel is unique to all 300SEs and to the 220SE convertibles. There are two slightly different versions of the wheels, the second version being a "well base rim" and "mid-centering". It is my understanding that the first version was "centered" by the lug-bolts, the second version is centered in the short axle stub that protrudes about a half inch through the center of the wheel and held in place by the lug bolts.

The changeover in the steel wheels occurred at the same time that the rear axle design changed from lug nuts to lug bolts.

All of the other finback sedans (190/D, 200/D, 220/S/SE, and 230/S) had narrower steel wheels, being no more than 5.0x13. It isn't easy to tell the difference at a glance, but, fortunately, the 10 digit part number is stamped on each wheel making it easy to determine if the wheel is the correct size and type. The part number can be found inside the bolt circle on the outer side of the wheel. Removing the center hub cap is all that is required.

See "Wheels" for specific part numbers for the two versions.

Headlights and front turn signals

There are three different styles of headlights (and front turn signals) that were used in production. These are the "Euro" style, the "Federal" (American) style, and what I refer to as the "English" style.

The Euro style light assemblies are beautiful, curved glass, all-in-one, works of art. For this reason, the Euro style assemblies are popular retro-fits for a lot of cars. All of the lights on the front of the car are contained in these assemblies, incorporating all combinations of head lamp, parking lamp, "side" markers, turn signals, etc. (The light output of the head lamps can be embarrassingly low, however.)

The Federal style front lamps take a wholly different approach. There are vertically arranged double sealed beam head lamps and turn signal lamp/parking lamp/"side marker " lamp in a separate assembly between the headlights and the radiator grill. The clear lens lamp holders on either side of the stacked head lamp assembly have no lamps in them.

The "English" style lamps are similar to the Federal style lamps in that there are the stacked twin head lamps, but the lamp holders on either side of the assembly have orange lenses and lamps for the turn signal/parking/"side" marker are installed. Many (if not all) of these style assemblies did not use the Federal style sealed beam lamps, but a separate bulb and reflector. Look closely because, with a casual glance, you can't tell the difference.

All I can do is offer some general guidelines on how to determine whether the front lamps on the car are original, or, more accurately, if not original. If the car has the Euro-style headlights but the instruments in the car are in English, the car is probably an American market car, and the headlight assemblies have probably been changed. If the car has factory air conditioning, it is probably an American market car and therefore would have the Federal style lights. (Some people have gone to the trouble to change out the instruments as well.)

Unlike the coupes and convertibles, the rear of the sedans are all the same, regardless of what market they were sold to, so there aren't any good clues there. One possible exception to this is that orange turn signal lenses, instead of red, were supplied for some European countries.

Perhaps the only definitive way to know what light assemblies where originally installed on the car is to check the factory's records, starting with the Build Card.


The owner that is changing to the Euro-style leadlight assemblies will occasionally also change the car's instruments to those reading in metric. (This is much more common in coupes and convertibles than in the sedans.) This maintains the appearance that the car is "original".

Like the headlight assemblies, the only definitive way to know what instruments where originally installed on the car is to check the factory's records, starting with the Build Card.

Because of the larger engine in the 300SEs, the cars were capable of much higher maximum speeds. This being the case, the 300SEs had a speedometer that read to a higher value than the models with smaller engines.

See Evolution of the Speedometer for more information.


The original type glass (windshields and side windows) has a  mark in a corner indicating the glass manufacturer's name. As a rule, the glass manufacturer was Sekurit. If any of the glass pieces are missing the mark, or the mark indicates a different name, the glass is not original

Air conditioning

For obvious reasons, the most common alteration to these cars is the addition of air conditioning. In most cases, this addition is very conspicuous. The lines in the engine compartment are poorly routed, the under-dash unit protrudes out too far, or the engine's fan clutch is the wrong type. It's the fan clutch that is the "give-away".

Cars with factory installed, or proper dealer installed, air conditioning have a hydraulic engine cooling fan clutch. Early engines not set up for air conditioning have a "direct drive" fan, with no clutch at all. Later engines not set up for air conditioning have an electro-magnetic clutch. If the car has air conditioning but does not have the hydraulic fan clutch, the air conditioning system isn't "original".

There are two "braces" that bolt to the under side of the dash and to the sides of the transmission tunnel. If the car wasn't originally equipped with an under-dash air conditioning system, the braces are the type that are relatively straight up and down. If the car was properly equipped with the under-dash air conditioning unit, the straight braces were replaced by two deeply arched braces. This allowed the under-dash air conditioning unit to be tucked well under the dash, such that the face of the air conditioning unit was flush with the lower edge of the dash. If the air conditioning unit protrudes out from under the dash by several inches, it implies that the car still has the straight braces and it's a sure bet that the air conditioning is not "original".

Air valves

All 300SEs built prior to August 1965 (see End of Life/Break in Production) had the early style air suspension system. Other than the appearance of the wheel valves, the primary difference between the early system and the later system was that the later system incorporated a means to automatically raise the vehicle an additional two inches. (This mode was not intended for normal driving!)

Since many more cars were built with the later style system, the prices of the later style air valves are lower than for the early style valves. In fact, if you buy a new early style wheel valve (if you can afford it!) it won't even look the same as those on the car. Even the prices of most rebuilt early style wheel valves are substantially higher than for the later style wheel valves. For this reason, some cars have been converted to the later style wheel valves. They work fine, but they are not original. If you want to bring the car back to "original", you could end up spending some serious money to do it.

Steering Wheel:

The steering wheel and center pad, turn signal knob, and shift lever knob were all normally provided in a black color. They could also be special ordered in a nice ivory color. Regardless of which color they are, all three were of the same color. Because of the nice appearance of the ivory color, a lot of cars with the black have been retro-fitted with the ivory. If you find a car were the three items' color doesn't match, something is not original. The turn signal is generally a good indicator of what the original color was because it's the hardest to change.

It's "restored"! (uh huh)

What's this "matching numbers" and "frame-off" stuff?

This is one of my "pet peeves". I can't tell you how many times I've seen cars advertised, or described, as having had a "frame off" restoration, or as having matching numbers.

Certainly other cars can have a "frame off" restoration, implying that the body has been removed from the frame, indicating an extensive effort. But 300SEs are of "unit body" construction. There is no "frame"! People that make claims of a "frame off" restoration are generally revealing themselves for what they really are: Sales hacks. They know less than you do about the car. They are just trying to pass it along to the next owner as quickly, and for as much money, as possible.

The same can be said of people who make claims of matching numbers. What is it that the numbers "match"? Unless they are referring to the numbers on the Build Card matching the Engine number and the VIN number, there is no such thing as "matching numbers" on a 300SE.

I'm not going to suggest that you not deal with these people, but watch out. These folks can be very slick, and you could end up paying more for the car than it's worth, assuming you want the car at all. It's these same folks that are most likely to have done questionable repairs to make a car look, or drive, better and get it sold as quickly as possible.

In the end, it's your responsibility to look out for yourself. As the saying goes, it's "buyer beware". As with anything, learn as much about 300SEs as possible in advance. Just like money, a 300SE can be an excellent servant, or a cruel master.

Know the beast (forewarned is forearmed) (update June 2006)

300SEs do have their drawbacks. The engine and the air suspension have been responsible for more of these cars disappearing from the road than anything else. That's not to say that either are troublesome, but when repairs are necessary, they can be outrageously expensive. The cost of a properly rebuilt engine can hit five figures, and a total rebuild of the air suspension system can easily cost $3000.00.

The engine block is an aluminum alloy. For this reason there are steel liners in the cylinder bores. When the liners are worn out, you don't bore them out, you replace them. As this is a very sophisticated engine design, the "plane" of the top "deck" of the block is not perpendicular to the direction of travel of the pistons. This means that the top of the liners is not cut "flat", so rotational alignment of the liners during installation is critical. And you have to have the special tool to "pull" them into the block, they can't be pushed in like normal liners. There are other issues with the liners too, but the intent here is to impress upon you that anything that involves the liners is going to be very expensive. You should assume that anything that involves the internals of the engine is going to be expensive. This is why so many of these cars no longer have the original engine in them. Compared to the price of a good rebuild, a functional used engine will cost you less than 10% of the price of the rebuild.

UPDATE!: I originally wrote this in 2001. Currently, in June of 2006, a proper rebuild of the engine costs over $17,000 US dollars, and that's if you can get the parts. Both pistons and cylinder liners went "no-longer-available" in 2005. Fortunately, the exact same parts where used in the late-production, aluminum engined 300SL, so we KNOW that this problem will be rectified at some point in the future. However, if history is any indication, these parts will be even more expensive when they do become available again. When last available, wholesale prices were $250 per piston, and $150 per liner. Plan for $200 per liner to have them installed and honed CORRECTLY.

Crankshaft bearings for the mains and the rods will total about $600, and the various bushings for the shafts that drive the distributor and oil pump will also cost about $600. The gear teeth on the driving and driven shafts are always bad on these engines (for whatever reason) and must be replaced as a set. In 2005 the set had a wholesale price of $1150. Add in the price of an oil pump and the necessary machine work on the crank and rods, new timing chain and sprockets, gaskets, seals, harmonic balancer components. All of a sudden, $10,000 for a short-block starts looking realistic. Then you still have to do the cylinder head! On average, it will cost about $2000 to have the head properly rebuilt, and that assumes that you won't need to replace any exhaust valves (these almost never need replacing). You still aren't done. Don't forget the water pump, generator and starter, and, you still have to have the injection pump rebuilt and the injectors tested/cleaned/replaced. Injection pump rebuilding/recalibrating averages about $1300.

Don't interpret all this as something meant to simply scare you. Other than the generator and starter, ALL these items really are going to need to be done. Even if you don't need to rebuild the engine, you should still plan on replacing the water pump and rebuilding the injection pump.

A total rebuild of the rear axle will cost over $3000.00 and will probably not include new clutches for the limited-slip.

Rebuilding the brake system on an early car with Dunlop brakes will cost around $2000.00

There are some individual parts costs that, by themselves, will make you blush. The most notorious of these is the distributor cap. A new one will set you back over $600.00. By weight, it compares favorably with gold, and you should treat it as such.

Another commonly needed expensive part is the aluminum flange on the front of the water pump shaft. The flange is very hard to get off of the old water pump (new pumps don't come with it!), and being aluminum, it gets damaged very easily. At last check (late 2000), the price for a new one was over $700.00. Because of the way the water pump is mounted to the engine, it is difficult to check its condition. Because of where it's mounted to the engine, it is also very easy to ignore. For this reason, the water pump on any newly acquired 300SE is usually bad. The price of a new water pump isn't terrible (you should be able to easily find a new one for less than $300.00), but if the flange is already damaged (very likely) , or if you damage it, the price of the water pump will be minor compared to the total cost. Whatever you do, don't use the two 10 mm attaching nuts to try to "press" the flange into the rear of the rubber drive coupling on the back of the generator. This is a sure-fire way to destroy the aluminum flange. Learn from my experience!

New front brake rotors can be found for very reasonable prices (less than $125.00), but the same cannot be said for the rear rotors. The rotors for the pre-1965 version of the rear axle are unique, and are more difficult to replace than most. (Rotors for the 1965 rear axle version are reasonably priced and easy to replace.) The last price I have for one of the early (pre-1965) rear rotors was $275.00, but that was in Jan. 1998. I'm certain that today's prices are much higher.

If the car is an "early" car, it will have the Dunlop brake calipers on both the front and rear axles. Personally, I really like the Dunlop calipers. I know a few people that think they are junk, but, everybody is entitled to their opinion. Regardless, the Dunlop calipers are very expensive to replace, or even rebuild. For example, there is a site on the Internet advertising rebuilt Dunlop calipers for $600.00 per axle. (as of Aug. 2001)

Common Ailments

Cold Start Valve. This is a very simple device but the consequences of failure can be catastrophic. The worst problems that develop are gasoline leakage, into the engine compartment and into the engine. Leaks into the engine compartment can lead to a fire, and leaks into the engine can cause oil dilution with possible engine failure. If the engine compartment doesn't look as though there has been a fire (look for small indications like wire insulation damage around the cold start valve), then there hasn't been an ignited leak, at least not yet.

Internal leakage is harder to detect. One clear symptom of a serious internal leak is a high reading on the engine oil dipstick. If you find a car like this, you'll have to make a judgment call. If you can determine that the engine has been operated like this for any period of time, assume that the engine is going to need serious help. If you are absolutely certain that the engine has not been operated under this condition, you may decide to go ahead and get the car. Be certain that the leak is fixed and the engine oil changed before trying to operate the engine again.

Air Suspension: When operating properly, the air suspension system provides an incomparable ride. Short of a major air leak in the system, the system will usually work well enough to allow the car to be driven successfully. The key phrase here is "major air leak". Just because the car doesn't have one today doesn't mean it won't have one tomorrow.

The most common cause of a "major air leak" is a blown air bag at one, or more, corners of the car. Sometimes a bag develops a small hole which will bleed air out slowly. Sometimes a bag will develop a large hole instantaneously with the resulting excitement of a loud explosion and the car slamming to the ground. This can happen at any time, including when you are working under the car. As part of purchasing a 300SE, you should budget for the replacement of all four air bags.

I won't say that they can't, but I have never heard of a bag in good shape that failed. Keep them in good shape and you will be able to avoid all the experiences that make those excellent stories that your car buddies have.

There are three types of air valves on the car. Check valves, the main control valve and regulator, and the "wheel" valves. The main and check valves can certainly have problems, but it's the wheel valves that are the most troublesome. These things are relatively unsophisticated and fail accordingly. The central moving mechanism is nothing more than a cam that rotates in bonded rubber mounts. This rubber also provides the seal against the pressurized air leaking out. The rubber has to stretch and twist as the valve lever moves up and down with the suspension, ultimately leading to tearing, and leaking, of the rubber. Assuming that the air bags are in good shape, the wheel valves are the most likely (but not only!) cause of leaking air.

Because the system operates on air drawn in from the outside atmosphere, if there is humidity in the air, there is water in the system. Most of this water collects in the bottom of the air reservoir tank. If the tank is not bled regularly, this water can start working its way into the rest of the system. This usually manifests itself as rust inside the steel air lines that run throughout the system. Replacing these steel lines is no small task.

The air reservoir is located inside the front left fender, behind the headlight assembly. The air reservoir tank has a Schrader valve on it for pressurizing from an external compressor. The Schrader valve is not in the most convenient place but it's better than nothing. The pressure has to be brought up to about 130 PSI to get the car to come up to normal height and, because the rear of the car is lighter than the front, the rear comes up first. The tank isn't very large so if there are any air leaks in the system, the car won't stay up for long, if it comes up at all.

There is a "bleeder" valve on the bottom of the tank. The tank should be bled frequently to expel the condensate so that it doesn't collect and start to work its way into the rest of the system. The tank is bled by pushing up on the stem protruding from the bottom of the valve. Don't do this in the middle of your (or the seller's!) clean driveway, or while wearing nice shoes. The stuff that comes out is a nasty, oily mess and can go everywhere (and certainly onto the hand you are using to do the bleeding). If the air compressor is in good shape and the system is bled regularly, then the quantity of expelled goop will be minimal. Note that bleeding the tank is only possible if the tank is pressurized.

The engine mounted air compressor is a "low volume, high pressure" unit capable of generating pressures over 200 PSI and are typically long lived. If you do have trouble with it, plan on spending next year's vacation money to get it fixed.

The compressor is lubricated by pressurized engine oil being fed to it through a flexible rubber hose. Plan on replacing this hose. It's not expensive and if it splits, all of the engine's oil will disappear very quickly. Don't count on noticing that the engine's oil pressure has dropped to zero soon enough to salvage the engine.

The compressor is driven from a fan belt, and the tension on the belt is adjusted by sliding the compressor in and out on its aluminum bracket. The oil that is pressure fed to the compressor drains out the bottom of the compressor, through the bracket and back into the engine. The gasket between the bracket and the engine doesn't usually leak (unless you have had the bracket removed) but the O-ring that provides a seal between the bottom of the compressor and the bracket gets torn up from sliding the compressor around to adjust the belt tension, and then leaks.

Air drawn in by the compressor is first sucked through the "anti-freeze" device. This device is a simple affair that bubbles the incoming air through denatured alcohol. The alcohol dries the air by absorbing the humidity in it. The intent is to prevent such an accumulation of moisture in the system that it could freeze during cold weather. The factory recommends filling the device when the outside temperature is below 40 degrees F. There is what looks like a small aluminum can mounted on the side of the anti-freeze device. This is the air cleaner for the incoming air.

The Main Air Valve is a somewhat complicated device that acts as a pressure regulator (147 psi max) for the air supply to the front wheel valves (the rear valve is supplied with full system pressure), as a pressure regulator for return air from the all the wheel valves (44 psi), and as a return air shut-off. Other than the brass shut-off valves, this device is relatively trouble free, so long as a frustrated mechanic hasn't changed the settings of the two regulators.

Contrary to popular belief, it is not wise to adjust the ride height of the car. This is done simply by adjusting the length of the steel rod between the wheel valve lever and the front lower A-arms or the rear sway bar. Some people do it because they like the  higher, or lower, look of the car. However, doing this has a severe impact on the ride of the car, especially in the rear. The system is designed as a very progressive suspension and is what accounts for the exceptional ride quality, when it is set up correctly. The height adjustment can be corrupted for more innocent reasons too, but the point is that you should expect to at least check the height, and be prepared to make adjustments as needed.

Starter: Fifty percent of the 300SEs that have been parked for a while have a defective starter. It may "sort of" work, but that just means it hasn't completely died yet. Most of the primary problems with starters are worn out bronze bushings in the nose of the starter. (If you were 35 years old and had never been lubricated, you'd be worn out too.) When the bushings start to wear (the holes becomes egg shaped), there is a lot more "drag" on the starter motor, causing the motor to turn slower, which causes the motor to consume more current from the electrical system. This makes the motor brushes wear out faster, and can make the heavy contacts in the solenoid burn down faster. And, the extra current flowing through the starter motor means that is operating hotter as well. All in all, a bad situation.

If you've already got problems, pull the starter out and check the brushes and the nose bushing. If you don't have problems yet, go ahead and buy new brushes and bushing, and schedule some time to put them in. The nose bushing is fairly easy to replace if you have any type of device that can be used as a press. (I've seen guys use C-clamps to do this!) The rear bushing is much more difficult to get out, but, fortunately, the rear bushing doesn't wear nearly as fast as the nose bushing. It doesn't have near the side loading that the nose bushing has.

Obviously, starters, like any part of the car, can fail for any number of reasons, but the nose bushing and brushes are definitely the most common reasons.

Fuel Pump: All 300SEs have an electric fuel pump. If the pump doesn't spin, the car won't run (not for long, anyway). The pump should be on any time the ignition key is on, whether the engine is running or not. When the pump is running, you can hear it spinning away back there behind the left rear wheel. If you can't hear it, it's not running.

Fuel pumps do wear out but not very often. What frequently does happen, however, if the car has been sitting for a long time, is that the pump vane gets stuck. This happens when the gasoline around the pump vane just sits there and turns to varnish. The cover plate over the vane can be removed and the crud cleaned out but this assumes that you don't break the heads off all six screws that hold the cover in place. Even without all the exposure to water and salt, the steel screws corrode inside the aluminum housing and are frequently there for forever. In the event you do get the cover off, be sure to replace the rubber O-ring between the cover plate and pump body. It's a sure bet that the old one will never seal again.

Injection Pump: In addition to all the other problems that injection pumps can develop, just like the electric fuel pump, they can have problems with varnish deposits causing the pump pistons to stick. The pistons are driven upwards by a cam in the bottom of the pump, and are spring loaded (with big springs) which is what is supposed to drive the pistons back down as the cam passes. The cam can drive the piston up through the varnish, but the spring can't push it back down.

It doesn't matter what else you do, if that pump piston isn't pumping fuel to the injector, that cylinder isn't going to run. If enough of the pistons are stuck, the engine won't even run. Assuming that the engine does run, the simple check is to crack open the connection between the steel injection line and the injector for the affected cylinder. If you don't start seeing regular pulses of fuel right away, the injection pump isn't pumping any fuel to that cylinder and, indeed, you have a defective injection pump. If you do start seeing regular pulses of fuel, but didn't initially, the line may have just had air in it and it needed bleeding. Tighten the connection again (with the engine still running) and perhaps now the cylinder will start to fire.

If everything is working properly, opening the injection line connection will cause the cylinder to stop firing, with a resulting change in the sound of the engine, because the fuel pressure in the line won't get high enough to cause the injector to open. Tightening the connection should cause things to return to normal.

If you have this, or any other injection pump problem, plan on having the pump rebuilt. The chances of your getting it working again without doing more harm than good is slim. Besides, considering its  age, the pump could use a good tune up anyway.

Intake valve recession: This is an aggravating problem. Every 300SE with the original intake valves has this problem so some degree, some worse than others. But they all have it, and I believe this is due to the use of unleaded fuel.

I had two different 300SE engines apart back in the '70s, both of which had always been run on premium, leaded fuel, and neither had any signs of valve recession. Twenty years later, when I started to acquire lots of these cars, I noticed that every single one of them had intake valve recession. And for a good portion of that twenty years, only unleaded fuel had been available. That is what my conclusion is based on.

Recession of the intake valve means that either the valve head, and/or the valve seat, is wearing away far too rapidly. This has two immediate effects; first, the mating surfaces on the valve head and seat become distorted and don't seal as well causing a loss of power, and, second, the clearance between the cam, rocker arm, and valve stem gets too small. Since this clearance is only .004" (0.1 mm) when set correctly, it doesn't take too much recession before the clearance is completely gone. When this happens, the valve cannot completely close and will create a larger loss of power. Visual evidence of this is the camshaft. If you see wear marks all the way around the cam, not just on the lobe, then that valve has no clearance. I have seen this get so bad that there is virtually no seal between the valve head and the seat, resulting in very low compression on the affected cylinder which, of course, also won't fire.

When this problem gets bad enough, you have no choice but to replace both the intake valves and the seats. Keeping the clearances set properly will usually keep the engine running on all six cylinders, but the engine will not have the normal amount of power. Don't let this problem go on too long either. See xxxx for a photo of an intake valve that was so badly receeded that it almost didn't even hit the seat!

I have not put all that many miles on any of the cylinder heads that I have rebuilt in the last several years, so I can't make any definitive statements about the new intake valves. So far, no problems. Mercedes does routinely make improvements in components, and I expect that the intake valves have also been re-engineered, and if so, then new valves should last for a long time.

Plan on replacing all twelve valve guides, these are routinely bad. New factory guides are silicon-bronze, so you will never have to worry about them again. I have never had a weak valve spring. I have had a couple of cracked valve spring retainers.


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