1991 300se W126 general problems

MBLover

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Feb 3, 2011
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6
Location
South Africa
Car
300se 1991
Hi,

I recently bought this car as the body is in immaculate condition and the previous owner said it was in excellent mechanical condition (however this is not the case).

I took it to the mechanic the next day to have it checked out and he found a few problems which he didnt think was a big concern.

I wonder if you could give me an idea of what could cause these problems to either put my mind at rest or let me know if I should try and repair what is faulty.

1) The thermostat has been removed, and the coolant is very dirty and rusty, there is no presence of antifreeze. The radiator was redone and has a sticker on it, but I know the company very well and they would not have released the vehicle without antifreeze. This leads me to believe that the vehicle has had over heating problems and the coolant has been replaced with plain water over some period. Why would someone remove the thermostat if this makes the car less economical and also causes overheating? I have yet to test the aux fans and their sensors (on radiator and a/c receiver bottle I assume), and possibly clutch fan). I wonder if they didnt remove the thermostat because one of these problems might be present. I am not sure it is actually removed, but the mechanic confirms it is not working but hasnt opened the housing. I havent heard of a thermostat that gets stuck in the open position either.

The car however does not overheat though in city driving, freeway driving at high speed does raise the temperature even after extended periods to just under the line I imagine would represent 100C. When you put the heater on, it does not seem very effective. Could that be due to low running temps of 60C or likely be the aux water pump. (when will the aux water pump turn on and off?)

2) What are the chances the head could be gone? It drives really nice and has plenty of power (easily drops a Golf TDi without effort)

3) Valve seals suggested to be replaced at some stage, mechanic says its no big deal and to check the oil periodically. It only makes a little blue smoke (barely noticeable) for a few seconds after very hard driving (say 180km/h for a few mins on the freeway)

3) Vibration at certain speeds/revs only, thinking propshaft center bearing or universals as the chassis vibrates, not a wheel.

4) There appears to be some oil ( or rather I think fluid) but the power steering is quiet. I am thinking this might have to do with the auto-leveling suspension (rear only). Could that be the pump leaking? Does the powersteering pump control both the leveling or is there a seperate pump on this model? I noticed the big "dump" tank was empty and I refilled it with the wrong fluid which the mechanic is going to flush.

I am wondering if the system is still active or if the shocks have been replaced which such do not have the hydraulic leveling capabilities, but I havent had the car since I bought it a week ago to verify this.

The shock (possibly) mounting makes a thud noise at times and am told this might be stabilisers, but I now also am considering this might be due to a changeover of shocks that dont work with auto-leveling as I believe they wont fit correctly.

Do these leveling systems give much trouble, and what should I look for. Cost of repair approximately?

5) The gearbox skips 3rd gear. It launches in 2nd gear which I am told is normal for this box, but then revs quite high before changing into fourth. The mechanic thinks it might be something small like a "switch box" as if you manually shift into 3rd it changes fine to that gear, but will not do so in when Drive is selected.

Also how does one make the vehicle launch in 1st gear, or rather, when would the car "know" to use 1st gear - I dont believe it used first gear in the short time I drove it.

I have a very steep driveway, and stopping at the steepest part, releasing the brake, the car does not roll back at all. How does the topograprical sensor work, or how does it achieve this.

6) On cold startup, the revs go to about 1200-1500 approximately for a short while and then come down to maybe 700-800rpm - is that normal?

As the engine performs very well even with a little oil leaking from the usual places, the mechanic is not concerned with a head problem and intends to do the following to the car.

Find problem with gearbox this coming week.
Flush cooling system and replace with proper coolant.
Replace thermostat (assuming it has one) else install new one.
Sort out prop vibration and shock / stabiliser noise if applicable.

Should I insist he do a chemical test to see if head was damaged? There is no white smoke, no sign of oil in water or vice versa.
Should i insist he check the sensors and full operation of the aux fans including relay etc?
Should i also insist he repair the self leveling suspension if the parts exist on the car and fix the pump if it is a problem - could this be very expensive?

Lastly, if the gear stick is placed in 2, is that second gear only or 1+2?
and in 3 is that 3rd gear only, or 2+3?

It looks like I may have bought a lemon, or rather something that will need some work. Mechanic took it for the roadworthy test and it passed fine.

Please offer any advice you may have. I am new to MB though have always loved this particular car, though I might have been a bit hasty to buy to it without proper investigation.

Thank you in advance for your contributions.

Mileage is 267000 Km's or 166000 Miles
 
Last edited:

Scott_F

MB Enthusiast
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Mar 12, 2010
Messages
4,094
You have bought a 20 year-old car that has covered 166k miles. Some problems are to be expected and are pretty much inevitable.

As the car doesn't overheat check for evidence of oil in the coolant and emulsified oil ("mayonnaise") when you remove the oil filler cap. If all seems OK proceed with your coolant flush and replacement of the antifreeze and thermostat.

Also check the condition of the ATF fluid. It may not have been changed for many years and changing the fluid and filter can greatly improve things.

If you persevere you will hopefully iron out these problems and have a car that will be a pleasure to drive and keep running for many more years.
 

grober

MB Master
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Location
Perth, Scotland
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W204 C200CDI Estate
THE MAJOR PROBLEM WITH THESE CARS IS BODYWORK RUST. If you have found a rust free example treasure it! All the mechanical problems are readily solved on these cars particularly since you have a cast iron block and not a alloy V8. The thermostat mystery is trivial - just replace it. If there is an overheating problem its probably the head gasket- known weakness on that engine--- get the garage do a coolant headspace sniff test with an exhaust gas analyser-- presence of combustion fumes in the coolant means the CHG is gone.Otherwise could be radiator/water pump/radiator/ engine fan / aircon fan/leaking -perished coolant hoses. Vibration could be engine mounts/propshaft coupings or centre bearing as you said.OR possible problems with the auto transmission-- check fluid level and condition- changing it might be a good idea as suggested.Changing gear problems could be the cable or vacuum modulator needs checking or replaced-- most of these are routine checks/adjustments if nothing is broken. The self levelling is the one area repairs might prove relatively expensive but I would urge you to get it repaired rather than converting it.
It sounds as if you have bought a basically sound car that the previous owner has just neglected to repair some minor problems and these have accumulated to significant level of expense rather than anything major.
PONTONEER or JAYMANEK are the W126 guys to ask on here about these cars.
 

ianrandom

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Feb 19, 2010
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Berks mainly, UK. Devon too a lot of the time.
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1986 420SE, 1990 420SEC, 1989 2CV6.
Ditto, doesnt sound too drastic.

As for 1st gear, is it a European model car? I dont see what country you are in. European cars have 1st gear start, you select kickdown from a standstill and you will feel it drop into 1st gear. Its also possible on those cars to selct 'B' on the gearshift gate and that will select 1st. You can select this at a standstill, and if you like then move the lever back to 'D'. Then the car will start off in 1st whatever you do, on that one occasion. It will hold the gears longer for better acceleration too.

Otherwise the car will always start in 2nd gear for smoothness.
Some markets didnt have this feature, the US for example.
 
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MBLover

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Joined
Feb 3, 2011
Messages
6
Location
South Africa
Car
300se 1991
Ditto, doesnt sound too drastic.

As for 1st gear, is it a European model car? I dont see what country you are in. European cars have 1st gear start, you select kickdown from a standstill and you will feel it drop into 1st gear. Its also possible on those cars to selct 'B' on the gearshift gate and that will select 1st. You can select this at a standstill, and if you like then move the lever back to 'D'. Then the car will start off in 1st whatever you do, on that one occasion. It will hold the gears longer for better acceleration too.

Otherwise the car will always start in 2nd gear for smoothness.
Some markets didnt have this feature, the US for example.
Hi ianrandom

I am in South Africa - The car gear shift gate has the following designations 2,3,D,N,R,P and has a selector switch labelled S/E which I am guessing is Sport/Economy?
 

kpon126

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Jul 10, 2008
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Location
Bangkok, Thailand
Car
1991 300SEL, 1991 190D 2.5 & 1996 C220
Hi ianrandom

I am in South Africa - The car gear shift gate has the following designations 2,3,D,N,R,P and has a selector switch labelled S/E which I am guessing is Sport/Economy?

Actually it's standard / economy.

The shift gate with engine brake "B" position is normally found in the V8 models. Yours is a 300SE and therefore does not come with it.

4) What is the colour of the leaked fluid?

The sensors and switches for the aux fans should be tested to ensure the proper operation.

A W126 can eat up miles if properly, I mean really proper, maintained.
 
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MBLover

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Location
South Africa
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300se 1991
Hi kpon126,

Thanks for the info. I can give you guys an update now on things done to the car so far.

The mechanic I found seemed to be far to busy to get around to the finer details of the car problems and only did what was considered "serious" and was also very cheap. I cannot complain and has a wealth of knowledge of these cars and plenty lying around which are being broken up for spares.

1) One self leveling shock was replaced (very lucky he had one lying around which was never used). The SLS is working perfectly now and no further leaks.

2) He says he took the gearbox out and replaced clutches however the same problem still exists (2nd gear shifts straight to 4th). He cleaned the valve body and replaced the transmission fluid. The old fluid was looking slightly brown but as mechanic said did not have a burned smell like you would find if clutches were burning. After he replaced the fluid he also adjusted the pressure (higher I believe) and it does now sometimes engage 3rd gear on up-shifting, and the more I drive it with the new transmission fluid in the better it gets. Yesterday it started downshifting to 3rd each time you press the accelarator and then swiftly going back into fourth (in economy mode) so I think whatever dirt was in there must be loosening itself up.

3) I replaced engine oil, petrol filter, oil filter, air filter (plugs were replaced recently so did not do those), diff oil (since I do not know the history). The diff oil was very clean though, but glad I did it anyway for peace of mind.

Since doing the oil change the car is like a new machine. It is suddenly more responsive and "happy". Actually quite zippy and now have to carefully monitor the speedo as almost always find myself at 140kmph since it feels like your only doing 60kmph. The car used to battle a bit to start and the idling was high for a few minutes, but after flushing radiator it has improved significantly and imagine there might have been rust caked around the temp sensor as now comes down to idle of 800 in under 30 secs from cold start (currently 10C at night or early morning). The radiator fluid had the same consistency as mud from the header tank (or whatever the plastic water bottle is called). I have rinsed out the block and radiator with a hose pipe and replaced the fluid and put in a good antifreeze (alumninium friendly).

So its no understatement that this car was neglected.

Today she is getting her prop shaft balanced as their is definitely a visible vibration when the prop specialist ran the car on the lift. The universals seem fine though.

The only thing currently concerning me is in the morning on first start there is a bit of a tappet noise for up to 20 secs which then dissapears and the car is fine for the rest of the day. I am taking her in later to just get the tappets tightened and checked though all mechanics who have heard the engine running ask to buy it.

I have been told to consider replacing guides and seals sometime in the future but that it is not at a critical stage yet. No smoking and plenty power, but recommended at this mileage. So will tackle that in the new year.

As the car is practically at a different mech each day I havent yet got a nice set of pics of her, but have some of the engine which I will figure out how to post shortly for you to look at. The engine has since been pressure washed (carefully by me) and looks so much better now. The power steering was leaking but solved with a seal replacement.

This pic was taking just after the shock was fitted on the lift and was parked here for bleeding as lift was needed urgently for another car, and one of the engine. Hood still open as dump tank resevoir being filled. I will post more pics once I have given her a nice wash and clean.

I also plan to remove those ugly things on the front windows, but its going to be messy as has been siliconed on, but feel it doesnt compliment the car.
 

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Scott_F

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2) He says he took the gearbox out and replaced clutches however the same problem still exists (2nd gear shifts straight to 4th). He cleaned the valve body and replaced the transmission fluid. The old fluid was looking slightly brown but as mechanic said did not have a burned smell like you would find if clutches were burning. After he replaced the fluid he also adjusted the pressure (higher I believe) and it does now sometimes engage 3rd gear on up-shifting, and the more I drive it with the new transmission fluid in the better it gets. Yesterday it started downshifting to 3rd each time you press the accelarator and then swiftly going back into fourth (in economy mode) so I think whatever dirt was in there must be loosening itself up.

I wouldn't run the car for too long if you have reason to believe that there is still dirt present in the ATF. Have it changed again soon and ensure that the filter and gasket are changed as you made no mention of the mechanic having done so with the first ATF change.
 

grober

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The tappets are hydraulic so cannot be adjusted- only replaced. Older engines often have noisy tappets on start up- nothing to worry about if they go quiet after a few seconds.

try this for autobox detail http://w126.pp.ru/akp722.pdf
 

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