I have a 1988 300 SEL that I have been having some trouble with and need some help.
The problem is the engine quiting. It is intermittent, and gives little or no warning. Usually does it when coming to a stop during braking, or when decelerating, but will also do it at any time. It always starts good when cold, but on a hot start it will crank good run for a second or so and then the engine will start running rough with the RPM's droping untill it quits. When restarted, it runs ok most of the time. Sometimes have to start it a few times before it will run. If I try to rev it when it is about to quit (less than500 rpm's) it will sometimes smooth out and run fine at its normal RPM.
During acceleration, it will start out pulling like it should, but sometimes will die down, and not be able to accelerate up to speed. During this, there is usually a slight drop in rpm's and the economy gauge (I assume hooked to vacuum) will peg out. The engine will still be running and running smooth, but just will not take the throttle input. The engine will either run like this for a while and quit or start operating like it should.
I generally do all the work on my cars myself. However I knew that I didn't know enough about it to fix it so I have taken it to several shops that were not able to find the problem. I feel that this is because of the intermittent nature of it. However even when it has quit while being driven into the service bay the problem could not be found.
Below are some things that I have done.

First of all I rebuilt the engine. I'm in the auto machine shop bussiness. It mainly needed a set of valve guides, and other head work, but I went ahead and rebuilt the engine to be sure.

I replaced all the fuel injectors with new ones from Mercedes.

I replaced a cracked rubber boot or duct that goes from the fuel injection unit to the intake manifold.

I replaced the fuel filter.

I checked the fuel pressure (3 Bar)

And it held this pressure for 30 minutes. I was told that this needed to be checked by a local mechanic so I borowed the gauge and checked it.

I'm open to any suggestions.

I like this car and would like to be able to drive it without being woried.

Thanks

Bill

Based on the way it shuts down on you I suspect that it is an ignition issue. Can't help much more than that - most of my cars don't use spark. You may want to start doing a search for your specific model and "shuts down".

I'm originally from Soperton.

__________________
Jim

Hi, Jim

Small world isn't it.
Thanks for your reply. I have searched for this with various key words with very little results, especially with regard to a 300 sel. I will keep searching and see if I find anything. I did find a couple of post's describing similar problems with different model cars. However there were not any replies to the post. Such as this one below.

http://www.peachparts.com/shopforum/showthread.php?threadid=47042&highlight=shuts+down

Thanks,
Bill

Well, I've searched the site for hours and still have not found anythind that relates directly to my problem. I did find a lot of posts where people were having similar problems, but no one could offer a fix to the problem.

Is my problem something that is not practical to fix?

I can't afford to replace all fuel and electrical systems from front to back. A few years ago I tried to obtain a service manual for this car and found that it was only avail from Mercedes for what I think was a high price.
On this site I have seen repair manuals on CD for about $50 or so. Do these provide good complete coverage of the whole car?

Any ideas on what I can rule out as being the culprit?

Any help greatly apreciated.

Bill

Not sure where you are at in your hunt for a fix. It seems likely to be ignition/fuel related. I would suggest getting it to a shop with the Bosch tester (the expensive one) that will check your fuel system and computer links. Otherwise, unless it's pretty obvious, you'll spend a lot of time and money chasing the ghost.

I'm not sure if my problem is exactly the same as yours, but on my 1989 300SE, I once had an intermittent loss of power problem. I changed first the over voltage relay, and later the fuel pump relay. Did so after doing the usual tune up stuff--new rotor, cap, wires, plugs, none of which solved the problem.

Anyway, because the problem was intermittent, I changed the relays on the advice of an MB tech friend of mine. Whatever it was, the problem seems to have gone away. Have not had it for probably 5,000 miles at least.

The over-voltage relay, I'm told, can cause many weird things. As has been suggested, it would be best to take it someplace with good test equipment, but if the problem is not in evidence when you do so, I suspect they will not find the problem. Both relays are easy to change, by the way--located between the firewall the the engine encapsulation wall. Best of luck.

Joe Bauers

Bill, sounds like you are and asset to the best MB forum there is! I would put your car on a code reader might be something simple..........
William Rogers........

Thanks so much for the replies...

Right now I'm trying to figure a way to set up a fuel pressure guage "T'ed" into the fuel line so that I can read it while on the road, and see if there is a drop in pressure when it quit's.

Joseph, is the over voltage relay the one with the clear top cover with a 10 amp fuse in the top?
I forgot to mention that I replaced the fuel pump relay. It was one of the first things that I did. If the price of this other relay is not too high I might replace it and see.

Thanks Again!

Bill

Hi, everyone

I drove the car a good bit today and had the engine quit a couple of times. It seems to be fairly consistent now. All you have to do is warm it up, shut it down, and wait a couple of minutes, then crank it up, put it in gear and try to drive off.

I did some searching for info on the OVP relay, and found one post that offered a way to check the OVP.

My understanding is that it can be checked by turning the ignition on, not starting the engine, and checking the voltage on diagnostic pin #3.
And the voltage should be between 2.5 - 5.0 volts.

So I checked it and it read 3.63 volts.

But, this evening I thought why not check the voltage just after the engine quits without turning the key to reset everything.

So I did and I got a reading of 7.5 volts, which is over the range according to this other thread.

Does anyone know if this is a way to check it?

Thanks,
Bill

I own a 91 300 SEL and have a similar problem. I searched the site and found several recommendations, it appears to be a common problem and the recommendations are the same. (Use "stall" in your search if you have not already) I have concluded that the best advise is to take it to the shop and have it hooked up to a diagnostic computer, as was suggested in this tread. I have not read on a shop forum of anyone consistantly solving the problem after all the replacement parts commonly suggested have been installed (OVP, spark plugs, injectors, et. al.)

Similar to you and others I replaced the OVP, spark plugs, wires, rotor, cap, tranny fluid, man I even switched to synthetic oil and replaced a license plate lamp too It all needed to be done, car ran better and I hoped to have solved the problem...but it died on the freeway when I stopped in traffic a couple weeks later. Thats why I stopped short of doing all the other stuff you have tried. I have found in this forum the OVP change to be successful for some (as was testified in this tread), but then again I find more threads were it is not the solution. I even replaced my MAS unit due to me shorting it out while changing my fuel filter in an attempt to solve this stall problem. So I have changed my relays and I still have the problem.

I have not helped you solve your problem, or mine either, but I find some reward in knowing that its not just me. Funny thing about this kind of issue is the confirmation that you have solved your problem is slow to come your way. Someday I will take the car to the shop and have it looked at.

One things for sure, our 300SEL's are one fine ride!

BMJR(Bill):

Have you looked at the fuel pump relay(behind the battery on 88-89 models). The 90-91 models have them within the MAS. Yours is easier/cheaper to replace.


There have been alot of write ups here on M103 motors exibiting the problem you describe and in many cases a faulty fuel pump relay was the problem. Some have even done some resoldering, but I wonder how long that holds.

Search around some more for faulty fuel pump relays on 103 motors, especially in 124 cars. They essentially have the same motor as your 300SEL and a fuel pump relay mounted around the battery.

This faulty fuel pump relay problems seems to surface once warm.

My 2 cents.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

Thanks for the info, Mike.
I'll have to go look and see if I can find it. The only relay's that I knew of were the 3 between the two wall's in the engine compartment, and the one's in the fuse box.

Lyle, you have made a good point.
I could end up replacing hundreds if not thousands of $ worth of parts and still not fix the problem. If I take it to a shop it's going to cost a lot, because I would have to leave it with them so that they could keep it long enough to find the problem. BTW, I forgot to mention that this is my only vehicle.
I've been thinking about fabricating a intake manifold so that I could get rid of all the FI stuff, put a carb on it and not worry about it any more. I know that the mileage would drop even lower than what it is now, but it doesn't matter. I just want it to run.
It seems to me that with all of us having this problem there would be more info on how to fix it.
I guess I'm stuck between a rock and a piece of granite.

Bill

I do my own work for the reasons most people on this forum do their own work. However, there are times when inexperience and/or lack of proper equipment will likely cost much more time and money than it's worth to 'chase the ghost'. Your problem is one of those that can be very difficult to solve - as you're finding out.

When I had a similar problem with my 90 SEL, I made an appointment with a local Bosch trained shop that I trust. They diagnosed the problem in 30 minutes and I did the repair myself. If I remember correctly, the diagnosis was $60.00.

Oh yeah - problem was that a mouse had chewed a tiny bit out of one of the computer links (cable) that runs on top of the left inner fender. It could BARELY be seen, and only because the diagnostic computer definitely prooved that the correct signals were not getting where they were supposed to be. With that info, it drastically reduced the troubleshooting and 'sport replacement' of various components.

As much as I enjoy keeping things running properly and doing the work myself, sometimes you've got to weigh the risks and decide if it's worth the time and effort and costs of potential mistakes.

Good luck - and let us know what happens.

Well, I decided to dig this thread back up and revisit the problem which I still have. I really need to figure out how to diagnose and fix this thing. Due to the manner in which it usually stalls I'm either at an intersection or backing out of a parking lot. Neither are a good place to be sitting while trying to restart the engine.
Recently I tried to back out from the parking lot at work to go home. It quit aprox. 20 times. This is the worst it has ever been, and I thought I was going to have to leave it that day and get a ride with someone else. I managed to push the car back in a parking space, and keep trying to restart. Each time it would crank fine or with a minimal amount of cranking time, run for a little and then quit. I finally got it running consistently sitting there in park. So I put it in rev., and just sit there holding the brakes. Within a second or two it would die, everytime I tried it. So I kept doing this and I noticed something that I hadn't before. Just before it dies you can hear the faint sound of the fuel pump running differently. It (or something making a similar noise) gets slightly louder and intermittent, then the classic stalling of the engine.
I did this several times and it did the same each time. I'm not sure that what I'm hearing is the fuel pump, but I don't know what else it could be. It almost sounds like another motor comes on on top of the fuel pump sound.
Eventually it started and ran fine in gear or not, and I headed out for home. No problems all the way back home (10 miles).
I thought that this problem had gone away on it's own for the most part, and I would live with it stalling from time to time. The incident in the parking lot the other day has changed my mind.
One thing that's sort of funny is how I've learned to drive this car..... Right foot on the throttle, left on the brake, and one eye on the tach to watch for a drop in RPM when around intersections.
Also I plan my driving while in town so that when it quits I'm not on a busy road. What a way to drive.

I'm still around. You alreay know my suggestion .