I am planning on getting this car to a shop ASAP, but wanting to check on one thing first.

I have a 1993 Mercedes 400SEL (W140 4.2L) that will start and run perfect when the engine is cold. However once the engine is fully warmed up and you turn the car off... when you restart it is either hard to start or when it does start it runs very poorly... it will misfire, splutter and barely run. This has been going on for awhile and has slowly gotten worse and worse. I finally had parked the car, but will start it and run it occasionally. The car has fairly new distributor cap/rotor button, etc. I also have already swapped out the old EZL with a good used one and the problem persists. The car also already has a new engine wiring harness (about 4 years old).

My normal MB mechanic can't look at the car until April 2 and I cannot wait that long. I do have another mechanic/shop lined up, but wanting to see if it is something I can troubleshoot myself. Someone told me it sounds like the ignition coil may be going bad, but I do not want to start throwing costly parts at it. An ignition coil for this car is over $100, so I do not want to replace that unless I know that is what the issue is. I let it start and run yesterday... it ran perfectly, smooth as silk. I let the engine warm to full operating temperature, shut it off. I restarted about 10 minutes later and it was missing, sputtering, etc. It was like it was literally running on 4-cylinders when I would put it in gear and it would try to stall out and eventually the exhaust started smelling like rotten eggs.

My check engine light does not work, no not sure if it would be on or not. Someone said there is a way to check for codes by a light/button under the hood and I have tried that before, but forgot how to do that and do not know how to read the codes.

Any help appreciated.

* I was also told it could be the 'output shaft sensor', but not sure how you check it or replace it?? Would that cause poor running on hot start-up?

sounds like a coil breaking down,How many coils does it have?It also can be a crank sensor if your model has one.
I buy all my parts thru EBAY.you could buy a used coil.My coils run 150 each,I bought 3 for that price,and they were bosch.
when it acts up check and see which plugs are not firing.before you buy a crank sensor check the wiring to it.Mine came loose

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

Thanks Oldsinner! I think it has a new coil(s).

I ended up taking it to another local shop and here is the update. I am rather frustrated. Four days passed and they never called, so we finally called on Thursday for update and the mechanic said he could find nothing wrong. He said at first he let it run for hours and drove it all over and it started back and ran fine everytime. He said since he really didn't do anything he was only going to charge $35. Well I thought that was too much. He did said that the oil leak (smell I was smelling) was from the valve cover gasket that is LESS than 2 years old, so that is frustrating. Makes me wonder if that shop actually replaced the valve cover gaskets.

Well I went and got the car today (mechanic shop was closed), but the sales department was open, so he gave me the key and said there was no charge that he only spent 35 minutes on it. Well that was strange. Also he told my mom when she called for me (I was busy at work and tied up all that day), he told her that he couldn't find anything wrong with it and even if he did he probably didn't have the tools to repair it and the car was so old, he didn't have the computer to check it? This is a place that specializes in German cars and they cannot work on a '93 Mercedes? They are also supposedly ASE certified. He told me before I took it that he could check it, so I am a little frustrated. I will never go back there and I will never recommend them to anyone. They apparently didn't know what was wrong with it and didn't want to mess with it.

Well I drove the car home (about 17 miles) and it ran like a brand new car. I got home and it restarted everytime and ran like new. I am honestly not sure what is going on, but the other day I couldn't even get the car to run good enough to get it to the lower part of the driveway.

I have an appointment with another MB mechanic on April 2, so hopefully he can figure something out.

Oldsinner, I remember you are fairly local to me - Do you know of any good Mercedes mechanics in the area? Benz Doctor in Bloomingdale is an awesome MB mechanic, but he is always booked up and backed up. He can check my car on April 2nd.

... and it puzzles me that the car never showed the problem to the mechanic and it runs like new on the drive back home and started back everytime. Now tomorrow may be a different story and it will leave me stranded if I decide to leave in it.

Sounds like a leaking fuel injector or two....

Leaking? How would I check? I do not smell any fuel under the hood... or would I not smell it?

Thanks.

fuel injector(s) would leak into the combustion chamber, so inside the engine. The rough running is the engine laboring to burn off the leaked fuel. Once the leaked fuel burns off then it runs fine. At a cold start, the leaked fuel has plenty of time to evaporate (overnight). If 5-10 minutes (say going inside a store) it's not enough time for much fuel to leak. But if the car is left for 20 mintues up to a few hours, the fuel can leak and then it sits there, unevaporated, until you start the car when it has to burn off.

So it would have to evap past the exhaust valves? Wouldn't that cause a backfire?

It don't backfire. In fact right now it is running perfectly. Two mechanics have not been able to find anything wrong... it goes to a 3rd mechanic on Tuesday.

I took it to a much closer mechanic that my uncle told me about that works on my uncles Volvo and BMW. He is said to be a very good mechanic.

He has been unable to find anything wrong either! He has let the car run, drove it, etc and it has not failed to start the first time. He put some seafoam in the tank and said it is running much smoother and said that some sort of temperature sensor for the fans looked like needed replaced and it was cheap and easy to replace, so he is replacing that. He said at 230K and original converters, it may be time to replace them. I am going to drive the car this weekend and see how it goes.

Like I said, I also have to take it to the other guy on the 2nd to check the converters and repair the check engine light and see if he can find anything out on it. What a mess.

Thanks!

Drove it on two long trips out today... ran like a new car. Started everytime when hot and ran smoothly. This car is such a mystery. There is currently seafoam in the tank, so maybe it is cleaning that crap out of there and that was my problem all along and not anything electronic.

I just got back from my MB mechanic #3 and he said that he thinks it is the cap and rotors. He said they were a little "crusty" and cleaned them up for now, but said I should replace them. They are only about 2 or 3 years old, but he said these cars were hard on these things. I have a couple of good used ones, I may try those. Even before the car hasnt stalled on me and has been running well, so not sure what was going on before I parked it all of those months, but it seems to have cleared itself up.

He replaced the CEL bulb, someone had put in the wrong bulb. He pulled old codes (there were several) and cleared everything out. No codes showing now. He said overall the car was solid, no major issues. The radio turning off by itself and going on when the left signal was turned on was an issue he thinks was caused by the battery cables not being properly tightened. The engine harness (main) was indeed replaced and no apparent issues with the others. The other problems noted was he things a couple of shifter bushings are bad - will replace for $36 parts and labor and he said the clutch fan was getting weak. He had it idling and put his hand in it to stop it and was able to. This is pricey - he said around $585.00. Parts were like $500, labor around $85, but surely I can find a fan cheaper than this? I thought I had bad engine mounts, but he said he didnt think mine were bad enough to worry about right now... if I did want to replace them however it would be $825, parts/labor for both mounts, including parts/labor for the transmission mount. Not sure what to do at this point - I may end up selling/trading the car, but need to make sure it is running good for now.

Thanks for all of the advise throughout this thread.