I have an 89 300se. Sometimes when you put it in reverse and start backing up, it will choke and stumble some and when you stop and put it in drive, it sometimes quits. Then, it is hard to start for a fews seconds. I can eventually get it started by pumping the gas. Could this be a bad fuel filter or fuel pump? It does not do it all the time, but when it stumbles and quits in reverse, you can almost bet 99% of the time, it will be hard to start until you get it started. Then it is fine. It will run fine and will not stumble anymore. It seems like it does it when the car has been sitting for a few minutes or overnite. Any suggestions?

That's an interesting observation Sorry, couldn't help it.

It might be useful for the diagnosis to know whether this happens right after you start the engine or after a long drive.

Sixto
91 300SE
87 300SDL
83 300SD

Start with the easy stuff first. Have you checked the distributor cap and rotor? I had a hard starting problem caused by a cracked cap. When was the last time the plugs were replaced? How do the spark plug wires and connectors look?

Check the ignition system first, then fuel delivery/injection, then compression.

Any other odd things happening? Loss of power during acceleration? any dash lights on? Do you smell gas?

__________________
Mike Heath
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino

Usually I will notice this problem of stumbling and quiting when it has sat overnight or has sat for a while(time to cool down). Once the motor is hot and has been driven for a while, I have yet to notice this problem. It seems like if you put in reverse and it seems like the idle is stumbling, you had better be careful when you put back in drive and whiggle the gas pedal(giving gas) until it starts rolling. Once the car is driving, you don't have to worry about it quiting anymore. I have not checked the plugs,wires, dist. cap yet, but can start down the list. I did put some gas treatment in the tank once and it seemed to be much better. I didn't notice the problem as much. The best I can remember, I found paperwork where the plugs had been replaced about 25-30k miles ago. Do you think running a couple of bottles of Techron through it might help, as I have seen in other posts? But, as stated before, I seem to notice it more when the car has been sitting for a while or overnite. Thanks for all the help.

Scott

I also, forgot to mention, in response to your questions, I do not notice any gas smell. I have not noticed any loss of power. There have been no dash lights come on. And there has been really nothing odd other than the problem stated.

Maheath had some good points - easy stuff first. The plugs in your car really need to be changed about every 15,000 miles. Use Bosch H9DCO. You may also use H9DC; however, the H9DCO has a slightly wider side electrode than the H9DC.

Mercedes calls for a .032 gap. Some folks here have reported that a gap of .040 provides a smoother idle. I settled for something in the middle -> .036.

I had a slightly rough idle awhile back and found the rotor to be dirty. I merely cleaned/smoothed it out with emory cloth. Did this on a weekend - parts shops were closed - smooth as glass after that.

You do not need to replace the entire plug wire assembly, just the resistors(piece that connects to spark plug). They screw off and back on to the wire fairly easily. A scope used by techs can help pin-point weak cyls.

It might be worth an hr. or two of shop labor to have the engine scoped and the fuel system analyzed. This way you don't waste a ton of money throwing parts at it.

Since it only happens when it has sat overnight or has had a chance to cool, my money says something's not right in the fuel system. One of the pressure holding or controlling devices isn't working properly.

Do a search on accumulator and check valve for the 300E/M103.

Could also be an injector that doesn't close completely.

Sixto
91 300SE
87 300SDL
83 300SD

I have noticed it stalling or choking sometimes even when the car has been run for a while. Can this be possibly caused by a vacuum leak? I know there is a line going into the air filter canister that is very loose and does not seal off inside the rubber hose(connection). Could this be part of the problem? I am currently running Techron in the tank. I have not had time to check the plugs or wire connectors.

Scottfish - don't know if you saw this post. Might be worth looking at:

When stopping, Car dies!!!