Stalling
 

My 1991 300SE has been stalling lately. It will shake when I start it after it has been sitting. I will let it warm up a bit. When I have to stop at a street light it will stall, but it will start right back up. When this happens, I will take it on the highway and drive it around 70 mph for a few miles. Once I do that, it no longer stalls at street lights. But then after it sits for a little while, the process starts all over again. I did get a tune up about 2 years ago. I have a second vehicle and do not drive this car all that much. Any thoughts on what might be causing the shaky starts and the stalling? Thanks.

Diagnosing over the web is a shot in the dark. My '91 300-SEL did the stalling bit some years back. Did not experience the shaking.

Replaced the OVP - over voltage protection relay. It's in the firewall area midway across. You can look up a photo at this sites parts section. An easy DIY replacement.

I would first look for disconnected/leaking vacuum hoses.

Possible causes:

- Vacuum leaks. Most likely scenario. Unmetered air is causing havoc
- OVP relay. Usually comes with dash light funkiness
- Sticking air meter plate
- Fuel filter
- Bad coolant temp sensor
- A wire fell off
- sticking fouled or failing idle air valve

Rule out vacuum leaks first. There are many many places for those to occur and the m103 has several custom molded air hoses that like to crack, particularly the one that goes to top of the engine next to the valve cover.

Thanks. I am not a DIY person so I need to take it into the shop. It does not sound like anything major, so that is a relief. I appreciate the input.

Since you must take it in, be sure the shop uses ONLY OEM bits for the two idle valve breather lines and the CIS boot. I also recommend OEM for the valve cover to head and valve cover to air cleaner assembly.

If you understand the concept of a nut on a bolt, you can at least remove the air cleaner assembly and take a look around. :)

M103+W126 cars have an issue where the exhaust downpipes come into contact with the idler arm bushing or the caster rod brace when the motor mounts sag. Changes are you are feeling this on start up. Use only OEM mounts, others are garbage. BTDT.

Very much true! My 300SE has been through several sets, even OEM ones don't last forever. One thing I have suspected but never proved is that the engine shock absorbers get worn out and fail to damp engine bucking on hard shifts. The engine bucking eventually tears up the motor mounts. If you do the motor mounts, do the engine shocks too just in case my theory has merit.

I still have not taken the car in. What I noticed today when I started it, it shook some and then I got out and looked at the back, it looks like water or something was coming out of the tail pipe. It looked clear, it did not look like oil or radiator fluid or anything like that.

Was the engine warmed up when you did this or was this the first start of the day in humid weather?

It's normal to have condensation coming out of the tailpipe after starting a cold engine.

Thanks for the questions, I am female and I don't know much about cars. It was not humid but yes, first start of the day. I have an independent Mercedes mechanic I am going to tow the car into. I could drive it there, but with it stalling at stop lights that could be dangerous and cause an accident.

No worries about being female, you can still be informed and not perform the repairs yourself. :)

If you feel like posting your location, you may be able to find references to an independent shop or forum folks to help you out.

Thanks Hit Man X. I use this forum only when I have a question on something going on with my car that seems out of the ordinary to me. I got the name of a local shop from this forum years ago and I still go there. I am going to give the guy a call next week. I was looking to see what others thought regarding if this might be a major problem or not. It doesn't sound like it is, so that makes me feel a lot better!