400E rough idle

[Bof]

Hi,
I have a 92 400E with 90K miles, which has picked up a rough idle, like small, quick surges. I pulled a plug and noticed it was somewhat wet. I seem to remember having read somewhere that transmission fluid can get into the engine somehow, and I have been having to add transmission fluid, (automatic), lately. Could that be the problem, and if so, how would I check to know for sure.

I have not yet changed the ignition harness, which from all I have read, will be required soon. If that might be the problem, how would I check that?

Thank you for all the good postings and for any advise you can offer here.

[rsmcmahon]

Finding the source of a rough idle can be the result of many things, but I would start by checking …………

A Possible vacuum leak
Replacing plugs, rotors and distributor caps – if needed
Check condition of Engine Mounts – at 90,000
A Faulty wiring harness which connects to various components

Do you experience rough idle only when the engine is cold or operating temperature? Does the engine stumble when you accelerate?

As for the transmission fluid, have you visually checked under the car for a leak? I suspect you have a leaking transmission pan gasket or cooler hose. Transmission fluid that finds its way into the engine is normally due to a fracture in the transmission cooler piping in the radiator. Actually, the coolant/water normally finds its way into the transmission fluid.

[Bof]

Russell,

Thanks for your prompt response. The idle is roughest when the engine has warmed up, and just recently, there seems to be an extremely slight stumble on acceleration, so slight in fact, I hadn't thought about it until you mentioned it.

I haven't actually looked under the car, because I have not seen any sign of fluid on the garage floor. I will, however, do so.

Thanks for your concern and reply.

Tom

[TimFreeh]

Is your car running rich? My cousin just got done troubleshooting a very similar problem on his 93 400E. The problem turned out to be a bad fuel pressure regulator - his fuel rail pressure was over 100PSI. This high pressure was permitting too much fuel into the engine when the injectors opened. The result was a rich-running engine that did not idle well and stumbled on acceleration and sometimes a no start condition due to wet spark plugs.

[Bof]

Tim,

Thanks for the tip. I'll check that and report back.

Bof

[fz500sel]

My rough idle was a result of a bad throttle body actuator. I got the part from Fastlane for about $1200. No problems since.