400E rough idle
 

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.

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.

400E rough idle
 

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

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.

400E rough idle
 

Tim,

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

Bof

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

If you'll look through the Air Mass sensor housing, down into the throttle body opening, you might be surprised to notice a "build up" of residue. You can remove the air mass sensor housing and clean the interior of the throttle body. The build up can keep the flap from opening and closing properly. You can use an engine carburetor cleaner and a plastic scouring pad. Don’t overdue the spraying of the carburetor cleaner.

Hopefully if your problem is the related to the throttle body, you don’t have to spend 1200 for a new replacement part!

As always ... try the least expensive 1st...

heh...My rough idling was caused by a failed ignition control module. $2250 later I was back on the road.
A few weeks later, my rough idling was cured by a new wiring harness.
A few months later my rough idling was cured by new caps and rotors. Right now, I have no idea what is causing the rough idling in the morning (cold) and the hesitation (all temps.). I have also noticed that the car feels like it has way more power cold, than after about 15 mins of driving.

I've also noticed that during heavy accelleration (as in merging onto the freeway, that once I get to around 60-70mph (and the trans. is still in 3rd), the engine startes to stumble/miss, as if the fuel supply was being cut off. - Do you have this happen also??

Larry makes a good point.... finding a cure for rough idle can be an expensive journey.

Sometimes you can go broke trying to save yourself money as a DIYer. After searching the forum and performing a few inexpensive attempts to resolve your problem, if the solution doesn't become obvious - turn the job over the professionals.

Find a Good mechanic and save some money and worry.

I'm a computer network engineer who has spent $$$$ trying to save money.