I have a 1992 Mercedes 400E, VIN# WDBEA34E3NB619468 (California Car I think). Idle started to hunt from 900 to 1100 RPM. After reading the forums, I inspected the engine wiring harness. It was bad so I bought a new one and replaced it. Visually inspected the MAF Sensor, it looked good. Cleaned the carbon deposits off of the throttle body and sprayed Carb cleaner into the throttle body and intake manifold with the MAF off. of the vehicle. Before I replaced the wiring harness I got codes 6, 17 and 18 from the LED in the engine compartment. When I restarted it, it seemed like the idle problem went away, but when the engine came up to temperature the idle shot up to 1500 without hunting. When I put it in Drive the idle went down to 700 (normal), but when in Park or Neutral it started hunting again between 900 and 1200 RPM. So essentially, I had the same idle hunting problem. After resetting the codes, the only code I had left was Code 6 from the LED in the engine compartment. I believe this is an idle speed control code but not certain. I have another 1992 Mercedes 400E so I thought I would start replacing parts one by one to see what part may be the problem, so I replaced the following parts and tested after I replaced each one: 1) MAF Sensor 2) Throttle Body (Electronic Throttle Actuator?) under the MAF Sensor After replacing the two components above, I had a leaking water hose on the water pump, so I replaced it and was flushing my radiator with Prestone SuperFlush. The weird thing that I then noticed regarding the: 1) it would idle fine at 700RPM when cold 2) after some temp increase about 30 second later, idle would go up to 1100 RPM (no hunting) 3) after some more temperature increase (couple of minutes) to about 80 degrees C, the idle started hunting between 900 and 1200 RPM 4) here is the weird part, I turned the heater on and the idle hunting went away, but it was idling at 1100 RPM 5) then with heater on, I turned on the rear dofrost and Idle went to 1000 RPM 6) then with heater on and rear defrost on, I can either turn on the Hazards or the Headlights, and the Idle would go down to 700RPM (no hunting) everything fine. 7) Now if the heater is still on (and Idle at 700 RPM ) I can turn off the Hazards and rear defrost and it would still Idle at 700 RPM just fine, but when I turn off the heater, the Idle hunt came right back (900 to 1200 RPM). So, I decided to replace the following components one by one and test in between: 1) 4-Wire Temp Sensor on front of Intake Manifold (not sure of its function) 2) 2-Wire Temp Sensor on front on Intake Manifold (This is for the electric radiator fans) 3) 1-Wire Temp Sensor on front of Intake Manifold (This is for the temp Gauge on Instrument Cluster) 4) Cam Sensor? (Wire going into a sensor on the Drivers Side Head from the engine wiring harness) 5) EGR Valve. Still have Idle hunting after warm up. Now check this out. When there is no coolant or water in the engine, I get no idle hunting at all. The engine runs fine idles perfectly at 700 RPM. I didn’t let it run for long because I didn’t want to warp anything, but I did this 3 times after it cooled back down and refilled it with water. With water engine idle hunts 900 to 1200RPM after warming up. Without water engine idles at 700 from cold start all the way up to 110 degrees C. I have also noticed that when starting it should have a higher idle and then after 20 to 30 seconds go down to a normal idle (cold start valve?) but it starts at 700 RPM and goes up. Questions, where is the idle speed control? Is the Idle speed control the throttle body (Electronic Throttle Actuator)? Could it be the Cold Start Valve / Sensor? If so, where is it? So, what do I do next? It sounds like it has something to do with coolant and temperature, but I am at a loss. I am planning on giving it a major tune-up but thought I would ask the forum for help first.

 

I came across your post while searching for help with my problem so I'll post my question here as it sounds as though you might have at least part of the anwser to my problem.

I have a 1993 400E and this was the first winter day. We we're visiting friends for the weekend so the car was outside. Temp was about -20C and this car did not want to start!! A cylinder fired every once in a while but at least 4 cylinders were missing. There was no exhaust noise so I am assuming that the problem is that no fuel (or not enough fuel) is being injected. I had to put the battery on charge but after MANY attempts the engine finally got warm enough that she started but idled really rough. Sounded like a whole side of injectors was plugged. Sounds like at least 6 cylinders at 2500 RPM but still having difficulty even staying idling.

Warmed the car up and drove home (180 miles). Coldest drive I can ever remember!! Temp Gauge reads about 55C (131F?) all the way, cruising at 80MPH. (Didn't warm up in traffic before we got to the highway, either). Drives ok, I think I've got all cylinders but I don't quite have full power.

Now I'm thinking that this car has been idling rough on occasions for a month or so. When I had driven it for a few blocks and stopped at a light the engine would miss when idling and shake the car enough that I would slip it into neutral to stop it. That didn't concern me because we often experience a similar problem with any car as it gets colder here. There is a point in the warmup where the engine still needs some choke but the choke has shut off. So I figured this was a similar problem - I don't any more.

Gas consumption on the way home was about what I would expect - a little heavy because the engine is running too cool (outside temp was -16C for most of the way also). So I believe I was running on all cylinders.

So I am thinking that there is a temperature sensor somewhere that is giving a false reading to the computer (hence not enough choke for cold start) but this sensor is also controlling the coolant temp and making it run too cool.

Anybody know where to look?

 

piggy backing on another thread isnt very nice, salfeola look at my answer to kenf thread, might help i really dont know

 

Sorry, but I thought this thread was dead - look at the date!! Just put it here to get the attention of the original postor. (Salfeola)