idle problems

[420-owner]

It idles rough when cold but good when warmed up. I disconnected the idle valve and it throttles up to around 2k rpms so I guess it works. Could it still be bad??? I was told it needed to idle around 800-900 rpms but I am sitting around 600rpms instead. I though maybe the idle relay could be playing a part too??? It is a little rough off the starting line. Sometimes just revving it up it will rev quick and at times it acts like its running rich and stumbles on the rev all the way to 5000rpms. I need a place to start as I am unsure what to do. Also my fuel pumps are very loud, and I dont think they are suppose to be heard that much but again I am unsure on that. Is it possible that one pump is not work but the other one is? It runs good down the road otherwise......Thanks Jamie

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[elolson]

Hi 420-O,

Cold idle/running issues might be caused by the warm up compensator, the cold start valve, or some other sensor/vacuum valve problem. There is a difference between idling rough and idling too low. Perhaps you can clarify. My experience has been that sometimes the symptom you are seeing is actually caused by many problems together and since the W126 fuel system is so robust, the car still seems like it is mostly running fine other than the nagging symptom that you observe and suspect is only one problem.

Frankly, I would find a shop and ask them to diagnose. Or you can start digging yourself. It can be fun but if you are not experienced you can get into the part replacement trap. This means that a misdiagnosis leads to parts replacement, that then does not solve the problem, etc...

How long have you had the car?

[420-owner]

I just got the car a few weeks ago. I am new to MB but have tinkered and restored other euro cars. I did have a 79 300sd but thats a whole other animal.

My fuel pumps are running very loud, I am going to change the filter to see if it is clogged. They seem to be working very hard. I hope that will be the problem. Maybe it will help the idle out.

SOMETHING ELSE I FOUND last night, the smog pump belt is missing. Will this affect the idle and other running issues?

Is the idle adjustable? I was told that a MB mechanic can connect up to it and adjust it electronic, but if I am having other part issues, this will not remedy anything.....I should be getting my cd manual in any day now, so this should help with my new toy. I drive it everyday so I need it running pretty reliable.......Thanks Jamie

[420-owner]

Oh, its idle is at 600rpm when warmed up and it goes up and down when cold.....

[elolson]

I can hear my pump whine when it is on and I am standing at the back of the car. If I walk to the front or am inside then I can not hear it running.

I do not think that the idle is adjustable, it either works the way it is supposed to or you start replacing/fixing stuff until it does. But then you have a newer and different model W126 and so I am not completely sure.

My idle does not bounce around, it may be have been rough or misfired at times, but it was still rock solid. That does not sound like your problem.

I disabled my smog pump while playing around with sensors and saw no difference in idle characteristics. Currently it is fully functional.

[Opa's 300SE]

I have a 90 300SE that will not idle correctly. She'll run fine over or above idle. Any ideas?

[Untertürkheim]

Originally Posted by Opa's 300SE

I have a 90 300SE that will not idle correctly. She'll run fine over or above idle. Any ideas?

What is the symptom?

[JayKinFL]

while we're at it, i too, have a idle problem. my 1991 420SEL runs at about 1,000 RPM in park, but drops down when in gear. everything operates normally, but the idle is too high. i was told that the idle cannot be adjusted. any advice? i'd rather the car didn't idle so high in park. i would also mention that the high idle remains whether the car is first started up or has been driven a couple hundred miles, the idle is not affected by engine temperature or time.

[richpron]

my car was idling rough also so I decided to start with a complete tune up, compression check(it was fine),tune up helped a little but with a little help from a machine called a leak tamer(a machine you plug into a vacuum line and it feeds smoke into the vacuum system and instantly tells you where all the leaks are).found out I had a leak from an injector-replaced the seal and what a difference! fixed the problem for a couple of dollars and about 15 min -hope this helps

[sparc100]

I solved my high idle problem on 89 560SEL by adjusting the idle air control valve(IAC). The problem had been: Engine was revving at 1000rpm, then drop to 600-700 when in gear, went back to 1000rpm when back in Park. I took the IAC out, spray cleaned with carb cleaner, then pry remove the very tiny little brown cap at nozzle end (opposite from electrical plug-in end) to reveal the screw thread, then I found a matching size screw with washer to thread it in, then gently and slowly to pull the core outward, need a vise and reverse hammer it out very gently. All it take was quarter of an inch. The more the thread core get pull out, the lower the idle, I pulled the threaded core out to flush with the edge of casing...and it worked. Now the car idles right at 600 RPM.
Hope this helps.