I'm at my wits end here. About a year ago My 1986 420SEL developed a mild rough idle when cold. I took it to my mechanic and he scoped it out, replaced the ignition coil and everything was fine. 6 months after that it started again. I moved to Ontario and had to have emmissions testing done. The car was passing everything put hydrocarbons. The dealer did a motorvac (cleaning or injectors I think). At that time we also noticed that the secondary air pump (smog pump) was shot and that the hose from it that goes to the engine was broken off. Fixing this (new air pump, hoses and valves) seemed to help quite a bit in reducing the rough idle. It would always seem to go away once the engine was hot. Now it is winter and getting pretty cold up here (below freezing everyday). The rough idle is very pronounced when you just start the car up, and it gets better, but doesn't go away even after an hour or more of driving. Whether the car is in park, or in gear at a stop light I can feel the whole car vibrating. I had the distributor cap changed, as well as the rotor and the wires recently (What was on the car was original and the distributor cap was in pretty bad shape at 180,000 miles). The new cap, rotor, etc made a noticeable differnce in driving, but did nothing to help the rough idle. What is left? The fuel mixture? (I use 92 octane Sunoco gas) The 02 sensor? When they were making my car emmissions aceptable they did a lambda adjustment and this was just four months ago. Motor mounts? How much to replace those? And is there anything else I should do first? My mechanic said to run some fuel system cleaner through the engine. I bought some Castrol Syntex Fuel System treatment and am just waiting for the tank to get down to 1/4 full. He said to put it in as concentrated as possible. I never let the tank go below half full and I use a gas line antifreeze additive this time of year every other fill up.

Any ideas?

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Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired

Have you check the motor mount?? I am not sure if the motor mount design on the 126 is the same as the 124. After I replace the motor mount on my 300E, the rough idle disappear.

jack

Resistor tip on spark plug wires. With engine off, pull one wire and restart. If the condition is worse, turn off engine and replace that wire. Move on through the wires one by one and see if you can find one where you don't notice too much of a change in the rough idle condition. That is your culprit.

Thanks for the help everyone.

Q. I just had the wires and resistor tips replaced last week with brand new one from MB. It has made no difference. Any ideas?

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Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired

"as well as the rotor and the wires recently"

Duh, if I could read a little closer, I could save myself some typing, huh?

I have exactly the same problem with my W123 280E, and several independent mechanics have failed to find the source of the problem.

But I recently took the car for a service to an M-B dealer, and they traced the problem to a faulty fuel distributor.

I don't know if your car has K-Jet, but in my case, petrol is leaking past a worn plunger inside the fuel distributor. It only causes a problem when the fuel and airflow are low, hence the car drives fine unless it's idling. At idle, it drops to four or five cylinders.

I haven't yet had it fixed. It's an irritant, but in a way, it's also a fuel-saving device because not enough petrol is getting through at idle!

So far the faulty plunger in the fuel distributer sounds like the most likely solution. I have had the motor mounts checked and am told they are fine. Does anyone know how much trouble it is to find out if my fuel distributor is a little worn? How hard is it to take it off and inspect, then replace worn plungers if necessary?

Thanks

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Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired

I understand that a rich running engine will idle rough. If one of the fuel injector plungers is worn and some gas is getting by when it shouldn't I think that might cause the rough idle. It is especially bad when very cold outside. By the time the engine is hot, the idle is pretty smooth except fot he occassional little bit that you can barely notice. I would also like to consider other causes. I have done searches and come across something called a cold start, and also over voltage relay. Could someone explain to me what these are, how a malfuntcion in one of them might affect the car, and what are the possible fixes. I I checked the hoses and they look OK.

Thanks

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Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired

Jason, my c280 had similer problem with rough idle
but not temp. dependant. After many hoops, turned out to be
the cam "balancer" (?) magnet at the front of engine.
No idea whether your engine has one.
Best.

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1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
********************
1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD

Thanks for the replies everyone. I still have the rough idle, altough it does seem to be less pronounced when the engine is hot. Still kind of rough when cold. Anyone have any other ideas?

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Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired

My ideas all involve diagnostic testing. The possible problems are too numerous to expect "answers".

One simple test that will give direction is to remove the aircleaner and lightly depress the airflow plate while idling. If this makes the engine briefly smooth out you have a lean misfire of some nature. A pair of needle nose pliers can lift the plate (by the ten mm screw in the center) to lean out the mixture.

A car that has a working lambda system isn't likely to be running too rich. The usual problem is that a restricted injector (or fuel dist circuit) or vacuum leak causes one or more cylinder to go over the line lean. Remember that the lambda control system takes the average mixture to a very lean condition. Deviations rich are counteracted causing some cylinders to go too lean.

The advantage to this simple test is to see if your problem is mixture based. There are still a bushel of possibilities. They all are as exactly testable with proper equipment and technique.

One point of feel: a proper running feedback control system (lambda system) will correct for your actions so as long as the oxygen sensor is connected the results of depressing the plate (enrichening) are brief, although if in a few seconds the miss occurs a further depression should again smooth out the engine.

BTW one very common vacuum leak is the line going to the vacuum modulator. It can be seen looking straight down at the back of the motor. It is a white plastic line that goes under the exhaust cross-over pipe.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Maybe you have a sticky valve, or a partially clogged injector. ? I'm not totally sure however a MotorVac made my buddies Blazer run great, and made the rough idle completely disappear.

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"Damn the torpedos, four bells full speed ahead"
-Admiral Farragut




-1987 300E Smoke Silver
-1997 S320 Black on Black
-Soon To be E55 AMG
-27 foot Grady White Offshore Sportfish.

Capt Alex,

Can you hear a sticky valve? I changed my oil this morning and when I started the car up after I was listening under the hood. There is a sound under the valve covers on each side that kind of sounds a little rough.

What does it take to fix a sticky valve (I assume its the lifter that is sticking).

Thanks for the idea, and steve thanks for the input. I know without testing I can't get any answers, I was just looking to know what kinds of tests can be done, and trying to get an idea of probable causes.

Thanks again and happy new year.

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Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired

Steve,

I tried that test you mentioned, depressing the airflow plate. I don't know if I depressed it too much, but as soon as I pushed it the car stalled out almost immediately and then was hard to resart. Also, when I took the lid off the air filter housing to do this test I noticed there was some oil on the lid and a bid inside the filter around the hole on the bottom where the hose that comes from the valve cover is. It wasn't much oil, a spot about the size of two fingers and not very thick. What do you think this means?

Thanks

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Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired

If a small depression causes instant slow-down/stall, then the engine is too rich. This means that your lambda system is probably not working. The next step is to monitor the current at the EHA or the duty cycle at the diagnostic port. Do you have a multi-meter?

The point is to use the lambda monitoring system to make and monitor basic adjustments.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician