1994 SL500 Rough Idle
 

I've had a rough idle problem for months and I'm at a loss as to what do about it. Here's the specifics...

- It idles rough only when less than 500 RPMs. It gets rougher when it's cold and/or damp outside. Everything else is great!
- I have had the following replaced in the last few months: Engine wiring harness, throttle body, distributor caps and rotors, spark plugs, spark plug wires, engine mounts. None of these had an impact.
- An engine compression check shows compression as good as new. There are no codes being stored.
- The car only has 67K miles on it.

The problem seemed to start after having my enginer steam cleaned, but I also had the engine mounts replaced right after, so it's possible it started before.

Any ideas?

You've replaced everything I was going to suggest.

I'm kind of stabbing in the dark, but did you replace the spark plug resistor boots?

Are you sure they are fully pressed on tightly?

What spark plugs are you using? You should be using plain old copper-core non-resistor Bosch or Beru spark plugs.

The resistance in these cars is in the spark plug end (or boot), so if you have installed resistor plugs, even platinum plugs, you have doubled the resistance.

Also, what about the mass air meters? Your car has two of them, and they can cause strange things to happen when one (or both) are going bad.

The spark plug wire set comes with the resister boot, so they were replaced. They all seem to be seated properly with the "snap" when installing them. The spark plugs are indeed the plain old non-resistive plugs. An additional comment... when the enginer first goes into idle, it seems fine. It's only after it's been idling for a longer period of time when it feels like it's missing slightly. (So slightly, that you have lay your hands on the air clean when the hood is up to feel it. Otherwise, it seems like it just purring.)

I thought about the air mass meters, but wouldn't that also impact non-idle speeds?

My neighbor across the street from me is a technician at Penske Mercedes Benz in West Covina, CA.

I just have had several conversations with him about mass air meters causing strange problems.

If it were my car, I wouldn't spend the money to replace them as it's too much of a gamble. However, a shop would have known good mass air meters on hand, and could swap them out too see if there's any change.

What a coincidence... I bought my car from Penske motors. Where are the mass meters located?

They are in the plastic pipes leading to the airfilter housing, or possible mounted in the air filter box itself.

Where do you live? I'm out in Phillips Ranch near Diamond Bar.

PM me if you'd like me to ask my neighbor if he can help you out.

He has worked on my car at a discounted rate in his garage.

By what means are you checking the codes? If you are really checking all the codes and getting nothing it seems to me it must fuel related.

I'm actually in Northern CA, but I appreciate the offer. Are you sure they are the plastic pipes connected to the filter? That would disappoint me, as it's not a very accurate way of measuring air flow. I had thought the their was onlu one.

When is the last time you had the fuel filter replaced , and fuel pump package resealed?

I would suggest you pay about one hour labor to have the car put on a scope to see if you have fuel or spark issues.
If you do ,the scope will definitely find it.

You said you had the compression tested, did they perform a cyllinder leak test?

This problem has been happening before my 60K service. At that time, the fuel filter was replaced. They did both a compression test and leakage test. I'm really thinking this is more of a fuel or ignition related problem, as it seems to be related to outside temp/humidity and not enginer temp. My dealer's scope is out, so I'll definitely find someplace else that has one. Not sure it matters, but it's more of a occasional miss than anything else.

94 sl 500 has only one mass air flow meter (lucky for you) I would have the fuel evaporator purge valve checked, also check egr valve, if I am
correct M.B. updated EA control unit to idle 50 rpm's higher to help idle,
but EA is expensive, and won't help that much IMO. Also check fuel
pressure regulator for fuel leakage out of vacuum source line. Good luck.

Thanks!!

I'll check all of these items. This problem seemed to start right after I had my engine steam cleaned. Are any of the items more likely to have been impacted during steam cleaning? You mentioned that MB increased the idle speed. What exactly was the symptom that they were trying to address?

I'm curious... it sounds like it may be normal to have some bit of roughness at idle. What is "normal"?

I hate to be the one to bring this up, but have you checked the condition of your wire harness insulation? The insulation used on these cars (1994 included) breaks down over time. In some cases it completely disintegrates. Both my MAF's insulation had deteriorated badly. The wires were cross-talking, and causing a very minor idle smoothness and surging problem. As "luck" would have it, the insulation on one of my ETA's was also shot. After I rewired them, problem went away.

http://v12uberalles.com/throttle_actuator_rewire.htm
http://v12uberalles.com/MAF_rewire.htm

In the first post he says the harness was already replaced in the last few months.