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.

Did you scan for codes?

True, but there are multiple harnesses.

The enginer harness was replaced and the other harnesses were checked and found to be fine. There are no codes being stored.

Are your spark plugs gapped correctly?

Yep... checked that too.

I see you replaced your motor mounts, but have you had them inspected again for the sake of it?

I think MB motor mounts are liquid-filled (could be wrong), and I've heard stories of new motor mounts being ruined during installation, allowing the liquid to run out.

They have been checked and found to be good. The rough idle is more like occasionally missing than anything else.

Read my post...

http://www.peachparts.com/shopforum/tech-help/123795-1994-mb-e420-issues-idle-surging-stalling-long.html

You can actually remove gasket sealing material with a pressure washer... Don't ask me why I would know something like this. The point is, I would look for some sort of vacume leak at and around the intake manifold. You may have a very slight leak causing one cylinder to run proportionally lean at idle. this is something that would be more pronounced on a warm/hot engine than a cold one. You can spray wd40 at the manifold mating surfaces while the engine is running. Any change in engine speed when spraying would be suspect.

This might be something to look at.

On my former car, a 1993 E320 with an M104 engine, the engine developed a massive vacum leak, causing the engine to surge violently. Since a vacum leak is false air in the system, the mass air meters were completely unable to do their job.

After searching for the cause of the vacum leak, it was found to be the intake manifold seal.

I think my mechanic sprayed soapy water at the intake manifold gasket.

Interesting thought, I'll check it out. This problem occurs when the engine is cold and hot. It's much worse when the outdoor temp is cold or it's humid. So it would seem to be intake air related.

That's interesting.

My problem only occurred when the car warmed up.

At cold start, it would idle fine, and then if left idling, after 3-5 minutes, the surging would start. It was a constant up and down surging, from about 500 to about 1,500 rpms.

The idle speed seems to vary very little, but I feel a miss. (It's a tad under 500 rpm.) It does not seem to do it immediately upon going to idle, but after 10 secs or so. When it's cold or damp and I'm sitting at a traffic light, it seems to miss bad enough occasionally that I think the car will stall... but it never does. Once out of idle everything is great.

Have you checked and see whether or not the engine control modules were updated or reprogrammed over time?

I remember reading that on the E420's, the chips were reprogrammed to slightly increase the engine idle. The E420 is also an M119 engine, and virtually identical to yours.

JimF has an '95 S500, and his website with a lot of this stuff detailed.

His site is well organized and makes for a good read.

http://pages.prodigy.net/jforgione/MB_S500.html

Check out this thread:

http://www.peachparts.com/shopforum/mercedes-benz-performance-paddock/17198-pad-chip-s500.html?highlight=chip


Contact JimF and see if he will reprogram the chip in your LH module.

It apparently changes your engines idle quality.

...and if you do a search under his user id for "chip", you get a ton of good reading:

http://www.peachparts.com/shopforum/search.php?searchid=989430

It also appears that were a service bulletin on replacing computers on E420's. Same basic engine, maybe applies to your car:

http://www.peachparts.com/shopforum/tech-help/15363-94-e420-check-engine-light.html?highlight=chip

Depending on the amount of false air being introduced, so would symptoms vary. It is as though you had a carburetor and were adjusting the idle mixture screw. Also, the colder/damper the air, the more oxygen per cubic foot being unaccounted for. It would take less than half an hour to rule 90% of intake related vacume leaks out. An 02 sesnsor does not tune per cylinder rather on overall output therefore you would also likely have the other cylinders running too rich. This would all but disappear after the throttle was opened due to the proportionaly minute amount of false air. On the 129's for instance, a plug reading is almost worthless as they run so lean at idle. I have chased lean stumbles on these cars many times that will exhibit these very symptoms. It is definately worth checking on before anything else. Steve

Sounds like a very likely scenario. I'll check it. Do I need to remove the air cleaner assembly to get to all the surfaces?

I did a bit of reading of the links that were referred to... wow, there are quite a few rough idle problems.

Last night was a chiily night and the problem showed up even worse. I did notice that if I slowed the car down by downshifting, the problem seem to show up when I wasn't at idle. That is, the rpms would oscillate between 500 and 900 when coasting to a stop in 1st gear. When I did finally stop, the engine seems like it's almost ready to stall. If I stop normally, the problem was as previously described. Does this sound like a intake manifold leak or EGR valve problem? (BTW... the problem is the same whether the engine is hot or cold.)

I checked the intake manifold using the suggested technique and found it made no difference. I'd like to check the EGR valve. I do notice it's leaking oil around it. Other than replacing it, how do I check if it's bad?

Check first to see if it holds vacume. Simply apply a vacum pump or you can even get a length of clean hose and suck on it. At idle and with the engine hot you should notice a slight difference in roughness when you apply vacume to the egr. If it does not hold vacume or if there is no difference when you apply vacume, look closer at the egr. They can get carbon caught between the valve and seat. You will only be able to see this via inspection and with the egr removed.

It's 4 months later and I'm still having the problem. What has been checked lately has been for manifold leaks... sprayed around the intake manifold, no difference noted. Checked for vacuum leaks, none found. Tried a new air mass sensor, made no difference. The engine has not EGR valve to check.

I only have a erratic/rough idle that gets worse when it's cool or damp outside. The car has plenty of power otherwise. This problem is costing me a ton of money to try to resolve. The dealer seems to be only able to fix something if it logs a code. I'm now seems down to two things... the coils or bad injectors, as everything else has been either checked or replaced. Any thoughts on which is more likely, given the symptoms? (I'm scheduled to have the injector spray pattern checked this week... should I do it, or save the money?) Thanks!