I've been having this rough idle problem with my car for over a year now. I've spent thousands of dollars with my dealer trying to get it fixed, with no success. They've checked and replaced everything that could be associated with it. I had thought that the problem only occurred when it was damp and chilly outside, but I've noticed in the past week that it appears to happen when the wind is blowing. In addition, when the air cleaner is off and there is the slightest of breezes, the idle also gets erratic. If a strong enough breeze occurs, it can even stall the engine. Is this behavior normal? My dealer tells me that the have checked the intake manifold gasket and it isn't leaking. Should I believe them? Any other ideas?

The last I heard of the wind impacting the operation of an automobile was my old VW beetles when being passed by a semi. And what was impacted there was the direction of travel, not the engine. If so it would point to something in the air intake or air mass sensor or throttle actuator but whatever it is it's rare. I would learn how to read fault codes and start reading them - try a search. There should be something there - a 94 LH motor has reasonable good diagnostics. Presumably the dealer has been doing this but perhaps they were somehow blinded by your checkbook.

There are no codes being stored. I wish it were storing codes, as that would help tons. He swapped out the air flow meter (at now charge) and it made no difference. He's monitored the engine while it's doing it and just just shows a variance in the idle speed and (I think) the fuel mixture. (The norm is something like 50%, but varies going as high as 60% at times.) He has tried finding the intake manifold leak by spraying something non-flammable like brake cleaner.

I've gotten to know the mechanic (he's the best there) and the dealer well... and I think they know what they're doing and charging a fair price. Any ideas would be much appreciated!

Sounds like an issue with the MAF to me, too, but if you've already replaced it... Have you tried cleaning the wire? There are special spray cleaners made precisely for this. The air filter is new and clean?

You must be way off on the air-fuel ratios you mentioned above. If you were squirting 50-60% fuel in there, the engine wouldn't run at all. Should be more like 8% -- 14:1 air to fuel ratio or thereabouts.

And be careful what you squirt in there. Brake cleaner can be VERY flammable. You don't wanna blow yourself up in the garage.

Cheers, John

__________________
'98 E300TD (W210) | Sold 6/09 with 205k and counting
'04 VW Touareg V10TDI

Check to see if the fuel pressure regulator vac hose has any evidence of fuel in it ..

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=23I0RAMEE24I12DIEL&year=1994&make=MB&model=S-500-001&category=C&part=Fuel+Pressure+Regulator

In addition to doing a 60K maintenance, which replaces the filters and plugs, I had the throttle body replaced as well. The engine computer was also swapped to see if it was a problem. As far as the 50%, I don't remember what it was. I just guessed it was the fuel. It could be Co2 or O2. Whatever the numbers, they were within the proper range.

Going back at another one of your post with the same complaint, I mentioned FP regulator on that post also, but you were inclined to think that the fuel pressure would not effect the idle and you notice no complaint at higher RPMs
Well, I will explain it to you and then leave it there.
The FP regulator is vacuum controlled. It's purpose is to keep the pressure from the fuel pumps at a pre-determined specification for the injectors , thru a fuel rail.
This is done with a vac actuated diphragm inside the regulator...BUT..what happens to these regulators is they get a leak in that diaphragm and that allows raw gas to get sucked into the engine thru the operating vac hose. So, you now have a supply of raw UNMETERED gas entering the intake of the engine,thru the regulators vac hose ..The results is erratic idle b/c this new gas supply is not sync into the fuel management as the Injectors are.
So, you may dismiss this fact, or you may want to see if gas is comming out the FP regulator port. It is not uncommon and ,as a matter of fact, it is standard procedure on your complaint.
One will not notice this fault at higher RPMs b/c the engine can then correct for this small amout of unmetered gas.
After doing all you have to correct the problem, I would take the 2 seconds time it take to check that regulator ...

Sorry... I really wasn't blowing it off... I was just trying to understand how it could cause the problem. Thanks for the explanation.

I just went out to check it and I don't see any gas coming out of the port. However, when I removed the vacuum hose and smelled it, I do smell gas vapors on the end that goes into the regulator. The car has been sitting for about a day. I started it up and let it run for a few minutes before I checked it again. Still not gas... but the smell remains. It's not strong, but it's definitely there. (But only on the end that goes into the regulator.) Should I be smelling anything? Should I maybe check it after running the engine for a longer period of time?

When reading of the air temp sensor was the same before and after the air cleaner was removed, and it did not vary... so I don't think it was the cause. I have a sense that the air flow meter is just sensitive. (I can blow into it and stall the engine.) I'll check the fuel pressure sensor this afternoon after the engine is warm. Thanks again for the suggestions.

OK
If the MAF is that sensitive , then I would still unplug it and see if the bad idle is corrects.
Also, as post #15 recommends, why not go in there and see if codes are present right now ?? At least Pin 17/DI module.
And....One last concern..do you have Plats in there ????

I just came back from testing the fuel pressure regulator. It makes no difference with the idle with or without the hose connected. (And plugged when taken off.) As for the codes, I have no way of reading out the codes. I take it to the dealer to do that. It was just in last week, and there were no codes stored. What are plats?

BTW... maybe I should explain what I mean by rough idle. When the engine drops below 500 RPM, it on a regular basis but erratically feels that it is missing. The mechanic tells me that the computer is not detecting a miss, or would store a code. I've had the vehicle for about 8 years and this rough idle starting happenin about 18 months ago. I gets bad enough when it is chilly and damp that it feels like it might stall. Sometimes I like I'm riding on a Harley.

Thanks again for your help!

It's very easy to build a code reader and read the codes yourself. Do a search for Arthur's code reader. Some dealers are not entirely forthright about what they find with codes. If you're doing it yourself you can be sure of what you have.

<
>>

Platinum Plugs .... do a search and you will find that these engines hate Platimum plugs , they idle like crap on Plats , and anyone who puts Plats in them is nuts..
Thy want F8DC4, reg/standard plugs.
Look at a plug..if you have put plats in there , your problem has more than likely been found.

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=23I0RAMEE24K0KKARI&year=1994&make=MB&model=SL-500-001&category=F&part=Spark+Plug

..and you are one of the fortunate owners who happen to have one of the LAST Benzs that the owner can go in and get the codes with a simple $2 LED from Radio Shack.
All the info is right there in the modules.
You say there are NO codes anywhere..well , I highly doubt that. ..like the simple FP regulator test, I would surely go take a simple look at the codes for myself ...

I'd describe that as an engine stumble at low RPM off idle.

I think "plats" probably refers to platinum spark plugs. And sometimes new plugs will cure a stumble on other gasoline engines I've owned, and is a very cheap DIY fix. If you haven't done plugs in awhile, it's good maintenance anyway, and might help with the problem. Make absolutely sure that the plugs are properly gapped for your engine or you won't gain anything.

Cheers, John

__________________
'98 E300TD (W210) | Sold 6/09 with 205k and counting
'04 VW Touareg V10TDI

Ahhh.... of course. This problem started before my 60K service. At that time, the spark plugs were replaced. Since then, I replaced the spark plug wires, rotor and distributor cap. About 4 months ago I had the compression tested and when it was done the mechanic checked the gap... they were all ok. The compression was smack on.

What do you mean by off idle?