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  #1  
Old 07-04-2007, 11:42 AM
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94 SL500 Rough Idle

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?

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  #2  
Old 07-04-2007, 01:19 PM
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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.
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  #3  
Old 07-04-2007, 04:47 PM
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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!
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  #4  
Old 07-04-2007, 05:41 PM
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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
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  #5  
Old 07-04-2007, 05:56 PM
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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
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  #6  
Old 07-04-2007, 07:39 PM
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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.
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  #7  
Old 07-04-2007, 08:25 PM
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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 ...
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  #8  
Old 07-04-2007, 09:24 PM
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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?
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  #9  
Old 07-04-2007, 09:35 PM
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Usually, drips of gas are evident from the port .
If you are smelling gas, yo may want to take the vac hose off and plug it with a golf " T". Then run the engine for a couple of min [ warm] and look for any evidence of gas at the reg vac port. If the reg is leaking, it should leak now b/c the fuel pressure will be higher than normal [ max]b/c the vac line is not on to regulate it.
Sometimes they will not leak under that condition, but they will leak with the vac hose back on b/c you now have the added force of engine vac suction on the diaphragm, combined with the pressure of the fuel pumps.
That is why we look into the vac hose for gas traces..

You may, in that case, get the car warm and at poor idle condition, then take the vac line off reg and plug it to see if idle condition corrects..if it does , you know the reg is likely sucking gas into intake..

You may also have a vac leak, but the reg also fits your complaint.
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  #10  
Old 07-05-2007, 09:57 AM
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Thanks.... I'll check it today.

The mechanic had started checking the vacuum hoses in the manifold area, but when he removed the air filter assembly and exposed the air flow meter, any sort of breeze would impact the engine idle, so it was difficult to see a difference. Is it normal for a little breeze to do that? (It was amazingly sensitive.)
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  #11  
Old 07-05-2007, 10:48 AM
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Maybe the LH sensor is picking up airflow w/fiter off...
Try unplugging it while it is running and the wind is blowing,....I guess that would narrow it down ...
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  #12  
Old 07-05-2007, 04:08 PM
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Sorry... I can't seem to find a reference for the LH sensor in the shop manual on AllData.... is it called another name?
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  #13  
Old 07-05-2007, 04:12 PM
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MAF

LH is Hot Wire
HFM is HoT Film Mas

Same thing, different heater..

Yours is LH style
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  #14  
Old 07-05-2007, 04:17 PM
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This could possibly be the cause .

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=23I0RAMEE24J0YT5ZQ&year=1994&make=MB&model=SL-500-001&category=C&part=Air+Temp.+Sensor

Last edited by Arthur Dalton; 07-05-2007 at 04:44 PM.
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  #15  
Old 07-05-2007, 04:47 PM
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And you're still seeing, with your own eyes, no fault codes on Pins 4, 8, 17 and 19?

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