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?

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

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

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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.

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?

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.

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.)

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 ...

Sorry... I can't seem to find a reference for the LH sensor in the shop manual on AllData.... is it called another name?

MAF

LH is Hot Wire
HFM is HoT Film Mas

Same thing, different heater..

Yours is LH style

This could possibly be the cause .

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And you're still seeing, with your own eyes, no fault codes on Pins 4, 8, 17 and 19?

The mechanic looked at the reading of the temp sensor. It correct and not varying. As for the codes... I looked over the shoulder of the mechanic when he went through the screens for the diag tool... and no codes were recorded. At the start of this problem there were some that indicated a problem with throttle body and it was replaced. (Personally, I think it just needed cleaning.... as it looked pretty dirty and was probably sticking.)

What I mean about the cause w/IAT sensor is that may be why you see an idle difference with wind w/air filter out...
I did not mean the sensor was bad..it may be reacting to air when filter is out..blow on it..

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!

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

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?

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.

<<What are plats?

>>

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.

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..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'll go to Radio Shack and get the parts for building the code reader, but I have to tell you that I trust the mechanic who I've been working with and have a great working relationship with him. (He's a German MB master mechanic.) The dealer is within walking distance, and I've gone into the dealer to just get the codes read out. They do it immediately and don't charge me. (As another data point, they have also sometimes reset the control module self-adaption feature to mean values.)

The spark plugs are not platinum, and are indeed the standard Bosch plugs. (I remember asking about the platinum plugs and the mechanic telling me that they won't work.)

I really think this is something similar to the track we were on with the fuel regulator. Any other ideas?

<< (He's a German MB master mechanic.)>>
Then he knows more than I/WE do.....he should have no problem fixing your car.

<<but I have to tell you that I trust the mechanic who I've been working with and have a great working relationship with him. (He's a German MB master mechanic.) The dealer is within walking distance, and I've gone into the dealer to just get the codes read out. They do it immediately and don't charge me. (As another data point, they have also sometimes reset the control module self-adaption feature to mean values.)
[/B]>>

You ask for assistance and everytime someone post a possible [ viable] idea or simple test, you keep coming back that you know the possible is not possible , etc, so I suggest you bring it back until they can fix it.

If they keep resetting back to mean, then there is something wrong with the trim, which could in fact , be the FP regulator..maybe you want to ask him about that..
I am sure a seasoned MB Tech has surely done a Fuel Rail pressure specification test , but might want to inquire anyway...

I agree that he should be able to fix my car. If there is a complaint about him, it is that he doesn't seem able to use traditional methods for fixing engines. For example, I've asked about using a scope. The dealer has one, but it's been broken for more than a year. As I also mentioned, I replaced the throttle body due to a code being read out. When I examined the old throttle body the throttle plate looked caked up a was probably sticking. A simple cleaning might have solved the problem.

My apologies if it appears that I'm coming back that it's not possible. What I'm really meaning to say is that the suggestion or problem area was looked at. In the future, I'll be more specific in what was done rather than say it's not possible. (The only suggestion I'm aware that I pushed back on was the fuel pressure regulator and that was meant more of a question to gain understanding. If I pushed back on more than that, please let me know.)

As for reading codes... I wasn't pushing back on that, I was just explaining that I had another way of reading the codes at my disposal. It's just easier for me to stop by the dealer to get them read, than it is for me to drive to Radio Shack. I do sense a bit of distrust from you in regards to having the dealer read them. I can tell you that I'm there when they are read and I've seen none being stored with my own eyes. As for resetting to mean values... that was done a few times. Most/all of which were when things were swapped out or replaced. (Like the throttle body assembly and HFM.) They've also done it at my request, as I wanted to see if it made any difference. This all said, I'll make a trip to Radio Shack for the items to build reader and read them myself.

The mechanic did do a nozzle spray test, where he took off the rail with injectors and did some sort of test. (He replaced the injector seals as part of the test.) Is that Fuel Rail pressure specification test?

My apologies again... I don't mean to come across that I'm not following suggestions. I very much appreciate them.

<Is that Fuel Rail pressure specification test?
>

Of course not. A Fuel Pressure test is a simple High pressure / Low pressure [ regulated], and pressure Holding test. There is a TEST port right there at the fuel rail to make it [again] REAL SIMPLE..but it requires a pressure gauge tool.. but maybe that is next to his Scope ..:)

Thanks... If it's that simple... I'll stop by the dealer and ask them to do it.

Please bear with me... I'm an engineer and need to understand (and I'm curious by nature)... what are we looking for and how does it impact the problem?

<<Please bear with me... I'm an engineer and need to understand>>

I know what you mean..I am just a Mb guy who has worked on these tubs for 45 years , so I have had to work with many guys like you.. so you can understand, I have/use sequenced test procedures [ mostly SIMPLE test] that I have the poster do and his reply tells ME what the next test step should be ..but when I have to tell YOU the whys/hows of each step b/c you do not think that the test is needed ,etc, then I have to go into the whole sha-bang of why I wanted to test the Regulator [example] and how it works . I do not need that info to diagnose your car, so I feel it is a waste of typing to have to do it in order to get to a fix ..but I will do it..just to satisfy the poster.[You]
So, I find it best/quickest for the poster to just do the test and report that finding back so I can help diagnose the problem..
It's a great frustration when I have a guys Schematic up on a screen and I am asking him to give me an ohm/volt/etc. reading at a diagnostic test point in the schematics series chain, only to have him come back with " Well, I don't see where that would have anything to do with my fan not working !!.. Duh.. " ..very frustrating, ... sometimes I just have to File that post..specially when I know all that info is in the Archieves, cuz I had just done one of them last week...
So, Bear with me...too
I just fix the stuff....
When I get someone that tells me "My MB GERMAN guy yada yada " , I can tell you that the old MB guys I used to work with were EXCELLENT Diesel Mechanics, but they sure were not equiped for todays Elecronically controlled Engine Management systems , no fault of theirs. Just the times and Tech advances have made it a new ball game a that some of us progressed with and others did not.

My DSO woks.

In this case, this guy is on one of the younger ones. He's second generation. His dad was also a MB mechanic... and sounds like the type you're referring to. Being Silicon Valley, I don't think he's fixed many diesels. (There are many more SL500s than diesels here.) I'm in the computer field, and he's impressed me a number of times with his in-depth knowledge of how the whole system works. However, this problem has stumped him, which is why I'm here (frustrating folks like yourself).

I hear ya.
Guess it is just hard for me to believe any shop working on these Full Elecronic systems has a broken scope in the corner...maybe they just don't need one.
I used to be able to see all those sensors wave patterns just looking at them too, until I got Blasted with that Kryptonite... so now I require the DSO on the tricky stuff..
Forget my sarcasm, ..do a FP test and maybe there are some codes in there ...and maybe not.