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  #1  
Old 05-14-2001, 11:49 AM
BENZ-LGB's Avatar
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Howdy. I've read previous threads addressing the issue of rough idle, but none of the threads seems to be on point regarding my specific question, rough cold idle. Hopefully one of you will have some suggestions that I can follow and that will help diagnose and fix the problem.

Here is the symptom:

My 1991 420SEL with 188K miles on the odometer suffers from rough cold idle. The engine starts fine, without any problems, but the idle is rough. Not quite to the point where the engine will turn off, but rough enough to be noticeable. There are no pings, backfires or other engine noises, just rough idle. If I put the car in gear to drive off, the car accelerates fine, again no missing, no sputtering, no backfire. When I come to a stop, the car will again idle rough. Once the engine reaches operating temperature, at around 80 degree Centigrade, the problem disappears and the engine then idles smoothly.

The only other symptom that I have noticed is that the "economy gauge" (a glorified vacuum gauge) sits a little to the right when the engine's idle is rough. After the idle gets to normal, the needle moves to the left (most economical) position where it remains until the car starts moving.

What I have done so far:

I tried using injector cleaner. The problem improved ever so slightly, but not 100% normal. I used a German brand cleaner, called "Moly-something" (sorry I forgot the full name). The O2 sensor, the EHA (it was leaking), and the ICV (idle control valve?--a round cylinder that sits on top of the engine, near the front, made by VDO) were recently replaced. All 8 injectors were also recently replaced. The problem, however, remains.

Someone had suggested the fuel distributor may be bad (but that's an expensive item to replace--are there rebuilt ones available? can the fuel distributor be rebuilt?). Someone else had suggested a vacuum leak at the intake manifold, but I have had cars before with an intake manifold vacuum leak and the symptoms were different. Although some sort of vacuum leak makes sense since the problem goes away when the engine is warm. If there is a gap someplace (causing a leak) then the expansion caused by heat may seal the leak, smoothing out the idle.

The car runs fine once it is warm. Fuel mileage is about 19-21 (combined highway/city) and oil consumption is less than a quart every 3K miles.

The problem is no more than annoyance at this point. Unless it gets worse, I can live with it and just write it off as another one of those endearing Mercedes idiosyncrasy, like crappy A/C controls (just kidding you guys!). Still, I'd like to get it fixed if it doesn't involve a lot of money.

I'd greatly appreciate your assistance in helping to diagnose the problem. Thanks.

[Edited by BENZ-LGB on 05-14-2001 at 03:21 PM]
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  #2  
Old 05-14-2001, 08:49 PM
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I would start by checking the ONE thing that will change the fuel mixture with temperature, the Coolant Temperature sensor.

Check or replace this sensor then have the lambda checked cold and hot.

Good luck.
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  #3  
Old 05-15-2001, 05:11 PM
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I have the exact (not similar - exact according to your description) same problem and no one has fixed it for me. I have been to four different Mercedes Dealers, one of which told me to junk the car (they're nuts). All the ignition checked out fine, including cap and rotor, coil and wires, etc.) Cold start system and O2 sensor are fine. I am taking it to my independent mechanic tomorrow night. He is going to specifically check for vacuum leaks and fuel delivery problems. He is also going to check out the valves and stuff. My plugs in #4,6 and 8 are fouling so I suspect too much gas in each of those cylinders. They will keep the car overnight so they have a cold car to start in the morning and I expect to have an answer by Friday at the latest. I will let you know what they found.

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  #4  
Old 05-15-2001, 10:31 PM
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check your ICU (ignition control unit), ...after I replace this baby, ...the system works fine. Hope this helps..
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  #5  
Old 05-16-2001, 01:11 AM
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ECT sensor values

Not had your problem with mb's but had the exact same thing with my Caddies! It was ALWAYS a bad ECT sensor as Benzmac said. The car would idle very roughly when cold but smooth as glass when warm. On a Caddy, the ECT sensor would drop from about 3K cold to 300 ohms cold. Thus the wrong timing and mixture. So don't rule it out.

Re MB: Here's what the manual says for the ECT's temp (deg C) vs resistance for my S500. Pull of the top and use an ohmmeter to measure the resistance between adjacent terminals. Readings should be no more than +/-5% from those below. You are especially interested in the COLD values. If not right, replace the sensor.

Temp(C)-Temp(F)---- Ohms
20 -------- 68 ----------- 2500
30 -------- 86 ----------- 1700
40 ---------104 ---------- 1170
50 ---------122 ----------- 830
60 ---------140 ----------- 600
70 ---------158 ----------- 435
80 ---------176 ----------- 325
90 ---------194 ----------- 245
100---------212 ----------- 185

Check it out! You should consider a Lamda test!!!

[Edited by JimF on 05-19-2001 at 01:01 AM]
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  #6  
Old 05-16-2001, 01:35 AM
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Benzmac, Jim, 420SEL & Rudy: Thank you for your pointers. Sometime next week I'll leave my car with my local, independent M-B mech and have him look into all of your suggestions. As I wrote, it is more of an annoyance than a real problem and since the car runs fine otherwise, I guess I can put up with it. I used to have a 1967 250SE (Euro) with mechanical fuel injection, same as the 190SL. That little old six-banger purred like a kitten. It had its own little idiosyncracies, but it was a very reliable car.

Donnie (Benzmac): Thanks for the tip on the A/C aspirator fan. I still haven't taken the plunge to look for it behind the dash area. When I finally do, I'll report on how it went. By the way, does a faulty A/C aspirator fan make a whiny little noise that can be heard only from the overhead console grill? How does the noise travel up from behind the knee bolster to the overhead console? Through the hose?

420SEL: Thanks for the suggestions, I read your other posts, on another thread, about your 420 rough idle. If your mechanic diagnoses the problem, and fixes it, let me know. I had read someplace here that someone suggested putting a silicone seal around the injector seals and that fixed the vacuum leak and the rough idle problem. Have you tried that?

I will keep everyone posted if, and when, the mechanic can diagnose and fix my problem. In the meantime I'd welcome any other suggestions from the rest of the gang.
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1989 300TE "Alice"
1990 300CE "Sam Spade"
1991 300CE "Beowulf" RIP (06.1991 - 10.10.2007)
1998 E320 "Orson"
2002 C320 Wagon "Molly Fox"

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  #7  
Old 05-16-2001, 12:54 PM
edbardzik
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How about trying to adjust your CO. It's the little tower next to your fuel distributor. You will need a long 3mm allen wrench to do this. With the car running, push down GENTLY and turn it about a quarter turn clockwise. You will feel spring tension as pressure is applied, but it will bottom out. If you push too hard, the car will stall. Just remember, a little turn goes a long way. Also, check your overvoltage protection relay. It is a little silver box next to your firewall. There will be a 10 Amp fuse in it. If it is blown, you will experience the exact symptom of which you speak of. Last, but not least, buy a BMW and wave bye-bye to inefficient CIS-E injection!
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  #8  
Old 05-18-2001, 08:26 PM
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OK, I got the car back and the rough idle is partially cured. What I mean by that is that the roughness is now a little less than half what it used to be. My mechanic looked it over and said he couldn't find anything really, so he took it out for a drive and said when he hit the gas all kind of black smoke came out the back and then it smoothed out pretty well. I've never gotten black smoke, so he really must have hit the gas hard.

I don't know if it could make a difference, but I notice he changed my oil cap. Maybe it was loose or worn and not giving proper vacuum. Either way the rough idle is reduce to the point where it is no longer shaking the car, but is noticeable if you are looking for it. Maybe I just need to drive it and clean out all the crud that had accumulated from the problem. I will keep everyone posted. So much for the dealer's idea of writing it off. My Mechanic only charged me $100 for his time.

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  #9  
Old 05-20-2001, 12:08 PM
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Standing By. . .

How about the standard 'stuff' such as plugs, wires, rotor, distributor, etc.? Standard fuel 'stuff' such as fuel filter, etc. ?

Those can cause problems although it shouldn't go away when it's warm.

The fuel distributor can be purchased as new and/or rebuilt. I've seen my tech use a small hammer to tap on it to 'dislodge' a stuck one. But again, this happens ALL the time. So a mystery ?
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  #10  
Old 05-21-2001, 10:21 PM
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COLD IDLE PROBLEM SOLVED!!!

OK, after going nuts with this cold idle problem I asked friends for a recommendation for a good, independent local Mercedes mechanic. I took my car to him and I told him all the symptoms that I described here. I also told him the dealer's recommendation was to fix an alleged intake manifold leak and replace the fuel distributor. The whole thing would cost over $1,600.00 [this dealer had already replaced the EHA (under warranty) and replaced all the injectors]. I also told the new mechanic about all the suggestions posted here.

He didn't think that the coolant temperature sensor was the problem so he replaced it at no charge to me. The new sensor did not fix the problem. Then he told me that the problem was probably with one of my exhaust valves that might be sticking (or seated too tight). The valves are supposed to be self-adjusting, but he said that sometimes they stick, or become seated too tight. Anyhow, he adjusted the valve and replaced a shim that sits around the lifter (he showed me what it looks like and where it sits). After he was done, voila! problem solved.

Now I have smooth idle, cold, warm or hot. He told me to also change the spark plug wires in order to get PERFECT idle. He suggested, however, that I wait till the next oil change because there is no hurry.

It is amazing that the local M-B dealer just wanted to throw parts at the problem (preferably expensive parts) and hope something would work. Had I followed the dealer's advice I would have spent over $2K without a guarantee that it would even work. This guy (who is also an airplane pilot) took the time to look for a solution to the specific problem, not just exchange parts. And he solved the problem w/o costing me an arm and a leg. He even forgot to add the cost of the oil (I also had the oil changed by him, I used dino-oil, no the syn stuff) in the final invoice.

I am very happy that the problem is solved. Now my 420SEL runs and idles as smooth as silk. If anyone wants the name of a good local mechanic (I am in the Central Coast, California) e-mail me.

I wish to thank all of you who offered your suggestions. In my case the problem was something other than what was suggested here. Still, this is a great site and the amount of knowledege being shared is just tremendous.

A/C fan aspirator:

Benzmac, I had the mechanic remove and replace the noisy A/C aspirator fan. I figured it was too much of a hassle digging around the dash area lookingfor the fan. Thanks for your suggestions and help. Boy, the car is so quiet now without all that chirping!!!

Wind noise issue (posted under a different thread):

I followed the suggestions given here and I used duct tape to cover the areas suspected of caasing wind noise. I was able to isolate the problem and I am getting the windshield resealed tomorrow.

Thanks ya'll.


P.S. The average age of a Mercedes owner is 52 years, 71% of all Mercedes owners have a college degree (highest of all luxury cars) and their mediam annual income is over $162K (also the highest).


[Edited by BENZ-LGB on 05-22-2001 at 12:41 AM]
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1989 300TE "Alice"
1990 300CE "Sam Spade"
1991 300CE "Beowulf" RIP (06.1991 - 10.10.2007)
1998 E320 "Orson"
2002 C320 Wagon "Molly Fox"

Res non semper sunt quae esse videntur

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  #11  
Old 05-21-2001, 11:02 PM
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Great!!

Nice to see 'happy' endings! Especially for the 'right' price!
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  #12  
Old 04-06-2002, 04:43 PM
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JimF,

I am trying to track down a rough cold idle problem in my '93 400E. I've replaced the rotors and caps and fuel pressure regulator. I am trying to evaluate the ECT sensor. Your post says to test adjacent terminals. When I did, they all showed infinite ohms, but if I test terminals OPPOSITE each other, the readings seem to be about what I expect (216 ohms at 80 degrees).

Am I doing it right?

Also, where in the MB CD-ROMs is this info? I can't find it . . .
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  #13  
Old 04-06-2002, 08:23 PM
brinzbenz
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A real mechanic

I was heartened to see your idle problem solved by a mechanic who knows his cars. I have had an idle problem with my 300E-24 for several months now and after several visits to an authorised service centre I am still no nearer to the problem being solved. Any conversation with the mechanic usually starts with "the test equipment indicates you have a problem with the......." It is nice to see a 'real mechanic' with 'real knowledge' fixing a problem based on experience and logic and not some technocrat quoting from a test set readout. Unfortunately the hit and miss approach (ie keep changing things till it is eventually fixed) to fault finding seems to be the norm nowadays, and it is definitely serving to sour owners perception of what is supposed to be a quality vehicle.
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  #14  
Old 04-06-2002, 10:04 PM
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Boy, can I attest to that!

I am a victim of the 'hit and miss' approach.

Just do a search of all the posts under my username 'suginami', and you'll find about $5,000 in the last 9 months to prove the hit and miss approach to engine diagnostics.

It sure is hard to find a good mechanic. I just wished I lived close to one of the moderators on this site.
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  #15  
Old 06-19-2012, 07:58 PM
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BENZ-LGB Im glad that problem was solved. Anyway I need your help, i have a 1991 M.B.420sel with the same problem. Could you please redirect me where the exchaust valve is located. Thanks, gp
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