ROUGH COLD IDLE PROBLEM SOLVED - PeachParts Mercedes-Benz Forum

BENZ-LGB · #1 05-24-2001, 12:55 PM

Howdy:

This is one of the posts that were erased ude to the system faulire. I am reposting it not so much because I want to accumulate enough posts to become a Senior Member, but because rough cold idle, or just rough idle in general, seems to be a concern for many here.

My car, a 1991 420SEL w/190K mile, was having rough cold idle problems. The car would start fine, then run rough till it reached operating temperature. When it reached 80 degrees C. or so, the idle would smooth out, but was never totally smooth.

I took it to a local M-B dealer who wanted to change (in order): (1)the EHA; (2) all 8 injectors; (3)the fuel distributor; and (4)fix an alleged vacuum leak in the intake manifold. I agree to 1 and 2, but not 3 and 4 because in my mind those other solutions did not sound right (plus it was getting expensive).

Even after the repairs were made by the dealer the car continued to experience the idle problem. I posted here, asking for suggestions. Most of the suggestions involved checking the coolant temperature control. Other suggestions included changing the spark plug wires and distributor cap.

I took the car to an independent mechanic, who checked the electrical and mechanical sides of the fuel distribution system. Nothing wrong there. Then he did a leak down test and discovered that the exhaust valve in cylinder #6 was sticking. He adjusted the valve and changed the shims on the rocker arm and voila! problem solved. The rough cold idle went away. There was just a little trace of roughness left, so he then suggested new spark plugs wires. We did that and now the idle is creamy smooth.

The idle will never be Lexus-smooth, I've been in enoguh W126 cars to know that M-Bs of that body style will never have Lexus-like idle. But the idle is Extremely smooth, the engine runs strong and it even runs cooler.

The total cost for all the work and parts was less than half of what the dealer's part-jockeys charged for replacing the EHA and the 8 injectors and far less than the cost of a new fuel distributor.

The above solution may not fix rough idle problems for all cars, but it may be worth looking into it before spending a lot of money throwing new parts at a problem.

I just have one (rhetorical) question: when did we lose the art of diagnosing problems and instead become a nation of parts-jockeys? It is such a pleasure to see a good, competent mechanic "attack" a problem and not give up until he finds the real root cause. My new mechanic is not even an "old" guy, he just has old-fashioned values and is really interested in saving me money and keeping me as a customer for the long run.

vrsmith · #2 05-24-2001, 01:17 PM

I'm happy to hear that your problem is solved. In this era of "parts-jockeys" it becomes very important that we recommend good independents. I told my guy that I listed him and I told a guy I'm using in California that I got his name from MercedesShop. This is the only advantage that we have. For my work in California, I wouldn't know where to start without MercedesShop. I believe that the recommendation here has already saved me $2000 and the mechanic hasn't turned a bolt for me yet. What about researching shops before starting a cross-country route? Armed with the locations of the great shops along the way, you are helping yourself should you have trouble enroute.

#3 05-24-2001, 01:23 PM

I share the same complaint about mechanics. They used to be a form of craftsman, now just an unbolter/bolter of parts. Forget repairing, they want to replace.

Reminds me of a time my father brought in a lawnmower that didn't start. Some guy wanted to replace the entire carbeurator. One of the guys (an older gent)in the shop overheard the conversation, walked over, and poked a hole in the gas tank cap. He gave it a pull and it started right up. You see, during a spring cleanup of the mower my dad painted the gas tank; the cap has a hole to let air in. With the hole painted over, the mower wouldn't start. The older guy just said , "you owe me a six pack" and walked away.

SANDEEP · #4 05-24-2001, 01:43 PM

BENZ-L-GB

I am glad that your problem is solved. I have a 300SEL 89 and I have a rough idle meaning that the car will idle at right rpm but the needle fluctuates a bit maybe 25-40 rpm. It does not do this all the time. Other wise ths car is fine. This rough idle is an annoyance. Some one told me that the mixture control unit could be bad.

Could you attribute your problem similar to mine and will a fix like yours be the solution.

Thanks for the reply

#5 05-24-2001, 02:44 PM

[QUOTE]Originally posted by rdurbeck
[B]I share the same complaint about mechanics. They used to be a form of craftsman, now just an unbolter/bolter of parts. Forget repairing, they want to replace.<>

Hey now. Some of us still treat the profession with a level of craftsmanship and enjoy the challenge of finding out "why" something happens. We're just too "crabby and opinionated" to deal with other human beings.
Randy D.

BENZ-LGB · #6 05-24-2001, 03:43 PM

Durrance:

Please don't be offended. There are plenty of crafstmen still left in the profession. And not all of them are either crabby or old. But it does seem, as many have noted here, that more and more of the new-breed mechanics just want to replace parts. And this is true in all fields, not just auto mecahnics. At home, the controls for my central A/C had stopped working. One guy wanted to replace the entire swithc at over $200.00 a pop. Another guy went outside to the central unit, took the switch out, cleaned the accumulation of ant residue (ants love electrical switches, them little buggers!!!) and charged me $50.00 for his time.

By the way, my new mechanic also flies and services his own plane. I figure that pilots tend to be very fastiduous about mechanical and electrical maintenance since one little engine "burp" at 10k feet altitude can spell TROUBLE. If he is that fussy about his own plane, he will also be that fussy about my car.

SANDEEP:

I am not sure what the diagnosis for your car's hesitation problem may be. Intermittent problems, however, usually are of an electrical nature.

420SEL · #7 05-24-2001, 06:19 PM

Thanks for reposting. I am going to have mine checked out. What bugs me most is that I have been to four diffent mechanics (including three Mercedes Dealers) and none have fixed the problem, and none have seemed to care about it in the least. One dealer noticed the rough idle when I was in for something else and had me book an appointment two weeks later. I took the car back and they told me there was no rough idle. Another Benz dealer told me to either pay 5 grand for a valve job or junk the car, and the third just didn't seem to care. "Live with it," they said. Hopefully my fix will be a nice and easy as your's was. Thanks again.

BENZ-LGB · #8 05-24-2001, 06:50 PM

Jason:

Let's know how it went for you. I sure hope that your mechanic can find a sure (and inexpensive) fix to your problem. These cars are too nice to just "junk" them.

BTW, VRSMITH had a really good suggestion. That is, to create a list of good, reliable M-B mechanics across the country. Such a network of well-known, and reliable, mechanics would be really useful when we travel outside of our "jurisdiction" and need emergency repairs. I know that Mercedes Benz already has such a network, i.e., their dealership system, but sometimes we want something more than what the dealers have to offer.

Aaron · #9 05-25-2001, 12:50 AM

It really is too bad that the younger generation of techs has become ignorant part-jockeys. These guys are my generation and it would anger me greatly to see the way some of them worked on cars. That's what finally prompted me to leave the dealership and return to the independent. It's a much different story around Precision! The owners and fellow techs are all concerned about customers and will actually diagnose a car and try to fix it rather than merely replacing parts. During my time at the Mercedes dealership, I singled out about 4 guys out of 25 mechanics there who, if I were not a mechanic myself, I would let even touch my car. Anyhow, within the next two years, I am going to be opening up my own independent Mercedes-Benz service and restoration shop. I haven't picked out a location yet, but I am thinking Cleveland, Ohio (home town loyalty). And besides, Precision already has Cincinnati covered! In addition to service, I would also like to be able to have one (and only one) restoration going at the same time. So I will be on the lookout for a very experienced bodyman (a hard thing to come by). I am thinking that the upholstery can be sub-contracted out and of course the mechanical work can be done in-house. Ok, enough talk. It's late. Good night all!

420SEL · #10 05-26-2001, 01:47 PM

Aaron,

Good luck opening your shop. You'll have to let us know when you do, I'm sure there are lots of us who are looking to add an older restorable MB to their collection (your collection seems to be about the biggest I've seen in a signature)

Benz-LGB,

How much time and labour was involved in fixing your rough idle? I have to have the car pass emmissions in a few weeks to re-register it. I took it in for a test this morning and it had miserably failed the hydrocarbon test (288 against a limit of 103). So I am sure I am looking at a valve problem, and I am pretty sure I know which ones. The car will visit the mechanic once again this week and I am specifically requesting they look for what was wrong with your car (so far no one has bothered to look). I am just curious as to how many days they will need to work on the car.

BENZ-LGB · #11 05-26-2001, 03:43 PM

Jason:

The total bill for diagnosing and fixing the problem came out to 4 hours X $60/hour. He ran a complete test on both the electrical and mechanical sides of the fuel distribution system. Then he did a leak down test on both banks. Then he adjusted the #6 exhaust valve and replaced the rocker shims (two little metal shims). The shims were $7.50 for the two shims. This was far, far less than what the dealer charged me t replace parts that didn't fix the problem. I also had him replace the spark plug wires. For a fraction of the cost, my car runs so smooth it is incredible. M-Bs will never run as smooth as Lexus, but the previous roughness in mine was unacceptable.

I hope this helps. I hope your mechanic can diagnose and fix your problem. My mechanic told me that a tight or sitcky exhaust valve tends to overheat and will eventually fail.

Mervyn · #12 05-29-2001, 09:24 PM

BENZ-LGB,

My 1983 W126 (280SEL) is also experiencing the same problems. I have 3 further questions to your post:

1. Does the adjustment of the exhaust valve and replacement of the 2 rocker shims constitute a head job?

2. What test did the mechanic do to find out which exhaust valve to be adjusted?

3. What plug wires did you get? Stock Beru wires?

I hope to get a similar diagnosis done by my mechanic soon, and I need your advice. Thanks a lot!

Mervyn

BENZ-LGB · #13 05-31-2001, 12:50 AM

For those of you who have been following this post (and for those of you who have written to me about this issue) here is an update. After the local M-B dealership was unable to diagnose and fix a rough cold idle problem I took my car to a highly recommended, local mechanic. The new mechanic was able to diagnose AND fix the problem for a fraction of the cost for the repairs recommended by the dealer.

I wrote a long letter to the service manager explaining the whole situation with my car and why I thought his service advisor and mechanic were trying to sell me unnecessary parts and service. At the end of the letter I wrote: "If you were me, and the roles were reversed, would you feel satisfied with the work done by the dealer?"

The service manager called me today and we had a long discussion about what I thought of the repairs made/service received. It was very cordial and at the end of the conversation he agreed to refund me for one-third of the charges incurred during my last visit. The amount was sizeable enough that the refund offered comes close to matching the tax refund that Pres. Bush has promised me (grin).

I am happy with the outcome. I got a nice refund and the service manager listened to my side of the story and promised to take steps to avoid a repeat of the situation.

Someone here has suggested that our best line of defense against shoddy repair shops is to be informed about our cars and to learn about what makes them run. I'd go a step further and write that we should be informed AND we should never hesitate to seek redress when things don't go exactly the way we think they should. The worst thing we can is to sit idly by and not speak up.

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