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Old 05-24-2001, 12:55 PM
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BENZ-LGB BENZ-LGB is offline
Strong, silent type
Join Date: Apr 2001
Location: Southern California
Posts: 1,663

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.
Current Benzes

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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