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Old 12-04-2001, 02:38 PM
Posts: n/a
cold rough idle 420 SEL

Thanks to Benzmac and Agupta for answers. I went to dealer to change timing chain and tensioner. Suggest to check for cold start injector and O2 sensor... Said it could not be the problem. I read all about cold rough idle in this forum : 120 thread in fact. Also heard about a throttle switch adjustment that never had been consider yet. What and where is that switch ???

Benz-LGB (apr 2001, no 166 post) described exactly my problem and fixed it with valve adjustment on no 6 cylinder! So I asked dealer's mechanic to check that possibility. In fact didn't believe me, but scope it as I ask for. Surprise : No 2 cylinder had evident and fixed missfire. Tried new wire, change gas line on fuel distributor: no effect; was then sure that it was a mechanic problem.He made a leak test that show an audible leak in valve cover, not elsewhere! So made like for Benz-LGB car, adjust both no2 cylinder valves till no or minimum leak (75% to 15% leak success)!

So my mechanic think that there is a valve or valve seat minor worn, at least partly corrected by valve shim adjustment, and that maybe cause a secondary carbon build-up explaining idle misfire till that no 2 carbonated sponge plently saturated; it must clear in a while, he think. But the engine was now almost warm and correction impossible to confir; I was sure that this was the solution, like on Benz-LGB 1991 420 SEL!!!

Few days and hundred km later, it didn't cure yet and I experience cold rough idle again !? In fact cold idle is little less rough last few minutes less, but hot idle seem to be a little more rough, specially when stop with transmission on drive ! Not satisfactory to me. And in fact I use again an ownmade switch to unplug the coolant temperature sensor what do the best job roughness till now!

But what seem sure is that no 2 cylinder is involved, and maybe no 7 a little bit, and that the problem is mechanic, neither gas or fire control problem... It is a pressure leak on second cylindre, in fact valve, head or manifold gasket problem, but I don't beleive the possibility of a hole in piston or ring leak only because it is a Mercedes and not Lada!!! The challenge is to understand why metal warm cured the symptoms, and the answer would make final diagnosis...

Someone have ideas ???

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