I'm at my wits end here. About a year ago My 1986 420SEL developed a mild rough idle when cold. I took it to my mechanic and he scoped it out, replaced the ignition coil and everything was fine. 6 months after that it started again. I moved to Ontario and had to have emmissions testing done. The car was passing everything put hydrocarbons. The dealer did a motorvac (cleaning or injectors I think). At that time we also noticed that the secondary air pump (smog pump) was shot and that the hose from it that goes to the engine was broken off. Fixing this (new air pump, hoses and valves) seemed to help quite a bit in reducing the rough idle. It would always seem to go away once the engine was hot. Now it is winter and getting pretty cold up here (below freezing everyday). The rough idle is very pronounced when you just start the car up, and it gets better, but doesn't go away even after an hour or more of driving. Whether the car is in park, or in gear at a stop light I can feel the whole car vibrating. I had the distributor cap changed, as well as the rotor and the wires recently (What was on the car was original and the distributor cap was in pretty bad shape at 180,000 miles). The new cap, rotor, etc made a noticeable differnce in driving, but did nothing to help the rough idle. What is left? The fuel mixture? (I use 92 octane Sunoco gas) The 02 sensor? When they were making my car emmissions aceptable they did a lambda adjustment and this was just four months ago. Motor mounts? How much to replace those? And is there anything else I should do first? My mechanic said to run some fuel system cleaner through the engine. I bought some Castrol Syntex Fuel System treatment and am just waiting for the tank to get down to 1/4 full. He said to put it in as concentrated as possible. I never let the tank go below half full and I use a gas line antifreeze additive this time of year every other fill up.

Any ideas?

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Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired