420SE V8 1986, 125K miles; Car was operating in top shape with smooth idle no cold/hot start problems, excellent power, just as a MB in top shape does. Out of nowhere after a short 5 mile run to the store, it started idling rough and idle surging just out of nowhere. Has had recent timing chain/rails, plug wires/cap/rotor 1 yr. old, plugs are all OK and just cleaned. Did electrical check plenty of juice to all plugs; fuel pumps working; changed OVP relay; did fuel check loosening injector lines all showed plenty of gas coming in; checked distributor all OK; no vacuum leaks anywhere. Fuel filter has less than 5K miles. Tapped idle selenoid in the engine and idle relay under passenger floorboard with no change. I'm puzzled because this situation arose immediately after I started the car at the store parking lot. Car still has plenty of power although it seems to have a slight misfire. Placing hands in the tailpipe discloses smooth idle then surge with a ppppt, ppppt, (uneven exhaust flow). I am puzzled and would appreciate some help.

I would look for a vacuum leak.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

how old is your crank position sensor? My '86 coughed or jumped while turning a corner a couple of weeks ago, got to the point it ran perfectly for a few mins, then died. Based on advice found here, I changed out my CPS; so far, so good. Electronics failures can be qirky: intermittant, etc. Based on my car's symptoms, I thought it was ignition related rather than fuel, because it would die suddenly, but restart and idle perfectly, it never spit through the intake nor had any other fuel-related symptoms. Hope this helps.

Carlo(s),
Did you find out what the problem was? My 300SE has a simular problem after changing the head gasket. I checked for vacuum leaks and found none. All hoses are still soft and plyable. It has very poor idle and stops running at red lights or when I take my foot off the gas. I can place it in nutural it will be OK and runs great above 1000 RPMs but stumbles getting there.