Can't find Dead Spot at Idle
 

My 90 420SEL will idle smooth then all of a sudden it will go dead for 1 cylinder. Very random also. If I drive it on the interstate for a while it does not do it. I have replace plugs with bosch copper core and ohmed the wires new cap and rotor. I hate throwing parts at it though. Plugs made it less. Every now and then it will miss on take off for about 30' then clear out. This started with a very bad miss and new plugs got me this far. Maybe one plug is bad? Thanks

Look for vacuum leaks -- sounds as if it's going overlean once in a while.

Suspect anything rubber, and use some carb cleaner or brake parts cleaner spray to spray the connections between the upper and lower manifold and the plastic part between the fuel distributor and the manifold. If the idle picks up and smooths out when you spray and area, it has a leak.

Peter

i recon dirty idle control valve