I have an engine miss that I can't find on my 91' Marquis with the V8 5.0ltr HO mods. I Replaced the following.....plugs, wires, cap, rotor, coil, fuel pump & filter, fuel pressure regulator, ACT, ECT, TPS, new distributor and spark control module. The miss is not constant it comes and goes. I have no codes showing. It doesn't matter if its a bumpy road, rainy day, etc. It does it most right when the temp is about to open the thermostat or just after it opens. I never notice it when its idling. Did a cylinder balance test and it passsed. It doesn't seem to be at any certain engine speed but is more prominent when under a load or big hill. The miss is usualy only a one time miss then a minute later it will doo it again just once. Kinda like the key is being turned off and on shutting the engine off for that split second.

 

SOUNDS LIKE THE ONLY THING LEFT IS A INJECTOR TRY HAVING THEM CLEANED WITH A MACHINE NOT JUST THE ADDITIVE YOU PUT IN THE TANK IF THAT DOSENT HELP THE DEALER SHOULD BE ABLE O PINPOINT WHAT CYL IT IS GOOD LUCK