I am having trouble locating the source of an erratic miss. It normally occurs under moderate to heavy acceleration. The plugs are good, wires cap and rotor also check out good. Fuel pressure is at 55psi static and 50psi at idle. I think I "blew something out" when I hooked up the MSD 6A for the first time. I got good idle at first then shut it off and restarted a short time later and took a drive. I had an almost undriveable miss just going around the block. I had to disconnect the MSD and go back to stock ignition just to reduce the miss and make it somewhat driveable. I am wondering if the problem lies in the condensor. I have never changed this and do not know how to check for proper operation. If I remember auto shop correctly, a bad condensor can cause an erratic miss. I have never heard of one going bad but after checking everything else who knows?!?
The motor idles o.k. This only happens under acceleration and not always at the same RPM. It also doesn't matter if the motor is hot or cold. Plugs are gapped at 0.044 and all look clean.
I know it is in the ignition system because air isn't a problem and the fuel system has passed all checks in the factory manual(pressure and delivery) and I do not get any trouble codes from the computer.
Any thoughts?
Thanks
Youngster
1990 F150 4.9 EFI

 

Well, I found the problem!!
I finally took the truck in and had it hooked up to a scope. Two plugs had cracks in the porcelin. 1/8" and 3/16" long and very hard to see. Scope showed it clearly and when I saw them the only way I could tell they were cracked was by feeling them with my fingernail! The technician said I would have seen carbon traces had I driven it much more. Replaced the plugs and what a difference!
Youngster