Puzzling distributor problem

[Tucson38]

1985 GMC, 350: For the last few years I've had a sporadic problem starting my truck. Finally figured out that if I remove the distributor cap and wipe it with a rag it starts right up. Suspecting the aluminum contacts might be a problem I replaced the cap and rotor with new units containing brass contacts. For good measure I replaced the coil also.

That seemed to clear up the problem for 8 months or so but I'm suddenly experiencing the same situation. Engine turns over easily but won't fire. Remove the cap, wipe it out, and it fires right up. The wiping rag doesn't show any kind of dust or debris, no moisture whatever.

Once it starts it purrs like a kitten, no stumble or anything. Just periodically won't fire up.

Anyone have any input?

[TurnNBurn]

Did you check the connetions on the cap and lead wire? They not loose and fouling the terminals? Long shot but maybe

[rs72z]

Pickup coil. When you take the cap off it probably moves the wires enough to restore contact.

[Weasel Diesel]

I'm with RS72, next time don't take off the cap.... just unplug and reconnect the module harness from the cap and see what happens. Sounds like an intermittent open connection somewhere.

[jhngardner367]

Another thing to check,is the rubber moisure barrier (rubber washer)that the coil sits on.
If there isn't any dialectric grease on it,it will sometimes arc,and prevent starting. It can also cause the spring that puts tension on the center contact(button) to stick,and as you pull the cap,it frees up,and contacts the rotor,again.

[Tucson38]

Thanks for the replies, I'll try your suggestions.

Didn't know about the dielectric grease, I'll put some on today.

[jhngardner367]

Just use a light smear on each side.It acts as a moisture barrier.

[Tucson38]

OK, I used the dielectric grease and polished up the brass part of the terminals (one was replaced a few months ago), now I'll give it a little time and see if that solves the problem. Thanks to everyone for their help.