Randomly out of nowhere running at 1500 rpm at 55 for half a hour i get a enormous backfire out the exhaust. Sounded like a shotgun going off. I immediately shut the truck off and coasted to a stop got out,looked around and started it back up and it purred like a kitten. I had my timing light with me and i check the timing. Its 4 degree's advanced like its been for a year...No lights on etc..truck is running great right after i start it back up.



1st idea i had was it was running super rich and the exhaust backfired from leftover fuel but i pulled 3 plugs and they look perfect.. This is a new one for me.

91 tbi 4.3

 

I've heard of this happening when the coil is going bad. Might have it tested.