Ok, well I've been trying to figure out this problem, but no one seems to know what it is. The truck idles and revs up perfect when it is in park or neutral. As soon as it is in gear, it runs like its only using 5 cylinders and backfires/mis-fires through low to mid rpm's. What all would cause this???Plugs are new, cap/rotor/wires are near new. Has a new EGR valve and new fuel filter. It's getting plenty of fuel, but I'm not sure about spark. When I changed the ignition coil, things got extremely worse, so i had to put the old one back on. Wires are not touching metal and there is no viible spark jump. No check egine light is on either. Any help here???

Ignition module?
Pick-up coil?
Exhaust? (it is broke at the y-pipe and missing tail-pipe)

 

well, having a new cat-back exhaust installed helped a little, courtesy of my boss. the cat wasn't replaced so maybe that will be next.

 

what style distributor do you have? do you have the one with magnets in it? if so, they tend to crack and then cause misfires like you described. very common problem but not many know about it. this magnet style dist was used until 1994 but i dont know how early it came out...

 

Um.. all i know is it's an electronic distributor for a 1989 S10 Blazer 4x4 (4.3TBI)

 

ok i just looked up the part number for a 1994 and a 1989 witht the same 4.3 and they are the same distributor part number. which means youhave the magnet type dist. so you may want to consider look into the dist. as the problem...just a thought

 

Yeah, i think i will pull the cap off and take a look, i was thinking about taking the ignition module in for a test anyways. hopefully i can get to it this sunday.

 

Well, i knew this problem would be something so simple, that i would kick myself in the *** for not having it fixed sooner. the new plug wires i had put on were little crappy 7mm's, so the insulation was not the greatest, causing almost un-noticable spark jump. I replaced them with higher grade 8mm wires and now things are running better than ever.