Hoss50

I am currently trying to diagnose a problem with why my truck is running poorly. It is backfiring through the manifold and has a hesitation off idle and through about 2000 rpm. The backfire only occurs under heavy throttle in park and when it is under load and you try and go WOT.

Background. 85 K5 Blazer w/MAF TPI setup off a 87 Firebird. Heated O2 sensor, no VSS. The truck believes it is in park all the time. It has run perfect in the past, this problem is a newer problem. There are no stored codes.

I have changed ECM's, EST's, ignition module, all relays, IAC is pretty new, TPS seems to be reading fine. I reset the IAC and minimum air already. I just cleaned the plugs and they looked ok. Timing was a 0* and I have bumped that to 6* to see if it helped but it doesn't seemed to have done much or anything. I have disconnected the MAF to see if that helps but it doesn't do anything beside set the code light. I have checked vacuum lines and they seem to be ok for leaks and such. PVC is ok.

I am to the point I am thinking about ripping off the manifold and redoing the gaskets on the TB and the runners.

Any ideas or help?

Harley

thekevin

i saw you post on pirate, im surprise nobody here has had any input on this.

jshepit

I have an 88 305 TPI that I'm trying to isolate (kinda) a similar problem. Mine starts and idles great, but hesitates and backfires during acceleration. Sounds like you've checked many key parts. How about the TPS, does it check good? And I would suggest checking around the manifold and fuel injectors for potential vaccum leaks(the propane tank technique works pretty well).