5.7 vortec backfire please help, 97 silverado Z71
 

I just bought a 1997 chevy z71 with vortec 350 its backfiring real bad. it cranks and starts fine though. i have changed plugs, wires, distributer and no change. I have checked the egr and pin 1&5 reads .01v and the pins 2&4 is reading 3,4v. I have checked the MAF, it seems good. I have no specific codes but cleared anyways. What flashes is the catalyst heat monitor, catalyst monitor and egr monitor. Im very mechanically inclined however this has me baffled. It popps and popps, sounds like its popping in the exhaust, the engine does not surge or misfire but occasionally lets out a huge gunshot like bang. prev owner said he blew out a cat baffle. I would usually just start replacing the sensors in a process of elimination but these sensors are pricey. codescould it possibly be a stuck valve and making it backfire through the intake? I'm at a stand still here.
Did compression test, 182 psi 13:1 on all 8 cylinders, checked ohms at pre-converter oxygen sensor and that was a 4.8 on both. Check fuel pressure and its 62 psi at key on engine off, but immediately looses pressure, this tells me it has a leak at one or more of the injectors. Engine running fuel pressure was a strong 55psi. Had to regap plugs and they were wet. The backfire is coming from the left bank only.
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in the middle of a complete rebuild intake up.

did you change the cap and rotor with the distribuer?

Sean

97 4x4 burb

Actually, the guy I bought the truck said he hadput all that in new, and he was right. But when I was removing some components outta the way to make room for the intake removal I noticed that only one screww was on the rotor and was wreeking havoc, sparkin and banging around, all chewed up, truck shouldnt have ran at all. Then to add, while I was undoing the fuel lines, I did so with my bare fingers, no wrenches required. YES he had the upper intake off it, YES another mess. The backfiring was so severe that some of the ends on the spider assemblyended up melting. Buying Delco replacement fuel meter, gaskets etc. going to do a fWhat began as project 'intake removal', wound up being upgraded to project 'intake removal and whatever else this guy touched'. I am gonna do a full once over on the rest of drivetrain so I can get a better look at everything. .