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Old 06-09-2011, 11:33 AM
CF Beginner
Join Date: Jun 2011
Posts: 15
Exclamation HELP!! popping & backfire, starts great, irratic idle. 97 silverado z71 5.7 Vortec

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.

Last edited by mrcleanmom; 06-18-2011 at 12:18 AM. Reason: title unclear so I included some symptoms so I can maximize help
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Old 06-09-2011, 02:04 PM
CF Senior Member
Join Date: Feb 2011
Location: Middletown DE
Posts: 345

i had a cap and rotor on mine cause starting problems it looked not to be made to spec replaced with AC/DELCO unit that did look to be made differently and it starts fine,old style cranck sensor also had problems,do a compression check


97 4x4 burb
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Old 06-09-2011, 11:15 PM
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Join Date: Jun 2011
Posts: 15
Default 97 Silverado Z71 5.7 Vortec

I will try the compression test, i will check o2 sensors...... Cap is the one with the plug wires plugged into the side of cap. I will also check the crank sensor. Thanks.
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Old 06-17-2011, 09:46 PM
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Join Date: Jun 2011
Posts: 15

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.

Last edited by mrcleanmom; 06-17-2011 at 09:49 PM. Reason: error correction
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