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ednoid
New User
Mar 28, 2012, 4:50 PM
Post #1 of 6
(2455 views)
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Hi. I have a '91 GMC 1500 w/5.7 It starts no problem, but when excellerating it will pop, like a back fire, then runs like it fouled a plug. I've changed the injecters in the TBI, checked vacuum lines, tested the EGR valve the old fassioned way...rev the motor and see if the diaphram moves. Plugs are new. Good wires...etc. Any idea? It almost seems like vacuum advance isn't funtioning properly, but not sure how to check.
Any help would be appreciated.
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Discretesignals
Ultimate Carjunky
/ Moderator
Mar 28, 2012, 5:05 PM
Post #2 of 6
(2441 views)
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Does it backfire out the exhaust or intake?
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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ednoid
New User
Mar 28, 2012, 5:12 PM
Post #3 of 6
(2433 views)
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Not sure, but thinking intake. It's more of a sputter, pop, like it wants to die thing.
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Discretesignals
Ultimate Carjunky
/ Moderator
Mar 28, 2012, 5:24 PM
Post #4 of 6
(2425 views)
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It might be a lean back fire on acceleration. Have you watched your fuel pressure when you accelerate to see if it drops out from lack of volume?
If you have a scan tool or volt meter, you can watch the O2 sensor voltage with the O2 hot and working. The voltage should peg at .9 volts (rich), when you get on it. If the O2 voltage doesn't peg, you don't have enough fuel getting into the combustion chambers.
You could have an ignition problem, but most the time ignition problems cause a backfire out the exhaust.
Another possibility is a leaking intake or exhaust valve under high pressure. Pressure in the cylinders is really high under WOT (wide open throttle) acceleration.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Mar 28, 2012, 5:32 PM)
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ednoid
New User
Mar 28, 2012, 5:56 PM
Post #5 of 6
(2399 views)
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It only does it while driving, not while in neutral, so I don't know if I can test the O2 censor. I do hear a ticking. Sounds like a valve or lifter. I thought it was the header leaking, but thinking the former now. Thanks for the help and for spelling acceleration. I thought I was spelling that wrong.
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Mar 31, 2012, 10:22 AM
Post #6 of 6
(2346 views)
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Just a thought - did you maybe drop any spark plugs when putting new ones in?
T
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