Backfire Problem
10-24-2008, 06:56 PM
#1
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Backfire Problem What would make a motor backfire through the pipes at 5400 rpms in any gear ?
This stupid problem has been rearing its head for 6 weeks now .
It usaly happens at wot but has backfired at 3/4 throttle .
The local shop had it for 2 weeks and couldnt figure it out either .
Ive tried swaping out the MAF , Computer , crank sensor , cam sensor , ive checked the wiring harness for issues and replaced a couple falty wires , changed spark plugs , swapped coil packs , built a new fuel system .
The car was running great right before i spun a rod bearing ,, had the motor rebuilt by Shaker , reassembled the car and now i get a backfire every time i go 3/4 to full throttle .
The car runs great other wise and there are no typical signs of a problem .
A/f is 12.750ish right up to the time the car does this .
When the car backfires it stops the logging and the car shuts off and stumbles from 5400 down to 4000 rpms then trys to catchback up to itself but cant .
Im at a total loss here and will try anything i can to get over this issue .
HELP ! ! !
This stupid problem has been rearing its head for 6 weeks now .
It usaly happens at wot but has backfired at 3/4 throttle .
The local shop had it for 2 weeks and couldnt figure it out either .
Ive tried swaping out the MAF , Computer , crank sensor , cam sensor , ive checked the wiring harness for issues and replaced a couple falty wires , changed spark plugs , swapped coil packs , built a new fuel system .
The car was running great right before i spun a rod bearing ,, had the motor rebuilt by Shaker , reassembled the car and now i get a backfire every time i go 3/4 to full throttle .
The car runs great other wise and there are no typical signs of a problem .
A/f is 12.750ish right up to the time the car does this .
When the car backfires it stops the logging and the car shuts off and stumbles from 5400 down to 4000 rpms then trys to catchback up to itself but cant .
Im at a total loss here and will try anything i can to get over this issue .
HELP ! ! !
Last edited by =ZoomZ=; 10-24-2008 at 07:07 PM.
10-24-2008, 07:25 PM
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the screenshot above is missing some information because the config is set up for my 06 GTO, not for his C5.
just before the car backfires it appears to start going lean. fuel pressure is static between 55-58 psi up to, through and after the backfire happens so it is not losing pressure. this is verified on 2 gauges.
when it was backfiring it appeared to be losing connection to the oxygen sensors and the oxygen sensor wires were pretty fragged so he rewired both sides. the screenshot above is from BEFORE the wiring was fixed so we're unsure if the oxygen sensors are dropping off at the point of the backfire or not.
injector duty cycle on another log was showing only 75-77% at the point of the backfire.
just before the car backfires it appears to start going lean. fuel pressure is static between 55-58 psi up to, through and after the backfire happens so it is not losing pressure. this is verified on 2 gauges.
when it was backfiring it appeared to be losing connection to the oxygen sensors and the oxygen sensor wires were pretty fragged so he rewired both sides. the screenshot above is from BEFORE the wiring was fixed so we're unsure if the oxygen sensors are dropping off at the point of the backfire or not.
injector duty cycle on another log was showing only 75-77% at the point of the backfire.
10-24-2008, 09:26 PM
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Does the vihicle have a high duration cam? Sounds like a valve timing concern. With a heavy cam with a lot of overlap timing can be touchy. I would say ignition and fuel concerns are not the best direction.
10-24-2008, 10:59 PM
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This same motor went 10.50's with the same cam , heads , springs and everything .
The motor was rebuilt and now i have this issue , everything is the same in the motor accept the compression went up a fraction from 11.8 to 12.0 .
