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Discussion Starter #1
I have a '92, 3.9L that backfires during acceleration until the engine light(right bank 02 sensor) comes on and the backfire quits. The engine is rebuilt (machined,new pistons, rings, bearings, cam, and lifters) with less than 100 miles. The only engine, exhaust or electronic parts that have not been replaced are the mass air sensor and the ICU. The mass air sensor was tested for voltage change and seems to be working. Compression tests indicate 120 psi in cylinders 3 and 4 the remaining are 145 psi. The only remaining items I think the issue can be is a leaking valve or the mass air sensor. Can someone tell me would the check engine light (O2 sensor) correct the backfire caused by a leaking valve?, seems unlikely. Last mechanic thought the vacuum advance on the distributor may be faulty, however I replaced the distributor with no change. Any ideas would be appreciated.
 

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Just throwing this out there
A constricted exhaust can also
Cause backfiring...
With these engines I believe the catalytic converters are a problem ...
Normally I would say the timeing or vacuum should be looked at
But seems you have that covered
 

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Think the cel will change the fuel map so it's possible issue is on the fuelling side of things

Sent from my LG-H930DS using Tapatalk
 

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Discussion Starter #4
Just throwing this out there
A constricted exhaust can also
Cause backfiring...
With these engines I believe the catalytic converters are a problem ...
Normally I would say the timeing or vacuum should be looked at
But seems you have that covered
Thanks for the reply. I have replaced the catalytic converters and O2 sensors and the back fire improved but still remains and causes the engine light and poor fuel economy. I did not replace the muffler and the rear muffler was replaced with thru pipe.
 

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Discussion Starter #5
Think the cel will change the fuel map so it's possible issue is on the fuelling side of things

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Thanks for your reply. I have not replaced the fuel pressure regulator, however the injectors, fuel pump and filter have been.
 

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Just to be clear it's a true back fire through the intake and not an after fire out the exhaust? Mixture and timing could be your only true culprits but if timing is spot on then mixture can be fuel map (bad sensor, vac leaks) or mechanical via pump or injectors. My style is the replace anything that's old as I still get value from the new part but quite disheartening when the symptom remains!

With timing have you any way to check for timing scatter? If distributor is quite poor then the timing might jump about even though at idle it behaves, a very early random spark will ignite the charge with the inlet valve open and fire back through the intake...

Sent from my LG-H930DS using Tapatalk
 

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Discussion Starter #7
Just to be clear it's a true back fire through the intake and not an after fire out the exhaust? Mixture and timing could be your only true culprits but if timing is spot on then mixture can be fuel map (bad sensor, vac leaks) or mechanical via pump or injectors. My style is the replace anything that's old as I still get value from the new part but quite disheartening when the symptom remains!

With timing have you any way to check for timing scatter? If distributor is quite poor then the timing might jump about even though at idle it behaves, a very early random spark will ignite the charge with the inlet valve open and fire back through the intake...

Sent from my LG-H930DS using Tapatalk
Based on soot in certain chambers of the inlet manifold and the sound it makes I believe it is an intake backfire. The backfire only occurs under load or quick throttle in park. I installed a new Petronix Distributor (original works better) with no change. The fact that only the right bank O2 sensor faults make me think it has to do with a valve, If the mass air flow sensor were bad I would think the faults would occur on both O2 sensors. Thanks for your reply
 

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Discussion Starter #10
Problem solved. Right bank connector to oxygen sensor wires had pulled out of connector and I had replaced them incorrectly. Runs good. All done for now.
 
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