Backfire under load

[hfaber329]

Hey everyone, happy holidays!

I have been recently having an issue with my 89 d100 5.2 tbi. I installed headers and dual exhaust on the truck and had one of the harness lay against the header on the drinkers side and melt into a couple wires. Long story short while trying to figure out the problem I tried everything even playing with the timing. I eventually found the issue and fixed it and the truck runs again.

I adjusted the timing and set to 10 BTDC as per emissions sticker. So my problem is the truck will start on second crank and idle and rev fine in park. While driving whether cold or hot if I accelerate slowly and bring the rpms up this way it’s fine. However if I get into it more the 25% of throttle from a stop it will hesitate and backfire through carb and almost want to stall but will quickly recover and accelerate again. Not sure what to make of it. Looking for some advice. My instincts are telling me timing related.


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[gogereaver]

I have the same problem

 

[hfaber329]

Let me know if you come up with anything.


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[reek]

if you're backfiring out of the intake, try dialing your timing to 0 TDC or close to it. if the plug fires before your intake valve closes, it will ignite and shoot out the intake valve and through the carb.

less common, timing is too ******** and the gas ignites out of the exhaust and keeps burning during intake stroke and flames out of the carb.

did you warm up the engine and disconnect coolant temp sensor lead prior to timing adjust? as well plug any vacuum advance if you engine has that.

the other reason could be too lean of air/fuel mixture. hopefully your harness repair is not the fault. you might be running leaner due to the new exhaust set up though so try 5 deg instead of 10 dge prior to TDC

unlikely a bent valve is the cause if the mod was just a new set of headers.

 

[hfaber329]

if you're backfiring out of the intake, try dialing your timing to 0 TDC or close to it. if the plug fires before your intake valve closes, it will ignite and shoot out the intake valve and through the carb.

less common, timing is too ******** and the gas ignites out of the exhaust and keeps burning during intake stroke and flames out of the carb.

did you warm up the engine and disconnect coolant temp sensor lead prior to timing adjust? as well plug any vacuum advance if you engine has that.

the other reason could be too lean of air/fuel mixture. hopefully your harness repair is not the fault. you might be running leaner due to the new exhaust set up though so try 5 deg instead of 10 dge prior to TDC

unlikely a bent valve is the cause if the mod was just a new set of headers.

Great tip. I appreciate that information. I’ll give that a shot and see how it goes.


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[hfaber329]

Update. The 5 degree advance worked great. No more backfires no more hesitation on acceleration. Just feel like I’ve got to adjust the throttle a little but not sure no rpm gauge.


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[reek]

Update. The 5 degree advance worked great. No more backfires no more hesitation on acceleration. Just feel like I’ve got to adjust the throttle a little but not sure no rpm gauge.


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glad it worked out. the idle speed on these trucks are usually 800 or so RPM warmed up. Playing with the timing, if you have issues with the idle and the ability to adjust the IAC, that might be the problem. Not sure on the specifics of the EFI on your model year. however, some of the more simple systems, the idle / throttle adjust can adjust the IAC.