backfiring after new cam install
06-09-2005, 05:22 PM
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backfiring after new cam install
I just installed a new cam. Specs are in the sig. The old cam had pretty similar specs. However, I can now hear backfiring in the exhaust almost constantly under light/medium throttle, but it is nonexistent at WOT. My wideband was reading really lean (like 17 to 1) during the part throttle stuff, but I might have an exhaust leak so I don't know if that is reliable. I pulled a plug and it was white which does indicate lean. At WOT the wideband read about 12.2 to 1. Do I just need to play with my chip and keep burning new ones, or could it just be my ignition timing is off. I haven't set it with a light but I can get access to one tomorrow. It doesn't really seem to matter how I turn the cap, it still backfires. Obviously I need to work on the tune, but it seems odd to me that with a similar cam it is backfiring so much. It never used to do this. I am not running a cat, but I wasn't before either. What do you guys think?
06-10-2005, 05:46 PM
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Car: '94 GMC Sonoma
Engine: 406 sd TPI
Transmission: 700r4
Did you set the valves with the engine not running (using pushrod feel) or with it running?
Whichever, you may want to recheck it. Could be an exhaust valve a little tight and not sealing - letting unburned gas into the exhaust.
Double check for the exhaust leak, also.
And don't give up, I'm also fighting an exhaust pop on mine with true duals - pop is on only one bank. I keep trying everything else before pulling the head.
David
Whichever, you may want to recheck it. Could be an exhaust valve a little tight and not sealing - letting unburned gas into the exhaust.
Double check for the exhaust leak, also.
And don't give up, I'm also fighting an exhaust pop on mine with true duals - pop is on only one bank. I keep trying everything else before pulling the head.
David
06-10-2005, 06:07 PM
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I set the valves with the engine off. If it was an exhaust valve not sealing, wouldn't it pop at idle and and WOT too? It only does it under part throttle. I know its not an exhaust leak, I know what those sound like.
06-10-2005, 06:35 PM
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Car: '94 GMC Sonoma
Engine: 406 sd TPI
Transmission: 700r4
A lot of different dynamics going on between idle, part throttle, and full throttle.
Hard to make a direct correlation.
This was not meant to be a specific diagnosis that is a do-all, fix-all. Just one of the many diagnostic procedures that are required to pin down a problem.
The exhaust leak issue has more to do with fresh air being sucked in through a leak giving unburned fuel and hot exhaust gases a nice atmosphere to burn in. Yeah, leaks can go both ways.
David
Hard to make a direct correlation.
This was not meant to be a specific diagnosis that is a do-all, fix-all. Just one of the many diagnostic procedures that are required to pin down a problem.
The exhaust leak issue has more to do with fresh air being sucked in through a leak giving unburned fuel and hot exhaust gases a nice atmosphere to burn in. Yeah, leaks can go both ways.
David
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