exhaust leak causing backfire??

I have a '91 Silverado and recently had a 97 Blazer engine put in. I stuck with a 4.3l v6 but now I have a small exhaust leak in my passengers side manifold. I was wondering if this would cause a backfire at idle. It's not a real constant backfire, its very random, but when it happens it'll miss a little and sometimes even shut off. Ive replaced the plugs, plug wires, and dist. cap and it still happens. It only does it after the truck has warmed up to temp. Im planning on gettin duals anyway and new manifolds but I was curious if this was why there is a backfire or something more serious? And this only happens at idle, not during acceleration.

When you did the engine swap, did you re-use the original electrical sensors, and componants from your 1st 4.3? I don't think there would be a differance between those years, but not sure. Any engine codes? ie "SERVICE ENGINE SOON" light? Try getting the exhaust system sealed completely, this MIGHT fix your problem, don't run too long with the exhaust leak, risk burning a valve. Also double check your timing. Remember to disconnect the connector under the relay cover before you set the timing.

The only thing we reused were all the accessories (compressor, alternator, ect..) I had another guy do it for me but i BELIEVE the wiring was all off of the new engine. The only sensors we used off the old engine was the oil pressure sensor because of the 2 diff. gauges. The service engine soon light does come on some when this happens but not everytime. It goes back off after u turn the truck off and turn it back on. I'm planning on gettin duals so ill have the leak fixxed soon.

If your SES light is coming on at all there is a code in your ECM telling you that something electronic is not right. My 91 Silver. had the SES coming on the same way, I pulled the code "21- TPS". Replaced the TPS and it runs great w/ 156,000! To pull the codes, jumper the last to connections(right side) on the upper row, on the diagnostic block just under your steering colunm, turn your key to "ON", don't start the engine! Count the flashes, first is "12", one flash followed by two flashes. It will flash this 3 times, then it will flash what ever codes it has in the ECM, then at the end it will flash "12" again. Turn your key off and consult your Haynes manual for OBD-I codes. If you need help Em me. We have the same trucks. Also compare your distributors, are they identical, might try installing the original one back into the 2nd engine. This is probably were your getting your back-fire problem.