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I have a 94 LT1 out of a chevy caprice interceptor with a painless harness installed. I pulled the engine and put it into a rat rod project I've been working on. While the engine was out I replaced the spark plugs, wires, coil, icm, optispark, and the fuel injectors. The air pump was also removed. The car starts up great but after running for maybe 10 - 15 seconds it will start to pop and backfire slightly out of the exhaust at idle. When it does it misses slightly. For the life of me I can't figure out what is going on. I double checked all of the vacuum lines and could not find any vacuum leaks. I also triple checked the wires to make sure they were in the correct order. The injectors were replaced with some off brand Most Plus 24lb injectors.

Any help or input would be greatly appreciated. This was a gift to my father and a project for us to work on. I really want to get this on the road so he can drive it to work and show it off.

Link to a video of the popping / backfire: 20210221_170410.mp4

Thanks in advance!
 

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Welcome to the forum.

When you replaced the optispark, if you used anything besides a GM, Delco, or Delphi, you problem probably lies there.

Does your painless wiring harness have a diagnostic link connector (DLC) ? If yes, put an obd 1 scanner on it and scan for codes. See if you have the ignition codes dtc 16, 36, 41, or 42. Dtc 16 is the opti low resolution pulse missing. 36 is high res pulse missing. 41 & 42 are ICM codes.

Does your rat rod have a check engine light as factory car does? If no, it would be in your best interest to wire 1 up.

94 f-body factory service manual is available at the following link. It's close enough to B body info to help you. It has troubleshooting, & factory wiring diagrams. www.mediafire.com/?40mfgeoe4ctti

I also suggest buying a PC based oscilloscope to check the opti low & high res signals going to the PCM. It's about $70 on ebay, amazon, etc.

Also buy a fuel pressure tester. Hook it up to fuel rail shrader valve and check key on prime fuel pressure. Should be around 43 psi. Crank engine and check idle pressure. Should be at least 35 psi. Run engine until it starts messing up. See if fuel pressure falls when engine starts to stumble.

For the misfire, try misting the spark plug wires with some water. Do this at night and run car in the dark. Look for spark leakage.
 

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Discussion Starter #3
Thanks for the reply Cocobolo,

I recently ordered a OBD1 dlc connector. It does have a CEL and I'm hoping that the connector will work with the painless harness. The fuel pressure is holding around 40 at idle and doesn't show any signs of any issues. I will take a look at the wires and I'll also look at getting an oscilloscope to test the opti. My father actually replaced the opti and I'm unsure of what brand he purchased.

Thanks again for the help and hopefully I will figure out what's going on this weekend.
 
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