Have just changed motor and box in 92 vp commodore. last engine ran but backfired on gas and struggled to run. got it going by moving fuse around on efi relay it ran for bit then dead, anyway ive taken most of the gas wiring out to put it back to standard got spark and fuel but wont run?

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Question moved to model category.

You can't, it is run by the ECM. Just test the fuse and the fuel relay.

https://www.google.com/search?q=holden+commodore+vz+fuse+box+diagram

https://www.google.com/search?q=holden+commodore+vz+fuel+pump+relay+testing

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Fuel, air, spark, compression. You're not getting one of those 4.

Someone gave me one in tbe fall and I tried cleaning out the carburator and it didn't help. I bought a new carburator made for that model on Ebay and it ran like new. Not sure if that's your issue, but that worked for me.

run codes and check for faulty TCM unit.

This may be due to a brunt valve or misadjusted valve, the back firing is due to the intake valve being open while its firing in that cylinder.
Make sure all wires to cylinders are correct and not crossed.

My first thing i would look at is the firing order, may be a crossed wire, even if it starts and runs you may still have a crossed wire, then if firing order is correct then i would move to checking the adustment on the valves for that cylinder,
Make sure rocker arms arm tightened correctly and push rods are in correct locations. i know that some engines like the 3.1 have diffrent size push rods for the exhaust and intake, remove the valve cover and see if that valve is closed, or try a compression test on that cylinder and if it is low then you have a bad valve or it is not adjusted correctly and last the timing chain is not aligned correctly.

Sounds like something big-time broken. I would recomment taking to your local repair shop for diagnosois and/or repair.

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Link: Fuse Box

More details can you find in autozone.com (free register).

I hope help you with this (remember rated this help) Good luck.

have you checked firing order on plug wires? could be problem.

timing chain/belt ?

I'm having the same problem with my 90 2.3L Ranger. I replaced the EGR Valve back in August and the truck ran fine (but the gas mileage was bad) until late December . I knew it needed an O2 Sensor back in August but didn't replace it until last Saturday along with the Idle Air Control Valve, Plugs and Wires. The truck ran fine for a couple minutes then started running like a VW bug! My code reader is now throwing out an EGR failure code, so I guess i'll take the 4 month EGR back and get a new one, and see what that does!