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problem with backfiring out of exhaust

456 Views 7 Replies 5 Participants Last post by  blue
I have a 73 240z turbo. Has a 81zxt engine and stock wire harness and ecu. I am running 18psi of boost with a 944 IC. Now when I floor it through 1st through 3rd it backfires out of the exhaust soon as I get half way into 3rd. But if I start in third and foor it through 4th sometimes it will happen and sometimes it wont. Fuel pressure at full throttle is 58psi. I raised it 5psi just to see if it would make a difference and it doesnt. A/F gauge says 3/4 rich at full throttle, but when it backfires it goes full lean for a split second and then back to where it was. I believe that is the o2 reading the backfire. My fuel pressure doesnt drop off before this happens, only during the backfire. Friend of mine said I should do a tune up, cap, rotor, wires, and plugs... do you think this will cure it?? It started all of the suddden, it wasnt a slow creeping up problem.
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Well it is always best to eliminate possible problems, and since you are running 18 PSI I would think you want all the performance it can produce.

As a minimum check the cap and rotor. Plugs are simply to cheap for me to worry about i just replace them. Now wires I tend to be cheap, I have been told the best cleaner is 409 for ignition wires then once they dry check to see if they are cracked, if they are time for new ones, if not I try to seal them.

Good luck
Full lean spike on O2 means a misfire

and the air and fuel charge is igniting causing the back fire

Check your plugs/ wires and ignition.

For more A/F interpretation go through these videos <http://www.innovatemotorsports.com/resources/lm101.php>

I just did it last week and learned the above point

btw nice observations!
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Also check to make sure your plug wires are all in there correct location. If the wires are not in the correct order that can cause backfire which can also result in damage to your Air Flow Meter.

A plug wire out of order will cause the car to run incorrectly all the time. NOT occassionally
Well I took it like it was running like sh!t all the time. He did do an engine swap so I figured maybe during this process, that could have easily happened by mistake and yes that will cause such a problem. It's a tip I learned back when I read my Bosch Fuel Injection Manual.

Nope this is occasional. Just in boost, and only during hard acceleration during a hard period of acceleration. I can go all out in 1st, 2nd, and half way through 3rd then it'll happen. Start hard accel in 3rd and go to 4th sometimes it will, and sometimes it won't... Never has done it in 3rd.
If you had a digital scope, you could capture plugs firing using an inductive pick up to try and find problem one.

Like I said, a misfire causes a lean spike on the O2 sensor.

I recommend a thorough inspection of your ignition system,
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