leave-eye

Ok here it is, Pulled my motor to reseal, put a cc305 and new valve springs in it, i used the twisting the pushrods method from shoebox to adjust the rockers. My car is a 94 z28 a4 with longtubes the cam solid motor mounts slp cai custom exhaust 2800 stall vigilante 3.73 gears, new plugs and wires reused opti reused o2 sensors. So my problem is when i start the car it idles good no problems whatsoever, until it reaches about 160 degrees coolant temp. It then develops a backfire which progressively gets worse as the car gets warmer. If i bring the rpms over about 2.5k no backfire at all. So at cold temp no backfiring, at warm temp backfires unless holding rpms over 2.5k or so. let me know what you all think. i checked all the wires to make sure none are crossed, and i wouldnt think it would run good at cold starts with crossed wires or bad timing. so there has to be a some kind of computer failure. and it hasnt been tuned yet. please any feedback would be great

Guest47904

Sounds as though you have a problem once it goes into closed loop. You want to compare what is happening with the O2 readings to the fuel trims. I had a wiring issue that caused it think it was running rich. The computer would pull fuel until it would hardly idle and then pop out to open loop, reset and start the whole thing all over. Until it went into closed loop while it was warming up, it ran fine.

If you can't do that, at least look at the physical connections of the O2s. From the PCM all the way to the sensor.

You can also force it into open loop by disconnecting one of the O2s. If it runs ok then even after being fully warmed up, you know you have a closed loop issue.

leave-eye

ok well the thing that is probably happening is that the o2s are super worn out. i have no idea when or if they have ever been changed since i just bought the car and havent driven it since ive owned it. just tore it apart right as soon as a i purchased it. it ran when i bought it but was dumping coolant out of two freeze plugs, never was able to get it up to operating temp. plus i had the o2s sitting out of the car for 3 months as i was working on it so possibly they have gotten really bad. im going to try to use the good ones on my cousins 94 camaro and see where that puts me. ill keep you updated.
another theory, maybe when i was doing the extensions on the o2s i didnt crimp them well enough or something. ill dig a little deaper and see what happens.