Backfiring at high RPM's/under alot of load

[MeatL0af]

97 Z28 A4. My problem started just a few days ago. I can drive it fine if I stay less than half throttle. However, first gear it usually doesn't backfire until it hits about 5500. Then in 2nd gear the car starts backfiring pretty badly at around 3500+,when giving half throttle or more, all the way redline 6100. The more gas I give it, the more it backfires. I have 2 codes currently (other than EGR and AIR; !!). They are both for the post cat 02 sensors and/or the cat's themselves. I replaced the bad 02 a while back, so I'm unsure how I could get this code already.. What do you guys think? Could this be a clogged cat or an engine missfire? I noticed the coil is loose on the assembly it sits on with the ICM. I checked all connections though and unplugged/plugged everything. No arcing was observed when I looked last night under the hood at the wires. I think this might have started right after I added oil to my crankcase (low oil light came on!! Bad valve guides I was told) and I believe I spilt some and maybe it got on the opti...? Would oil hurt the opti? Any input would be helpful. PS: All parts I've replaced within the past 6 months: Coil(new OEM) ICM(used one my friend had layin around) Plugs and wires, fuel filter, air filter, fuel pump(used also but factory), Injectors(used, but from a 97 also), Opti was replaced about a year and a half ago (i believe just cap/rotor but not positive).. Whoever decides the read all of this and has any input, thank you so much and god bless you

[Paulster2]

Hey Jeremy. I think you'll need to datalog it to get a good idea of what is going on. It could be the Opti breaking up at high speed (if the shutter wheel inside is starting to vibrate at that speed, you might be getting some timing issues from it). If you only spilled a little bit of oil, I doubt it hurt the Opti ... besides the Opti is aways away from the oil fill hole. Another thing it might be the is the TPS if it has a dead spot in it at the higher throttle positions. Datalog would tell you what the voltage is at the higher rpm levels ... actually, I don't even think the engine has to be running to see what the TPS is reading as long as the ignition is on. Could be your coil. There is a procedure on Rob's site to check it. I'm sure there are some other things as well, but that should get you started.

[MeatL0af]

Awesome, thanks alot i'll check it out.

[Gatorhead]

You state the coil was loose. What concerns me about this is if it is loose so are your engine grounds.

[Paulster2]

Originally Posted by Gatorhead

You state the coil was loose. What concerns me about this is if it is loose so are your engine grounds.

Excellent point, Jack.

[MeatL0af]

Yes that is a good point. Well then I guess i'd better check that before I even mess with the opti. Thanks!