I recently added a set of headers to my L03. I added a set of hooker 1-5/8 primaries, with a 3" y-pipe, catco 3" catalytic converter, and a flowmaster 3" force II catback. I love the sound. But have noticed it seems down on power, after warm up. I broke 3 plugs when installing the passenger header, but replaced them(the others will be replaced soon). And I also had to replace 4 plug wires(again, I have a new set of mallory 8 mm in the car, just have to wait a few days to put them in). Anyway, I also broke the knock sensor. I broke the porcelain shell, off. I tried plugging it in, but the wire falls out. I also deleted my A.I.R. system. But I did that 2 weeks ago. My point is, after warm up, the car seems down on power, to a point I think its worse than before the headers. If I drive it cold, it runs great, but after it reaches operating temp. its a dog. Could this be the knock sensor? I only noticed it a few days after the header install. And it is fine, even fast, when cold. What do you all think?
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I almost forgot, I recently changed tires too. In the rear, I went from the stock, 215s, to a 275/50/15, and a set of convo pro 15X8-1/2. They are actually lighter, than the stockers, I believe, but do have quite a bit more bite. Other than that, and the air pump, and the knock sensor, Im lost. I was thinking, since its in closed loop, while cold, it doesnt read the knock sensor. When warm, it doesnt get a reading, so it messes the timing up. Does that sound right? Or am I way off. The difference between cold and hot, on the seat-of-pants scale is big. Maybe a 2-3 seconds, on 0-60.

Perhaps since there is no contact (sensor information) for the knock sensor, the ECM is not advancing your timing under load. I would definitely replace your knock sensor.

Also, with all of the exhuast work you have done, the engine is breathing better. The ECM should be able to adapt since this is a basic mod.

Im not getting a light, though. And I havent replaced the aircleaner yet. Again its on the to-do list. I will change the sensor anyway, cause I know its broke. I am just wondering what else it could be. This is my first guess. A friend said he thought the air injection bypass was the culprit, cause the exhaust temps would be higher, and give a bad 02 read. IDK.

o2 sensor should be checked too. Although the knock sensor is very important.

Did your performance degrade after you remove the AIR system but before the header install? Or after the header install, when the knock sensor broke?

AIR system should not cause the problem, but if you installed a cat with the AIR tube and you are not using the AIR system, the car will not like that in the long run.

Get your timing light and check the advance to make sure it is working.

The cat has no air injection tube. And it started after all the upgrades. I actually noticed it, after the rear tires were installed. I dont know if it started after the headers, but I didnt notice till about 2-3 days after. And the air injection was disabled about 2 weeks before the header install. I really dont think the tires are the problem, they are only 1/2" taller, and quite a bit wider, but not enough to cause this. And I kinda babied it for a few days after the header install. Thats what I always do after an upgrade. I baby it, and listen for strange noises, or smell for anything unusual.

Also my idle is low. Like in gear its 450-500rpm. At idle, in park its only about 550-600.