Problem with custom exhaust

[DGvr4]

I build a custom exhaust system for my 2002 mr2, no catalytic with a straight magnaflow flow race series silencer. The first time I start the car runs perfect but 5 minutes later it starts to fail. The cars dos not accelerate upon I accelerate to full throttle. I scan the car computer and says 3 codes.

Randomly miss fire
Miss fire on cylinder 1
Miss fire on cylinder 4

I replace the sparks but nothing

[MGalo]

First, why get rid of the CAT? I just don't get it...

Second, there are 13 possible problems that cause P0300/1/2/3/4 codes. Among the easiest to check are the MAF, ignition sys and injectors.
Try swapping the coils, say, #1 to #2 first to see if the P0301 moves to P0302 etc. Do the same with #4 coil. These tests will either show whether it's the coils or not.
Have you ever cleaned the MAF? If not, get some CRC ELECTRICAL Cleaner(or equivalent); DO NOT USE CARB/TB cleaner!
If you're not aware of this site, SpyderMagazine.com - The Premier Source for MR2 Spyder Information , DO go there and read and learn. Click on "Articles" and scroll through the list to find a good answer to just about any problem or how-to.

[DGvr4]

Sorry for the late reply I have to much to do. I exchange the coils and do almost everything you say and finally found I have a bad O2 sensor. May be ruined when the welder use it like a guide to weld the O2 plugs. I replace the sensors. Now the car runs almost ok, but It start to fail when warm up and do almost the same type of fail than before but less intense. Also the cat failure code turns on, what is expected. May be the fail have some relationship with the cat code??

I live on PR and the laws for the cars are not so strict. I ever say that PR is the best place to have a race car on the streets without so much problems, so why not get rid of the cat

[MGalo]

Originally Posted by DGvr4

Sorry for the late reply I have to much to do. I exchange the coils and do almost everything you say and finally found I have a bad O2 sensor. May be ruined when the welder use it like a guide to weld the O2 plugs. I replace the sensors. Now the car runs almost ok, but It start to fail when warm up and do almost the same type of fail than before but less intense. Also the cat failure code turns on, what is expected. May be the fail have some relationship with the cat code??

I live on PR and the laws for the cars are not so strict. I ever say that PR is the best place to have a race car on the streets without so much problems, so why not get rid of the cat

No problem. Just how did you "find" the bad sensor? Also, does your Spyder still have the stock exhaust manifold, and if it does, are the "pre-cats" in good condition? Other areas that could be the problem include coolant temp, vacuum leaks, PCV Valve and hoses, ECU, valve timing/clearance, fuel pressure...It could be ANY of these.
I hope you get it figured out and running well!!