Yesterday I went to an autocross event, and on my second run the car started misfiring. It misfires only at wide open throttle, and from 3-5.5k rpm, and again at 7k rpm. But I can rev it freely to 9k rpm if its not wide open. I also do not have a check engine light on yet, and it didn't even flash. Has anyone experienced something like this or know some things to check? The only thing i've done recently was replace the upstream o2 sensor with the cheapest one I could find on ebay (maybe it went bad? I saw one thread where a guy had a similar issue and he had pinched his o2 wire between the bell housing, and that was his issue).

So far, I replaced my spark plugs, checked the crank sensor for dirt/debris, and checked the maf for dirt or debris. And today, I switched back to the factory tune and drove it around the neighborhood and made it break up a decent amount of times, but still no CEL. I plan on doing a compression check when my buddy with the tester is available- the car still hot starts like a champ.

Related modifications-
BHR ignition setup (d585 coils, msd wires)
Catless resonated midpipe
Racing beat intake with snorkel
Tuned by mazdamaniac on accessport
119k on original engine

 

If it's not flashing the CEL how do you know it's misfiring?
3.5 and 7k is when the intake valves change over so might be something there.

Does the tune you have move activation points for intake valves around?

I would reset NVRAM just in case.

 

How many miles on those coils?

 

Bet the coils are dead.

 

Mystery solved.

Only about 10k on the coils. But the CEL managed to show itself this morning when I let the car warm up before shuffling around the vehicles. P0030 for upstream o2 sensor. That sensor only has about 200 miles on it. Moral of the story, don't buy the cheapest ebay stuff for important components. Here is a video I took this morning on it.

 

Thanks for the update

 

Mystery not solved. The new sensor came today, I replaced the previous sensor with a denso sensor, part number 2349102. The car still has the same issue with the same code. Any advice?

 

Another update (still not fixed). So I went to my buddy's house and did a compression check. about 92 on all 3 faces of the front rotor, and about 90 on the rear rotor (At about 200rpm). So that's not too hateful. I checked the throttle body, and cleaned the ess. The MAF had some oil residue on it because there was some oil in the intake boot, but I could not find any MAF cleaner at local stores (all sold out).

Then, I cleared the codes and went for a 35min drive. Codes p0481 P0480 P0335 and P0171 popped up. In order, they are cooling fan 2, cooling fan 1, eccentric shaft sensor, and running lean. Gonna try to find some maf cleaner, but after that, i'm stumped

 

Maybe some electrical gremlins, but you are definitely having a problem with oil getting on your MAF. Install a proper, baffled oil catch can to eliminate the issue. Next, check for a weak or dead coil. Following that verify your coil ground is still properly secured. Finally, clean and inspect your cooling fans. Ensure not only that their connectors, relays, and fuses are good but also that they spin freely and turn on at the correct temperatures.

 

Car is officially fixed. So I had time to mess with it today and long story short, it ended up being a coil pack. I was in denial that any of the packs could have gone bad that early, but sure enough, the first pack (for leading plug on front rotor) was not firing. And I guess the erroneous codes came from the idle occasionally bouncing low enough to turn the battery light on. Thanks for the help everyone.