996.2matt

My 02 c4 has had a very strange issue for about two weeks, it started off intermittently, but has slowly gotten worse to the point that it’s constant now. The issue is, in high rpm’s ( Around 3.5-4k ) it’ll throw the psm and abs error lights and also bog down REALLY bad. Also when it bogs down it backfires and shoots flames. My only thought is since It’s catless but dosent have a tune yet maybe the 02 sensors are fouled??
Does anyone know what could be causing this? Thanks in advance! Link to videos of the issue: https://youtube.com/shorts/gD5X1OJ8o2I?si=jqyt3DFpGatybV4A

DBJoe996

Have you ever replaced the MAF sensor?

sublm8

Don’t know if this will help much, but disconnecting the MAF on my car causes those two lights to show up. For what it’s worth.

996.2matt

yes I replace the Maf sensor yesterday and it didn’t help the issue at all

allcool

Any actual trouble codes to go by..?

hardtailer

Your Monitoring O2 sensors that normally sit downstream of the cats see O2 levels that had there still been cats installed would indicate a too lean mixture. With cats that oxygen would have been used inside fhe cats to convert HC --> H2O (= water vapour) + CO (and that to CO2) and all would be fine. Since you dont have cats, this ixygen is not used but since the ecu has no means of taking this fact into account, it richens the mixture to reduce O2 levels in the exhaust gases. Your mixture is now so rich, that it doesn't ignite through spark inside the cilinder (therefore no power but bogging down of the car) but needs the heat of the exhaust to burn and that's why you hear the bangs and see the flames.
Engine no longer responses to control inputs as would be required by PSM so it throws the Amber light and ABS for extra good measure too.
So put the cats back and enjoy the car instead of whatever you're trying to accomplish without cats.

Error codes were developed to diagnose problems on a stock engine not for the purpose of trouble shooting a modified one, so chances are slim that they are relevant to the actual cause: the missing cats...

996.2matt

I cant put stock cats back, my cats blew and an x pipe was much cheaper than new cats, i guess its just finally time to get a tune fore catless.

damage98MO

@hardtailer If you're running that rich won't you be washing the cylinders with fuel? If that's the case then oil change ASAP as you figure this out right? Or bore scoring?

e30rapidic

Get some mini cats; they fit right into the O2 bung, then the O2 sensor threads into them. I have them on mine and have zero issues.

hardtailer

Most probably oil will be washed from the cilinder walls, yes.

996.2matt

yeah I w finally messed around with it a bit today, only codes were MAF hot film and misfires on every cylinder, so I replaced the Maf with a brand new one AGAIN and the code didn’t go away. My only thought is maybe it’s the throttle body sweeps??