daveHy

some advice please technical boffins... my euro spec 96 vario 993 has recently been treated to a set of DACH cat byepasses. Sounds great and more grunt at high revs but now has an annoying backfire on every upchange (when the change is over 2500rpm). Now I can handle a bit of burbling and popping on the overrun but this making me want to put the cats back on.
As far as I am aware if all is well within the motor then when you change up the fuel should be shut off instantly and there should be no unburnt fuel ending up in the exhuast which then ignites. Is something wrong / out of wack in the motor ? where do I start please ? injectors ? fuel reg ?
The car has a steve wong chip and runs on 98 octane. I have changed every gasket twice in the exhuast to make sure there is no leaks.
The winning answer wins a dive holiday on the great barrier reef and a night with the bar maid of there choice down the local pub....

black ice

Your cat bypass will exacerbate whatever condition you have. if it's that bad, there are a couple of things I could think of.
- all the timing in your Wong chip might do that - If you have the original you could put it back in to see if it gets better (can't remember if you can do that on a 96)
- i used to think it was all rich conditions that did that but someone told me a lean condition will give you a backfire out of the tailpipe - like an intake leak somewhere


dave

CalvinC4S

"The winning answer wins a dive holiday on the great barrier reef and a night with the bar maid of there choice down the local pub...."

So in on this.

Research the clearing of OBDII memory then drive the trips off, your car is used to more back pressure. It has some learning to do.

Edit: steve wong chip, car is lean.

David in LA

You could try to unplug the ECU and let it "relearn".