AKrlic

What could possibly cause a car to start backfiring when shifting at high RPM? It has never done this in the four years that I've owned the car, and even now it doesn't do it regularly. However, on two or three occasions in the last couple of weeks, I've noticed a slight *pop* from the exhaust pipe when upshifting during hard acceleration.

My friend's K04'd B5 S4 does this occasionally, too, and the tuning shop told him that it was unburned fuel in the exhaust quickly burning off/combusting (causing the pop), due to a momentary richness from getting off the throttle quickly... Or something like that. Makes sense to me. But how does this start happening out of nowhere? I haven't made any changes to my motor set-up.

Any comments/suggestions/theories?

UrS4boy

Is your aftermarket bypass valve functioning properly?

AKrlic

But in the end I decided (erroneously?) that it couldn't possibly be the BPV, unless it was slowly coming apart... But being that it has no more than 5K miles on it, I couldn't see why it'd be failing already.

kinderutz

aftermarket or stock unit?

UrS4boy

You need to check it to make sure it is functioning correctly.

UrS4boy

Read his sig. Aftermarket.

kinderutz

aha....hey i got the same one! read my sig! read my sig!

AKrlic

So let me guess... You think it might be "leaking" air that was metered and should've either found its way to the intake manifold, or been recycled back into the airbox...?

What's strange is that I'm not noticing any loss of power or slowed throttle response or additional turbo lag or anything of that sort. (But I guess it could simply be in the beginning stages of failure..?)

Jason C

if those have a rubber diaphram, it may be torn

AKrlic

Ahhh. Comedy soon turns to tragedy....