I've got a 2008 GT, automatic, 23k miles, fresh plugs with the Bullitt CAI, FRPP tune, and FR500S mufflers. About a month after I installed the tune, it started to backfire at WOT (or from a slow roll) when it makes the 1st gear to 2nd gear shift.

Any idea what it could be?

 

If you can tell where it's coming from this may help

http://www.2carpros.com/articles/wha...ngine-backfire

 

Backfire or just a louder pop like the KR's do from the unburned fuel?

 

cdynaco:

It does sound like a pop. Tell me about this with the KR's... I had never heard of such a problem/issue of them.

 

I have the same mufflers as the KR, for the record.

 

They typically pop on decel much more than other cans. I really wanted them because although I love the Bullitt tone from the GTB's, they are subdued. The KR's keep the tone the same but a notch louder at idle and really sing at throttle. The GTB's pop a bit, but the KR's quite a bit more. Too much for my taste. My F150 pops coming down the pass - but that's due to an exhaust leak. The KR's would be popping all the way down the pass.

If the FRPP tune is like other aftermarket tunes, they prevent/reduce that 'run on' that the stock tune has to meet emissions. You know, you rev and let off but the rpm's come down a bit slow. But with a tune, when the throttle snaps shut the rpm's come down promptly, leaving unburned fuel in the system for a bit and due to the temp of the exhaust, it pops till that fuel is gone.

And IIRC, the FRPP tune is a bit richer than BAMA or other tunes to avoid lean conditions so it probably compounds it a bit.

 

Right. KR's, M-5230-S, FR500S - all the same can but 3 names for some reason. Like I say, I really wanted them so I've learned the names.

 

Thanks but it pops on acceleration. I've called FRPP Tech Line and of course they've never heard of this before.

 

Yeah I can't explain that. But you do get popping on decel right? Like when coming off a freeway ramp to a stop...

Plus when you let off to shift 1-2 as you first explained, would have a brief closed throttle condition followed by an immediate WOT fuel dump.

Have you tried reloading the stock tune, driving a bit, then unhooking the battery to reset the ECU? Then reload the FRPP tune. Maybe the ECU has "learned" something that's a bit off from how you're driving it now.

 

No popping on deceleration. I don't let-off on the shift - it's an automatic. But I'm sure it does cut out slightly. I may click the "Octane Adjust" in the Procal tuner to dial back some timing. Maybe that'll do it.

 

Oh - totally missed it was an auto.

That explains why no popping on decel (unless you manually have it in a lower gear for engine braking).

 

Just thought of something. Maybe it is just the mufflers making a popping sound at the shift. There's no soot on the inside of the tips and with the high polish, you'd likely notice. Am I crazy?

 

I've never heard anyone reporting soot in the tips with the KR's. They are the same as my Bullitt tips and I have no soot even though mine does pop a little on decel and start up.

I don't know enough about the fuel mapping for an auto. In the old days you floor it and the tranny shifts without affecting throttle. With all the new computer stuff perhaps there is a brief change in fuel load that creates a momentary rich condition in the exhaust system.

Popping is harmless and nowhere near as loud as a backfire.