Loud Pop In Exhaust .. Misfire Or ?

[Wacky Dee]

Stock everything except, turbo back exhaust, EBC (12psi) and remapped factory ECU with safe tune (11.5 AFRs iirc) and new spark plugs gapped at 1mm (lower gaps caused constant misfire at idle)

Anyway, when I gun it to 4500rpm or so (usually don't go higher than that) there's a loud pop at gear change or as I take foot off the throttle as I'm about to shift. It sounds like it's coming from the exhaust.

What could this be ? Is this misfire or unburnt fule being dumped into exhaust and what would cause that .. or something else ? By the way, when I got my ECU remapped the timing was slightly off 17 degrees instead of 15 (IIRC) and the tuner only found out AFTER the ecu was remapped and said to leave it as-is now. What sort of effect would this have in general .. less power ? more fuel usage?

Edited December 2, 2008 by Delta Force

[.:: GimpS-R34 ::.]

yeah mine used to misfire on gear changes.. a tune fixed it..

[Wacky Dee]

yeah but the car's already been tuned .. ecu remap ($450) and I've only noticed this started happening lately.

A possible related issue .. about 3 months ago I noticed my fuel usage go up about 20% so I took it to Unigroup who said the engine was knocking slightly. This was a couple of months adter ecu remap (there was no knocking after remap) and Unigroup said that the increased fuel consumption was due to the ecu running richer to stop the knocking which may have been caused by a batch of bad fuel.

They fixed it apparently... not sure how, because the ECU is not programmable (it was remapped by another tuner). I'm pretty sure this loud banging started some time after Unigroup fixed the knocking

Any ideas?

[R31Nismoid]

11.5 is fairly rich, possibly at some points its going even richer which could easily explain it.

i'd get it back on a dyno and see what the AFR's are doing, could have changed due to a number of things

[Wacky Dee]

after the 'fix' I mentioned above they did dyno the car and the AFRs were fine and the performance was the same as after the ecu remap (about 190rwkw) What can cause AFRs to change ?

[R31Nismoid]

Anything.

Leaky injector, fuel reg on the way out, fuel pump on the way out, split hose somewhere even, fuel filter blocking up

there is more, and the above aren't necessarily related to you (like a dying pump is leaner), but you get the idea.

Either way 11.5 is beyond safe and you could bring it back a bit and that would most likely fix it