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I've recently acquired a 2001 Corsa Milenium (a Brazillian saloon version of the Opel Corsa B, with a GM 8V 1.0 MPFI engine). It has a performance air filter (which I plan on replacing with the original filter soon) and a performance exhaust. I noticed on reductions, when I'm using engine braking or decelerating that there's some backfiring in the exhaust.

From some quick research I've found lots of candidate reasons for that, including ignition timing being a little off, valves adjustment slightly opened, amongst others. But I also read that "Decel Pop" is a known issue with performance exhausts1,2, since it's natural that some fuel mixture will come out in the exhaustion during a deceleration, but with a performance exhaust more air reaches the inside of the exhaust pipe and the rich mixture makes it burn and make that poping sound.

Questions are:

  • Is it really normal that some fuel comes out during the exhaustion step?
  • Should I be worried about exhaust backfiring on deceleration considering the car has a performance exhaust installed?

[1] http://www.thunder-max.com/techdocs/DecelPop1.pdf

[2] http://www.allthingsmoto.com/forums/f-13/deceleration-popping-explained-10/ (This considers a motorcycle with a carburator, but I believe it's an issue with electronic injection cars as well?)

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Is it really normal that some fuel comes out during the exhaustion step?

It's quite common - the unburnt fuel has to go somewhere and the exhaust is the natural route. This will be happening even with standard exhausts - just not quite as much and not as loud. It can be tuned out (to an extent) with adjustment to both ignition timing and injector managment but pretty much every four-stroke engine is going to punt some fuel into the exhaust on overrun.

Should I be worried about exhaust backfiring on deceleration considering the car has a performance exhaust installed?

The performance exhaust is likely a big contributor to the popping happening - as you correctly mention in the question the more air can get into the exhaust the more likely you are to get pops and the louder they will be. Performance exhausts usually are freer-flowing than their stock brethren and that means more air.

Another possibility is that there could be an leak in the exhaust system which is letting air in at the leak under deceleration - particularly if the popping sound is coming from somewhere other than the exhaust exit. So I would investigate this as a exhaust leaks only ever get worse.

Overall it's not typically something to worry about - if it's at excessive levels there's a possibility of damage to the catalytic converter (if the exhaust has one!) but otherwise if it's not annoying you it's not doing any real harm, my car (stock exhaust, complete with cats) will do it if I've got the engine/gearbox in their most aggressive settings and I've been..ahem.. enjoying the performance. To be honest my inner five-year old actually quite likes it!

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