Exhaust backfire at speed in stock GXP?

[Eidolon]

When letting off the gas in most any gear, as the engine winds down, the exhaust tends to pop, resulting in what sounds like thuds against the rear suspension of the car. When I first heard it, that's what I thought was happening. But it's just the exhaust system.

It sounds to me like there's unburnt gas getting into the manifold just past the valves and it ignites there, resulting in the rumbling pop I'm hearing. I'm guessing the engine is running a bit rich, as factory tunes are wont to do.

Just curious if any of you other GXP owners out there have experienced this. Is this normal for stock? Think the dealer would be able to do bupkus? Or would the only way to get rid of this be a tune?

Yeah, the exhaust rumble and pop isn't entirely unpleasant to the ear, but when it does it this much, I worry for the exhaust system. It also means that's wasted fuel that I paid for!

[Devilish34]

The gurgle sound??? If so I think it's normal

[Eidolon]

The gurgle sound??? If so I think it's normal

"Gurgle" would be a good way to describe it. I see by your sig have a tuned/modded GT. Are you getting the same thing?

[99-LS1-SS]

Not a gurgle sound. It's actually a popping, like a back fire almost. Mine does it too. I don't like the way it sounds but it did it before the tune and after the tune.

[Eidolon]

Not a gurgle sound. It's actually a popping, like a back fire almost. Mine does it too. I don't like the way it sounds but it did it before the tune and after the tune.

We're gonna have to come up with a noise glossary for the car! It's definitely a backfire of some sort.

AND you've got the ARH LT Headers, which I was hoping would help! Oh well. I guess it's unavoidable with cylinders and valves that move as much air as these do.

[Devilish34]

gurgle slight popping sound been doing it since day one

[chuckwi11]

GXP here... VMS tune and VR CAI. Was fairly subtle without mods but it is pronounced now. Wouldn't call it a pop, it's more like a rumble in my car... definitely exhaust and gets people's attention..

[norm8332]

I get it while decelerating. It's not loud. I like it. I can even control it to a certain extent by giving it a tiny bit of gas then pop..pop. I do it for pedestrians sometimes.

[R.Penguin]

When letting off the gas in most any gear, as the engine winds down, the exhaust tends to pop, resulting in what sounds like thuds against the rear suspension of the car. When I first heard it, that's what I thought was happening. But it's just the exhaust system.

It sounds to me like there's unburnt gas getting into the manifold just past the valves and it ignites there, resulting in the rumbling pop I'm hearing. I'm guessing the engine is running a bit rich, as factory tunes are wont to do.

Just curious if any of you other GXP owners out there have experienced this. Is this normal for stock? Think the dealer would be able to do bupkus? Or would the only way to get rid of this be a tune?

Yeah, the exhaust rumble and pop isn't entirely unpleasant to the ear, but when it does it this much, I worry for the exhaust system. It also means that's wasted fuel that I paid for!
Learn to love it! It is exhaust music! :p


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Deceleration Backfire is caused by fuel burning in the exhaust manifold or header.

No ifs ands or buts, that’s what causes it. But the bigger question is how does gas get there in the first place, and that’s a bit more complicated. Generally, there are a variety of ways it gets there, and a variety of things that can make the backfiring worse. But there’s a kicker, and something you should understand before we go any farther:

A motor in perfect tune will exhibit deceleration backfiring.

Therefore, just because your motor is banging it up, doesn’t mean there’s anything wrong. And consequently:

Getting rid of the noise means de-tuning your motor.

Yup. If you’ve jut got to eliminate that popping, you’ll have to accept the fact that your motor is going to be forced to run rich to do it, and that isn’t necessarily a good thing.

Ok, so you’re driving along at some given rpm, and suddenly you decide to decelerate, and you reduce the amount of throttle. This causes an “overrun” – that is, the motors rpm is turning faster than the fuel provided can support, so the motor begins to spool down. This causes a couple of things to happen.

First, when you close the throttle, you are also closing the throttle plate. This reduces the air and fuel flowing into the motor, and increases the vacuum (lowers the pressure). This results in less air and fuel in the cylinder during the power stroke, which in turn results in a lower pressure in the combustion chamber. The A/F mix burns faster in proportion to the pressure applied? Well, when we reduce pressure this way, the mix burns slower. This results in two things happening.

1. The lower burning fuel generates less heat, and the cooling effect of the non-burning fuel tends to “quench” the flame front, or slow it down even further. Because the mix is burning much slower, the exhaust valve can open before all the fuel is consumed, and the unburnt fuel is ejected into the exhaust.
2. The engine designers, in order to promote smoother idling and better combustion, retard the spark when the throttle is shut, and this results in the mix being lit later.

So, now we end up with unburnt fuel in the exhaust, and burning fuel being ejected into the exhaust, and bang! Backfire.

So the bottom line, is: That backfiring is perfectly normal and expected. If you’ve just got get rid of it, that’s up to you. You’re entitled to set your motor up the way you want, and your goals are your goals. But don’t refer to it as “fixing” the popping. Rather, the correct way to think of it is “de-tuning a bit to get rid of the popping”.

You will get worse gas mileage by getting rid of it because you will have to run richer on decel inorder to raise the chamber pressure engough to burn all the fuel.

[Eidolon]

So the bottom line, is: That backfiring is perfectly normal and expected. If you’ve just got get rid of it, that’s up to you. You’re entitled to set your motor up the way you want, and your goals are your goals. But don’t refer to it as “fixing” the popping. Rather, the correct way to think of it is “de-tuning a bit to get rid of the popping”.

You will get worse gas mileage by getting rid of it because you will have to run richer on decel inorder to raise the chamber pressure engough to burn all the fuel.

I don't mind the rumble and pop. I much prefer this car's exhaust character to the sedate stock GT. I just mind that it does it so frequently! If it's fully expected, though, then that answers my question. :) No way would I be willing to de-tune the engine to get rid of it.

From what I've heard, engines run richer from the factory, anyhoo. So a tune may actually increase the noise. I can live with that. :D

Now I just need to get some headers so it's a proper bark rather than a pop...

[Steve]

no to hijack but anyone hear of the term the 'after-fire' - someone got into a discussion saying that fuel in the exhaust 'after-fire' and fuel in the intake is 'back-fire'... wtf?!?

[kjmccart]

My 2009 GXP also does it, My buddy has a 2009 Corvette & he has the same sounds!

[sharkie]

Remember the movie "Uncle Buck"? That was back fire coupled with run on caused by fuel leaking on the intake side. The pops you hear on the GXP are caused from unspent fuel making it to the exhaust side. It's just unused potential energy. My GXP has always made some pops on deceleration, but it is more pronounced with headers and tune, especially when driving on hilly roads. It's just a reminder of the awesome power asleep under the hood when you baby it. My kids think they are hearing fireworks everywhere we go! Gotta love Independence Day!

[TyG8GXP]

At some highways speeds when I let off the gas I can hear slight pops. Nothing out of the ordinary.

[XXFire]

Sounds like sex to me... :)
Can't wait for Corsa, and hope to hear it louder !