What causes a car to backfire underdeceleration

Thats nature of glasspacks, and alot of aftermarket exhaust systems.

As long as its nothing more than the exhaust setup, its no worries. You can easily tell the difference between it and an actual problem most of the time.

Unburnt fuel. My Cobra gurgles and pops a lot under deceleration and I think it sounds pretty cool.

Yes unburnt fuel is right, Before I had my car tuned my exhaust would pop, crackle, etc, on deceleration. After the tune It doesn't crack one little bit. I was pretty amazed. I got better MPG, Faster, and It cleared up my idle problem and the cracking problem.

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.

If it shoots flames, I would leave it alone.

As others said, some popping and gurgling is expected and can sound bad ass on downshifts. Ford seemed to tune it out of the 05+ mustangs, because I've never heard my dad's 06GT gurgle and pop on decel. A bit disappointing really.

If you get a REAL backfire, it will sound like a gunshot and scare the crap out of you. The head on my 4cyl was cracked and dumping oil into the cylinder after letting off from boost. On more than one occasion, I've gotten a gun shot style backfire, and I have a near stock quiet exhaust system too! I can't imagine if it were to happen when the cutout is open.

You say its not idling great either? You might have a vacuum leak.

Sometimes an exhaust leak will make them pop also.