Does anyone know how to fix a backfire problem which happens on deceleration?

 

I take it that you do not have the stock exhaust? Perhaps no cats at all and an ECU mod?

 

Fix it? Why?!?!? It sounds amazing!!!

 

Sounds like you are running extremely lean.

 

Exhaust is stock. I think there may be unburnt fuel getting into the exhaust that cause a "pop" sound. Will cleaning the injectors help?

 

Yeah I would get your car tuned, and if its stock bring it to the dealer. On the older 911 turbos like the 930s this is common. And yes your car is spitting out unburnt fuel.

 

Alot of turbo cars do this because of all the heat (exhaust manifolds and turbine housing get quite hot when under load) and to keep EGTs down turbo cars like to run rich, so in decel the EGTs are high to the car adds more fuel, which some is ignited down the exhaust path sometimes. This is why with alot of turbo cars with no cats (mine for instance) will shoot flames under WOT when I let of the gas to shift.

 

If you are backfiring through the exhaust with throttle closed and it is a fuel probem, you are most likely running lean, A backfire through the intake would indicate a rich condition. What happens is that the fuel/air mixture in the cylinder(s) is so lean that all the fuel does not burn coimpletely and is expelled into the pipes where it ignites. If you were running rich, you should experience bogg down at WOT.

However, look at your engine temp and timing too. Excessive heat can retard the timing too much and cause backfire. Have you change fuel suppliers lately?

 

What type, I assume its a turbo. Popping on decel, would indicate
an exhaust leak, or a leaking injector. Lean misfire generally pops through the intake, or a bent/burnt intake valve. Have not encountered, intake popping since racing carbed 911's. Good luck on this. Jayster.

 

With all due respect, you've got it backwards.

Backfiring in the exhaust is caused by over-rich conditions. When the mixture is too rich, not all the fuel gets burned. So, as it enters the exhaust tract, the excess fuel ignites, causing the "explosion", or backfire.

 

Beat me to it.

 

How would back firing up the intake happen (do to lean condition)?

 

Well, some say is normal and some say is not. I think the bottomline is will the popping or backfiring damage the engine.

My friends said that this may be caused by a clogged fuel injector and suggested me to use some injector cleaner which they think will fix the problem. Any views?

 

It's not really "bad" in that sense, but it does mean that you are probably running rich for one reason or another and not getting the full power out of your 911 turbo. The "explosion" takes place after the turbos inside of the exhaust. Your engine is fine. A clogged fuel injector might make you run lean, but not rich.

Do you have the stock ECU or did you modify it? If everything is stock, then you might have a leaking diverter valve or some other component of your system. The reason why I propose this issue (if everything is stock) is because your MAF measures air as it enters the system. If some of that air is lost (leaking diverter valve or hose), then the ECU will squirt in an appropriate amount of fuel based on what it measured. However, air has been lost and the ECU is not aware of that. That will produce a rich condition, especially between shifts. It could be any number of things, honestly.

Are you at stock boost? If so, how does your boost look in 3rd/4th gear at WOT? What are the peak hold values? What does it fall down to? If you are not a stock boost, what was done to your ECU?

I run a stock ECU and a modified exhaust. I get slight popping at times, but nothing major. By default, I would guess that Porsche tries to run the turbo on the rich side. That is typical for most turbo cars, honestly. It protects (the best it can) against detonation by doing so.

 

Thanks for being respectful, but I DO NOT stand corrected. It is a LEAN condition that can cause a closed throttle exhaust backfire and NOT a rich condition. Think about what happens when the throttle is slammed closed while at high rpms. Fuel is cut off and the vacuum in the engine jumps up. Since the valves are still opening and closing, air is still being sucked into the cylinders from any intake system leak there might be. Now, I ask you, how can a rich condition exist?

Next, a lean mixture will burn eratically and unpredictable. Most often it does not burn comletely and the unburned fuel is expelled past the exhaust valves into the pipes. What can cause the "pop" is that some of the lean mixture will begin igniting and pass by the exhaust valve while still burning and ignite the fuel that is in the pipes. Extreme exhaust temps can also cause this lean mixture to expode inside the pipes.

The more fuel there is to air, the more BURN there will be. The more air there is to fuel, the MORE explosion there will be. An over rich exhaust will burn (flames) if conditions are right but will not cause backfire or a "pop" with throttle closed on deceleration.

Do not confuse a closed throttle exhaust backfire with a WOT backfire. The causes are very different. You can not have a rich condition when the throttle is closed.

I suggest you look closely for leaks on the intake system. If the backfire IS fuel related, chances are the problem lies there.