Deceleration Popping or Backfiring-Cause
 

I have recently purchased my first Porsche, a 1983 911SC Cab. I am easily troubled by noises that I do not recognize or do not know why they occur. While decelerating down a hill, and using engine compression for braking, I hear a popping, or backfiring, out of the exhaust. What normally causes this noise and/or is it normal.

Thank you for response to this possibly stupid question........But, I will feel better once I know what this is.

it is running lean. Need to richen the mixture.

Don't worry about it too much, most older Porsches do it. It's when it's backfiring through the intake that you need to worry.

Check out the above suggestion.

Not a pcar expert, but actually, isn't it running rich? Above the upper explosive limit and the mixture leans out in the exhaust and detonated/lights off as it leans into the explosive range as it enters the exhaust system?

also could be a hole in exhaust upstream.......usually will never be cause for concern (the back fire) if not to obnoxious.
Is it a stock P sys. on it ??

I have copies of the service records back to when the car was 8 years old, and there is no record of replacing any portion of the Heat Exchangors or Exhaust. I think that the care is pretty much original. I have been looking at a SSI & sports muffler system?

Thanks for the replys!

Hole in the exhaust or leak in the flanges...

My SC is set rich and I've heard the popping (a backfire is a much more extreme event) ever since I gutted my muffler.
Many race cars do the same thing.
I don't think it's a big deal.

It sounds like a lean condition. Where? Who knows, could be a vacuum leak, hose off, gasket on the intake manifold or many other things.

Lean causes backfires through the intake, rich through the exhaust.
Check or replace all exhaust gaskets, then have the mixture checked.

Run the motor, wet your hands and pass it over the bottom of the engine feeling for a leak.....best to do so while car is on a rack....or take it to a muff shop.

Note that the big cooling fan exhausts out the bottom of the car (that's how it cools the head and cylinders) so there will be a lot of air flowing in that area.
-Chris

popita-pop-popop softly is normal and I like it! You might have an exhaust leak or be running a little rich. I probably have both. Better too rich than too lean. Do you have a working catalytic converter?

KAAPOOWW!! Is bad and can blow off exhaust hardware.

Isn't deacceleration poping a result of the CIS not having a fuel cut-off when closing the throttle?

Cause: Too much fuel on overrun (exhaust leak and/or gutted muffler will amplify it)

The beauty of the internet. Guy asks a seemingly simple question, and gets 3 or 4 different responses--2 of them totally opposite. FWIW, I vote for too rich. My 930 does it, too. I love it, as it makes the car sound all snarly and mean. And minor popping on decel is not supposed to be harmful.

Probably what MIKEZ posted is cause for the backfire.

I just added a Fabspeed cat-bypass and Fabspeed muffler last week, installed by John Walker.

I get a slight burbling and small popping on deceleration, sounds great and healthy. However if i am going really slow or stoped and I rev it up a few times it will POP on the deceleration portion and it sounds to a regular person like an out of tune car.

I am guessing this is normal and there is unspent fuel igniting in the cat area that now you can hear clearly. I am going in for a new O2 sensor in a couple of weeks and will have John check for any leaks. I dont see any when i start the car in the morning.

I have not ordered my SW chip yet, would the re-mapping help in excess fuel burn-off?

Popping on deceleration, if not from intake or exhaust leaks, is usually caused by a lean part throttle condition just prior to letting off the gas pedal. Removing exhaust restrictions such as by removing the cat, and installing premufflers and sport mufflers create the lean condition. The lean mixture has a difficult time igniting in combustion chamber, so the unburnt fuel goes out into the exhaust manifolds. On the next cycle, the same condition repeats, and eventually enough raw fuel accumulates in the exhaust to ignite, and thus the backfiring and popping. Remapping additional fuel in the lower rpm part throttle range typically alleviates the condition.

For example, if one were to evaluate the factory mapping for the European spec 911s, which have the lower restriction premuffler exhaust, and don't have the exhaust popping, you can see Porsche did the same thing to the fuel curves.

Yup