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

I suspected as much, thanks for the clarification Steve. Just one more reason for me to get my order in to Steve. :)

I just have to get the energy to take my seat out to see if I have a 24 or 28 pin chip.

--Bill

...now I'm confused...on a CIS 911, I had always heard that popping through intake means lean (and is bad), but popping through exhaust (assuming no leaks) means a little rich (not a problem). Which is it????

Very good question?

Ya still gots to verify "NO LEAKS".....if there is no change on your intake or mixture or anything else and you suddenly get popping on decel.....assume leaks in the exhaust system.

Eliminate the obvious......

I'll check for leaks next, and go from there.

Thanks

In most real world applications the cause will be a lean condition ...from a a whole lot of of diff conditions and most will not be harmful to the lil recip eng. You just do not want lean in the power band....that would be a no no

I will not try to argue with Steve, especially about computerized cars.

But..... on my CIS engine, leaning the mixture out reduces the popping in the exhaust, richening it up makes it worse.
My car is not far out of tune either, it is very nearly optimal.

My experience was just like sammy's - had popping through intake and adjusted mixture screw (turned to right to richen) and cleared it up, but went a little to far and had popping through exhaust - a little left turn of the adjustment screw and no more popping through exhaust. Thus my conclusion that popping through exhaust (with no /other problems) meant a little rich.

I had this trouble with an early car I purchased, 2.7L with weber's, sport exhaust... during deceleration the car would roar off a 'boom' and scare the hell our of me, any one with me and anyone else within a 100 ft... no amount of adjustment would help this thing. Finally I had a leak down test preformed and found the trouble... the engine heads were leaking all over the place... shoty machine work... Reworking the heads would have fixed this problem... I elected to have a total rebuild preformed.

In most instances it's almost always a lean AFR that causes this (subtle popping) not rich. It's most notable on high RPM overrun conditions especially when going downhill in gear with foot off gas. It's during this overrun that the AFR can go high enough that one can hear it. But on level street one won't (different engine condition).

I can vouch haven seen it on the LM-2. Once I set the carbs for (LBI ~13.0) if I coast down a hill at say 40mph in 3rd gear (high RPM) foot off throttle the LM-2 shows lean 14-low 15's. Which is normal. Attempt the same thing on level street and the AFR won't go high enough...different engine condition.

Could only hear it or get it to be heard on downhill high RPM condition. This also lead me strongly believe it wasn't an exhaust leak. Since exhaust leaks are also heard during many other driving conditions (heavy accel & shifting points) and are almost always loud.

This lean condition (LM-2) while lean still shows fuel flowing thru the engine. But since the lean mix burns slowly it requires more timing. But given my foot is off the throttle timing is too retarded so the small amount of fuel makes its way into the exhaust (collects until next cycle) where it's heard out the tailpipe (pop, pop, pop, pop) a bit like popcorn being popped. No harm no foul.

I'm also running a Bosch SVDA but since it's connected to ported vacuum as it should be... there is no added advance because of the closed throttle = no vacuum. Hence...no additional advance to help with the lean burn. Timing at idle is too retarded to burn the lean mix fuel.

Initially I thought it was an exhaust leak but I soon confirmed that it wasn't after crawling under and not finding any tell tale signs of an exhaust leak.

An exhaust leak will almost always be accompanied with a loud bang or boom due to an excessive rich mix coupled with an exhaust leak/s. They (exhaust leak/s) would also be readily apparent during other driving conditions aside from just on the overrun. Such as during acceleration & shifting points.

VW's & older Porsches are fairly common to this condition (lean popping) they (VW/Porsche) would come up with ways in hopes of reducing it by adding devices such as the following:

Throttle Positioner
Dashpot
Gulp Valve
Decel Valve


But for me none of the above devices apply because I have none of them. I'm running dual Weber 40 IDF's. The above devices generally allow for more air to be passed to bring the AFR up very high (20:1 or higher) to the point where there is no O2...no O2 no way for the fuel to go pop, pop, pop.


1972 Westy Hardtop/Type-4 2056cc
96mm Biral AA P/C's~7.8:1CR
Headflow Masters New AMC 42x36mm heads w/Porsche swivel adjusters
71mm Stroke
73 Web Cam w/Web solids
Dual 40mm Webers (50 idles/125 mains/180 air corr./F11 tubes/28mm Vents - Float height 10.45mm/Drop 32mm
Bosch SVDA w/Pertronix module
Pertronix Flamethrower coil
S&S 4-1 w/Walker QP 17862
3 rib 002 Trans

The exhaust mixture is lean. Check you AFR's and if OK, then don't worry about it.

Learn to read the post.......
 

Are you guys know how to read dates? This post was made 9 years ago!!!!! Where have you been all these years?

Tony

I have GOT to start looking at the thread start date. Now Steve is going to wonder what the hell I'm talking about when I pm'd him about my 3.2 popping under deceleration with a cat delete tube.

I guess I could have "hijacked" this thread since it's older than dirt and it wouldn't have mattered.

The worst part about this thread is the OP never posted if they resolved their problem and how. That drives me crazier (not crazy, I'm already there)

The date is irrelevant...it isn't a 'closed' topic. No reason for a new thread when this one exists.

Good threads never die.

Make sure this vacuum hose is connected. It's purpose is to limit the amount of manifold vacuum when decelerating to avoid backfiring/popping and, at the same time, reduce emissions. Excessive maniflold vacuum means insufficient oxygen present to support combustion during deceleration resulting in unburnt fuel leaving the combustion chamber igniting in your red hot exhaust system...pow sputter bing bang boom!

The later DME controlled Carreras have an even better strategy. Fuel delivery is totally interrupted during engine overrun, preventing any possibility of sputter and backfire altogether regardless of how lean or rich they happen to be tuned or even if vacuum leaks are present for that matter.

http://forums.pelicanparts.com/uploa...1390528313.jpg

Cheers,
Joe
82 Targa

sub'd

will have a look at the vacuum hose as in above pic

I too have popping on deceleration after installing cat bypass

The pop is awesome. I would never change it ' my fabspeed is like its engineered to kick ass,
Always perfect , just rev it pop the clutch at 4 or 5 k and whammy ". In the Baltimore tunnel
Its priceless . It's like a bomb went off.

Damn Steve just brought logic into the picture ". Those genius guys can be such a buzz kill