Problem w/ deceleration backfiring

[Kurt B]

This happens pretty sporadically. No Cat and bursch etc are likely the cause, but is there anything I can do about it?

It doesn't happen often, but it does happen after hitting very high RPMS and letting off the gas to shift. A nasty BOOOM that scares the shi!t out of people.

If anyone knows how I can tone it down a bit without putting everything back on, I'm open to ideas. Any other causes?
O2 sensor anything related to it?

[911nut]

I have a bypass pipe and a sport muffler also and backfires too but it's a mild popping rather than a big boom. Have you checked your mixture and is your O2 sensor functioning?

Bruce Anderson writing in the "911 Performance Handbook" stated that the only way he found to eliminate backfiring on throttle closure was to add a premuffler in place of the bypass pipe. Sounds like you have a CIS/mixture related problem though.

[nhromyak]

In all my other cars, this hi rpm decel pop or boom was due to the mixture being too rich, and/or a leak in the exhaust.

Good Luck.

[Superman]

My mechanic freinds tell me exhaust leaks are the leading cause of backfiring, by far. I did not find exhaust leaks when I disassembled my engine, but I did find that about half the heads were not sealing well on the cylinder tops, so maybe this was the cause of my popping.

If what you get is a loud boom, then I'd suspect that your ignition system is shutting off briefly. Seriously. Ignition shorts are about the only cause I've ever seem for BOOMing. coasting down a hill in a carbureted car, if you turn the key off for a moment and then turn it back on, you'dd get exactly the same symptom. BOOM!

[makaio]

Well I'm positive it can't be a CIS mixture problem, since you have DME.

I leaning towards exhaust leak.

[nhromyak]

DME's can be adjusted just like CIS... FYI

[911nut]

Originally posted by makaio
Well I'm positive it can't be a CIS mixture problem, since you have DME.

I leaning towards exhaust leak.

Ooops! Yup, ain't no CIS in that one!

[70flatsix]

I had the same problem with deceleration backfiring. My kids loved it! Traced problem down to leaking/rusted heat exchangers. Temporarily patched hole(s) in the heat exchangers and backfiring stopped. I'm looking for good used set of heat exchangers to fit a 70 911E with original mfi if anyone has a set looking for a good home!!

[beepbeep]

Deccel' popping is sweet! :-)

I have 930 and it pops beacuse of sport-exhaust. It will happend on avery car with little muffling. Check deccel-switch tough...it might be busted if you hear loud bangs.

[Kurt B]

BeepBeep: decel popping is sweet. I have that, but what I'm talking about is BOOM.
I was racing a 'vette the other day and I really punched it straigh up the revs, and when I shifted the BOOM was so loud, it frightened the hell out of this pedestrian.
That's when I knew it was a problem...it went BOOOM, and the guy who was walking the other way, jumped around as though he'd heard a plane crash. So that part of it, I don't want

[beepbeep]

Ooopps....you are dumping too much fuel into engine on deccel...

CIS? Decell-switch is first one to check...

Do you mean boom like when rally cars shift gear?

Mine does that occasionally when I accelerate very hard and shift up. Just as I let off the gas pedal and shift gear a loud and quick bang comes from the rear. I think it's unburned fuel igniting in the exhaust pipe.

It never caughs, burple, hickup or pop when I lift off the gas pedal from high rpms. Always pure and clean sound.

[Kurt B]

It's a very clean sound...don't get me wrong. But it is loud at times, and I mean loud. Just one BOOOOOOOOOM.

The car will out accelerate about anything I come across in the roads, so I know it's running tough. But the chip, lack of cat, bursch etc...you can just imagine.
However, I don't want to give some poor old soul a coronary!

There was no hint of it before the cat-pipe.

I suppose it could be leaking out of the o2 sensor port. Maybe I should jbweld that thing closed and drop the o2 sensor...and with the speed at which exhaust gasses go through that pipe (which must be faster than they did through the combed cat), I imagine the sensor can't function as well anyway...

[Superman]

Kurt, I don't think you can just ditch the O2 sensor on a Motronic car. According to what I've heard.

Now, if it were an SC......

[Kurt B]

Unfortunately, I have no way of testing whether it works without an exhaust gas analyzer. How do I test it?

Will the idle change...so I shove my nose in the exhaust pipe or what?

[nhromyak]

When I disconnect my 02 sensor, I noticed my RPMS go to exactly 950 rpms. With it connected my rpms go to ~ 800 rpms.

[tobluforu]

If your getting a very loud BOOM! Then it's probably what most have said and that is it's running way rich. When i was 16 my friend had a modified vw w\ dual exhaust. He would roll up to bus stops, put it neutral, turn key off, pump gas pedal a few times and just click the ignition while pumping gas pedal one last time. The back fire it produced was increidible. Ruined the exhaust after a while. Ahhh, the good old day's.

[Brian993]

Kurt,

I am sure this problem is bugging you, but the thought of the poor pedestrian is cracking me up. Sure would have liked to see that scene. I can imagine it though.

[Eric Coffey]

Kurt-

Like Superman suggested, I would start looking for exhaust leaks. Also, it is not widely known, but another common cause of decel backfiring is a faulty fuel pressure regulator.

[Superman]

Exhaust leaks cause popping, no doubt. But the BOOMs are caused by internittent ignition. Notice that the VW trick involved shutting off the ignition.

I doubt that it's a rich running problem. I think ignition is shutting off briefly.