What causes Back firing on deceleration

PK 77 911 · PK 77 911's Garage

Like when coasting down a hill in first gear? Tried a search but did not find anything

echrisconnor · echrisconnor's Garage

Too rich. Unburnt fuel is getting into the exhaust and igniting.

Blanco · 05-13-2005, 11:37 AM

Is this like a big backfire or more of a rumble like an 18 wheeler jake brake type

FrayAdjacent911 · 05-13-2005, 11:43 AM

My 911 exhibits the same type of thing... some backfiring when decelerating in gear.

I think it's something to do with the distributor/points.

Once I shift into neutral, it goes to idle and idles normally.

Oh, and mine's more like a gurgle-gurgle-pop-gurgle-pop-pop... kinda like the 18 wheeler thing. Not real loud backfires, but clearly audible 'pop's.

Ron.G · 05-13-2005, 11:54 AM

I do not think at has anything to do with your points.
Sounds like fuel mixture as stated above.

When you decell in gear your pulling a vacume so you are getting some additional fuel. If your mixture is rich you are sending unburned gas through the hot exhaust causing the sputter/popping thing.

FrayAdjacent911 · 05-13-2005, 12:25 PM

Ron, that makes sense. I think my engine is running a little on the rich side. I have not gotten to (or even figured out) the mix adjustment.

I do know the distributor in my car is not the correct one, so the cap and rotor I got are not the correct ones either. Apparently a PO put a Bosch distributor from a slightly older engine on it. It's got a two peice points setup. I ordered the two peice points, and what I think are the correct cap and rotor for it.

ruf-porsche · ruf-porsche's Garage

leaks in the exhaust system also contribute to backfiring.

wholberg · wholberg's Garage

Quote:

Originally posted by ruf-porsche
leaks in the exhaust system also contribute to backfiring.

A leak allows air to get sucked into the exhaust. With more oxygen available, unburnt gas can ignite and burn better.

An engine does not have to run rich to backfire under heavy enging deceleration.

FrayAdjacent911 · 05-13-2005, 12:35 PM

Well, the exhaust leaks would make sense. The undercarriage looks pretty... well.... WORN on mine. Eventually she'll get new heat exchangers and exhaust/muffler stuff.

PK 77 911 · PK 77 911's Garage

I coasted to a stop in gear the other day and a guy thought I was doing a drive by. He gave me a pretty funny look. I does not happen all the time.

Joe Bob · Joe Bob's Garage

Quote:

Originally posted by ruf-porsche
leaks in the exhaust system also contribute to backfiring.

Yeah..what he said...

sammyg2 · 05-13-2005, 01:15 PM

On mine I went all the way through the exhaust, all new gaskets etc and found no leaks. It was like the jake brake thing, not loud popping, just rumbling.
Since the repair to the lamda sensor fixed the noise (or at least compensated for whatever was wrong) I'd have to say mine was mixture related.

rjay · 05-13-2005, 01:45 PM

Quote:

Originally posted by Ron.G
I do not think at has anything to do with your points.
Sounds like fuel mixture as stated above.

When you decell in gear your pulling a vacume so you are getting some additional fuel. If your mixture is rich you are sending unburned gas through the hot exhaust causing the sputter/popping thing.

All true, but it is also possbile that if the timing is wrong or the spark is diminished then you can have an excessive amount of unburned fuel in the exhaust without the mixture being generically too rich. In the olden daze before CD and optical ignitions we used to have to deal with this stuff all the time.

deoxford · 05-13-2005, 03:03 PM

Adjust your micro switch.

jstobo · 05-13-2005, 07:25 PM

There is another thread that asks the same questions. Your mixture may be correc t and it will still happen. It happens because there is incomplete combustion in the cylinders on overrun conditions. Overrun condition is when the RPM is high but you have your foot off the accelerator(throttle closed). The microswitch on the early MFI and Zenith carb cars activated a solenoid to stop this when the throttle is closed and RPM s are aboce 1500 RPM for MFI and 1350 for the Zeniths. Under these RPM limits the RPM tranducer(speed switch) deactivates the solenoid and the car works normally. TO stop the gurgle(backfires) put in your clutch and the RPM will drop and the overrun condition stops. On Zenith cars the solenoid actually adds fuel to increase combustion to stop the backfires. Hope this helps.

JIM

masraum · 05-13-2005, 08:31 PM

2 things, exhaust leaks, which in my experience make a few big backfires, and gas in the exhaust which is usually the popping and gurgling that some cars exhibit.

the first should be fixed

the second is normal and expected for most older cars like SC's and older, but is not normal for 3.2L cars and newer.

jaydubya · 05-14-2005, 04:44 AM

Another data point - after installing SSIs and Mony muffler this now happens (gurgle gurgle pop pop [small pops]). No exhaust leaks. I chalked this up to something different between the new exhaust and the old exhaust since every SC (and even Carrera) that I have seen/heard with SSIs or B&B exhaust does it. Especially noticable at autox.

I kind of like it

masraum · 05-14-2005, 06:31 AM

Quote:

Originally posted by jaydubya
Another data point - after installing SSIs and Mony muffler this now happens (gurgle gurgle pop pop [small pops]). No exhaust leaks. I chalked this up to something different between the new exhaust and the old exhaust since every SC (and even Carrera) that I have seen/heard with SSIs or B&B exhaust does it. Especially noticable at autox.

I kind of like it

With the stock exhaust you normally can't hear it, but it was there, just very muffled. What you are now hearing is because of reason 1 which is normal.