Deceleration Backfire

[Superman]

Okay, some of you know that I am curious about my local trusted mechanics' conclusion that backfiring on deceleraton almost HAS to be caused by an exhaust leak. I spooged (technical term) high-temp RTV on both sides of the final three exhaust gaskets on my car. These three gaskets touch the CAT. There is just one gasket, besides the ones against the heads, that I have not touched, and that is the one before the crossover pipe.

the popping and snapping on deceleration did not change at all. I see that an exhaust leak may still be present at the start of the crossover pipe, causing this. Okay.

But my feeling is that an exhaust leak at the head would spit all the time (like other intake manifold leaks I have had on other cars), so I am not ready to conclude that there is a leak there.

So, here's my curiosity: Do you think my broken head stud is allowing air to get in, causing the decel popping and snapping?

One more thing: occasionally (about 1/5 of the time) the engine makes a single loud SNAP just before it fires on startup. Do you feel this is related?

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'83 SC

[john70t]

I believe backfiring on decel is raw gas thats burning in the muffler.

I know on air injection systems, The "decel" vale diverts the air from the cat to atmosphere to keep extra oxygen from the mostly inert-gas exhaust stream(i.e. blowing up the cat) so I suppose an exhaust leak is possibly causing it. Diverter valves are known to fail often too.

On a different tokien, carburators leak and are notorious for drawing in gas on decel (high manifold pressure) so mabye the "air-bleed" circuit could be what to look at.(never mind-I forgot it was CIS.)

[john70t]

Worn valve guides can create a vacuum leak that doesnt burn completely in the cylinder also.

[Superman]

Thanks, John. My car has no air injection nonsense, so it's not that. I agree that air may be getting past the valve guides. They are worn.

My local mechanic said there can be leaks at exhaust port joints that do not cause acceleration spitting. That surprized me. I thought they would. He suggested plugging the exhaust pipe with my hand (most convenient when not hot) and listening. sometimes I miss the simplest troubleshooting techniques.

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'83 SC

[stlrj]

I may be wrong, but aren't SC's supposed to have a decel valve as part of the intake plumbing to prevent this sort of thing?

Joe

[Doug Zielke]

Ahhh...Superman's Obsession (ie:exhaust popping) rises, yet again, to the fore.

Hey Jim...Unless your car sounds like the 4th of July, it's normal! Especially if your cat is gutted or removed. The Decel valve also has an effect on this as well.

This "normal" bit of popping is caused by a momentary lean mixture as the throttle is lifted, along with a simultaneous high manifold vacuum.

And if you want to listen for *real* exhaust leaks, do as your wrench suggested, but use a wet rag, rather than your bare hand, over the tail-pipe.

(So when are we dropping that motor?)

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'81 SC Coupe (aka: "Blue Bomber")
Canada West Region PCA
The Blue Bomber's Website
"If it ain't broke...we'll help you fix it 'til it is!"

[Blackdimonds]

My SC was doing this after a rebuild last year, and it got worse this year. I ended up finding a vacuum leak from the Decel Valve back to the intake (it was leaning out too much under decel and was backfiring into the intake). Once I corrected this, the car runs like a dream, with no backfires.



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Paul
1981 911SC/RS
2001 Audi A4 1.8T Quattro

[Superman]

Thanks, dudes for th excellent info. I will be investigating my decel valve. My decel valve's vacuum line was plugged when I bought it. When I unplugged it, it caused my engine to return to idle WAY too slowly. The valve basically keeps the throttle open a bit so deceleration (of the engine only, not under load, such as when shifting) is slower. With the vacuum line plugged, the throttle is allowed to snap shut and idle is achieved quickly.

The reason I have not imagined that this valve is my problem is that it's job (when working) is to allow air, and fuel to get into the engine when the throttle should be closed. It is this air and gas that can cause the popping. If the throttle truly snaps shut, then only a teeny tiny amount of air and fuel (just enough to maintin idle) gets into the engine on deceleration. I will investigate further.

Doug, I will indeed implement your wet rag suggestion. Thanks. FYI, I am repairing the brakes on my junker truck because I am getting tired of thinking about the rebuild. I want to get started. We'll see, schedule wise, and I will certainly work with your schedule in advance. John Walker made some interesting remarks about the profound effect of porting the heads and switching to early SC intake runnders and sensor plate.

So, I am trying to figure out how I can build a HI-PO Porche engine for under $2000. Yeah, right!

Ah, and no, this is not just a little, subtle popping. It spits and pops as long as my foot is off the pedal, and other motorists notice. It's fairly obnoxious.



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'83 SC

[Early_S_Man]

The spitting and popping **ALL THE TIME** on decel is most likely NOT a leaking cylinder! With only one broken stud or loose stud, they do tend to spit when cold, but warming up to normal operating temps puts more tension on the good studs and the leak does lessen or close up to a great degree! Unless, of course you use WOT at low revs ... then you get the banging/barking noise.

And, typically, the spitting is worse off-idle when the throttle is opened, not closed!

So, I think your CIS-system is creating your current worry!

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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

[Jcon]

I have the exact same problem. I'm sure it's not an exhaust leak. Where is the decel valve????

Jeff C
81 SC

[Kurt B]

Let me check my signature here.
Add a 'view signature' Wayne, and i'll buy the book!
I'll buy the book anyway, of course. I'm gonna, just haven't gotten around to it.

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1984 Rockin' Cab
http://www.pelicanparts.com/pmpre/images/Kurt_B/kb84cab.jpg

[This message has been edited by Kurt B (edited 10-06-2001).]

[RoninLB]

I am not a CIS expert. IMHO the deceleration valve is a non necessary part of CIS. IMHO the unit is a problem waiting to happen.

[Doug Zielke]

Originally posted by RoninLB:
I am not a CIS expert. IMHO the deceleration valve is a non necessary part of CIS. IMHO the unit is a problem waiting to happen.

Agreed. It's emissions baggage. (The Decel Valve is located on the right side of the engine, near the middle intake runner ??).

When I bought my car, the line to the valve was disconnected and plugged. For the sake of curiosity, I re-connected it. All it appeared to do, was to keep the motor from dropping back to idle immediately after letting off the throttle. This, I believe has some esoteric effect on emissions. Car runs fine without it, but I did read somewhere it can cause the car to "pop" on the overrun.

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'81 SC Coupe (aka: "Blue Bomber")
Canada West Region PCA
The Blue Bomber's Website
"If it ain't broke...we'll help you fix it 'til it is!"

[This message has been edited by Doug Zielke (edited 10-06-2001).]

[Kurt B]

Yo, deceleration pops. Sorry superman for interruption here.

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1984 Rockin' Cab
www.geocities.com/carrera_cabriolet

[wckrause]

The decel valve is also called the vacuum limiter. It's job is to limit or reduce the high vacuum that is caused by a closed throttle at high RPM.

Here's my theory for what it's worth. If you have disconnected the decel valve, then your WUR is seeing the very high vacuum levels caused by deceleration. This causes the WUR to lean out your mixture way too much, which can cause intake backfiring. A possible way to prove or disprove this would be to disconnect the vacuum line to the WUR and see if your condition improves. If the popping does go away, then fix the decel valve (don't just leave the WUR disconnected).



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Bill Krause
'79 911SC Euro
MY PELICAN GALLERY

[Superman]

Thanks, dudes. thanks John, for reviving this from Page 2 with 0 replies. All are great ideas. I sure does not sound like intake popping but I'll look at the WUR/DV conspiracy. It sounds like exhaust popping.

I know that intake popping is a lean mixture deal. I also believe that exhaust popping means air is getting into the exhaust. As far as I know it does.

I put the wet rag in the exhaust pipe and there is no exhaust leak. I did notice that exhaust gasses and their surrounding pipes get hot in a hurry but I knew that. The engine sounds pretty interesting without all that exhaust noise. It's like you can hear every part working.

Again, I sealed the last three exhaust gaskets, but was wondering about the gaskets at the exhaust ports. I had expected that any leak there would spit during acceleration. In my experience intake manifold leaks cause spitting, particularly on acceleration, and particularly also when cold. But recently I have been told they may not spit. Well anyway, the issue is settled. There apparently is no exhaust leak. Every joint was whisper quiet on cold engine.

I now believe air is getting past valve guides. Could be either exhaust or intake, since there is great pressure on both. I think I need to just take a look at those heads lined up on a bench.

BTW, there is a Superhero Support Group and the guy with the big brain told me he thinks the beautiful blue startup clouds may be caused by intake valve guides allowing oil into the cylinder. Rather than a ring problem. I tend to agree. We superheros have to stick together. Also, I have valves that tick no matter what.



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'83 SC