popping on deceleration

On my 82sc I get small popping from the exhaust when decelerating in gear. The car runs great other than that. Idles smooth and constant, pulls strong. Any suggstions or is this normal? doesn't seem like it to me.


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Gunter
'82 SC Targa

There's a device called an RPM transducer that slows down fuel delivery as RPM's go down on low throttle. If this isn't working your getting too much fuel delivery and exhaust detonation...

Where is this RPM transducer and is it easy to repair or replace?

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Gunter
'82 SC Targa

The RPM transducer is on the left wall(when facing the car from the back) inside the engine compartment. On my 73, it was right next to the voltage regulator for the alternator.

Bill

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William Armentrout
1973 911T
2.7 carerra rs specs
www.geocities.com/william_armentrout

Gunter,

My 80 SC does the same thing that you describe. I have heard that this is common and even "normal." I have not done anything about it. Let me know how you make out. I may look into this myself.

Thanks!

Don

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Donald Weintraub
1980 Porsche Red 911SC Coupe

I don't knwo anything about an RPM Transducer and have not noticed such a device on my car or any drawings. there is a Decel Valve which simply adds air, to increase idle for a moment. Mine is deliberately disconnected (when I lift off the throttle, I want the throttle to CLOSE).

I had some popping when my mixture was not correct. Popping can also happen with a misfiring ignition system, but it sounds like you don't have one of those.

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'83 SC

Bill- what did your rpm transducer look like? I've bought the transistor that supposedly is the bit in them that goes wrong, but a lazy hunt for it the other night just revealed loads of relays!

I'm not sure if the later models have this set up. Probably that Decell valve does a similar job. I believe the popping is indeed from unburnt fuel.

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'72 911 TE

this is normal. mt 924 does it, the 944 does it, my friend's 911s do it. before you adjust the fuel delivery... look over every millimeter of your exhaust system. you'd be surprised what a tiny leak can do.

when my 924 started popping a while ago, i ignored it then it got worse. it doesn't damage the engine, and if you were to take the exhaust off of your car and run it with only the manifols, you'd hear a LOT more popping. the reason you don't notice it most of the time is because of the muffler. in my 924, it turned out that one of the gaskets in the exhaust system had developed a TINY leak and that is what was causing the pops. that could be the problem in your car. the only way of detecting the leak in my 924 was putting a clean cotton cloth by all the seams of the exhaust and running the car until it slightly heated up. we were then able to see that at one of the seams there was a little bit of black exhaust on it. $3 later i had new gaskets and the car has not popped like it did before.

good luck!

obin

Popping on deceleration is not always normal. You may get a slight popping under heavy down shifting situations. Under light deceleration or just when you let off the throttle if you get major popping it usually means that your air/fuel mixture is not correct. Have your exhaust CO level checked and adjusted if possible. It happened to my SC and with the turn of a screw it was running perfect.

Popping under deceleration is not normal and you can burn a valve

It can be a couple of things my Sc was doing it and it turned out a lock nut on my valve was loose and the valve was so out of adjustment it was not opening fully, it can also be a symptom of a broken inner or outer valve spring

Last it may be a leak in your intake system

I'd have it checked ASAP


Jeff
78Sc

Well, I have found out that the CIS cars definitely do not have an RPM Transducer so that one goes by the wayside. I did recently take the cat off and put on a test pipe. Thats why I think I notice the popping more. The Cat muffled it pretty well. ( its hard to describe in words... it just a light pop..pop..pop not POP.POP.POP.) I will check the connections on the exhaust system as descibed above. The car was just tuned and the mixture was checked yesterday on a machine and all is well there. The mechanic I trust says that this can be pretty common and that there is no need to worry about it. ( he has gone over the car ). I'll let you know if there are any exhaust leaks that solve it.

Other than that I'm not sure where to go from there.

Thanks for the input.

Boy did we have a shaker up here yesterday. We all got sent home early so engineers could check our buildings.


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Gunter
'82 SC Targa

Originally posted by retnug:
Well, I have found out that the CIS cars definitely do not have an RPM Transducer so that one goes by the wayside. I did recently take the cat off and put on a test pipe. Thats why I think I notice the popping more. The Cat muffled it pretty well. ( its hard to describe in words... it just a light pop..pop..pop not POP.POP.POP.) I will check the connections on the exhaust system as descibed above. The car was just tuned and the mixture was checked yesterday on a machine and all is well there. The mechanic I trust says that this can be pretty common and that there is no need to worry about it. ( he has gone over the car ). I'll let you know if there are any exhaust leaks that solve it.

Other than that I'm not sure where to go from there.

Thanks for the input.

Boy did we have a shaker up here yesterday. We all got sent home early so engineers could check our buildings.

i'm willing to bet the farm that it's a TINY leak somewhere in the exhaust. did you use BRAND NEW gaskets when you changed the exhaust components. when i first did this, i did not use brand new ones and that's when the popping started. also, i found a TINY leak back by the muffler. fixed those and the popping went back to a "normal" level.

also, check the torque settings on all the exhaust hardware. i once didn't torque one of the bolts down to the recommended specs and the vibrations were just enough to shake loose the exhaust and it make the popping louder.


obin

[This message has been edited by Obin Robinson (edited 03-01-2001).]

I used new gaskets but I need to check all the bolts. That is my task for this evening.



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Gunter
'82 SC Targa

exhaust popping is usually caused by an exhaust leak, allowing air into system where it creates a lean (hot) situation which helps explode unburned fuel in the exhaust system. exhaust going thru a pipe creates a vacuum at the hole, sucking in air.

I am going to try tightening the exhaust bolts and if that doesn't work I'll try another set of new gaskets. I was going to do that last week but earthquake damage at my house created a more important project for me last weekend.

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Gunter
'82 SC Targa