Surging/Hunting Cured!

Strangest thing. After getting my engine rebuilt 3000 miles ago, car has been "hunting" when fully warmed up for the last 1000 miles or so. When I lift my foot off the accelerator to stop, the rpm's would fall to zero, rev to 1200, fall to zero, and cycle like that until I manually regulated with the pedal. Really irritating.

Woke up this morning and started the car only to be greeted with a "mild" backfire. Not the ear splitting crack that destroyed my airbox last year, but scary. Fouled the plugs momentarily, but hunting is cured and Idle is right on! Hmmmm.

Joe
'80 SC Targa

I wonder if that is because your pop-off valve wasn't seating properly before, causing a vacuum leak. Then when the backfire occurred, it reseated itsself properly and cured the vacuum leak.
I found a poorly seated (bent pop-off valve pivot pin) pop-off valve on an '81 SC that was causing that same symptom. My guess is that it got bent originally by a very large backfire at some point.
Also don't forget that the rubber seal on those things don't last forever and harden after awhile. Something to check if you've exhausted all the alternatives.

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Tyson Schmidt
72 911 Cabriolet

I didn't really like the idea of a popoff valve for this reason. Vacuum leaks I mean. But it's a necessary evil and mine seems to make a very good seal.

I believe that with SweatGlo, or I mean DOW CORNING 111, I'm not sure the rubber o-ring won't last forever. Not sure at all.

I still suspect sensor plate issues and perhaps warmup fuel pressure issues causing these idle fluctuations. The air boxes on these cars are no doubt the largest ones ever dreamed up and I can't imagine the air does not cavitate.

Does the adjustable WUR upgrade affect this?

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'83 SC

Joe,
Amazing. The idle on my 78SC has been hunting for about a month. A couple of days ago a backfire. While my idle is still hunting it has greatly decreased and I was able to lower the idle to about 950 rpms (I had raised it to 1100 so it wouldn't stall). Previoulsy I had noticed that the pin holding the pop-off valve flap to the base were not aligned properly. I think you are right on! I'll check tomorrow. Perhaps a little red-line grease on the pop-off valve seal will do the trick. Thanks for the realization.
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Paul
78SC Targa911

[This message has been edited by pbs911 (edited 01-29-2001).]

I read somewhere, probally in the bible (Bruce's handbook) that you should put a little epoxy on those pins to keep them from moving.

Zimmerman, in his book "Used 911" claims that adding a pop-off valve to an SC's airbox is a bad idea and leads to more problems than it solves. He advocated leaving the airbox standard and being careful how you start the car in order to mimise the risk of a backfire.

Does anyone agree with this argurment or are we all devotees of the pop-off valve ?

James

Last year my 80sc backfired and blew the pop-off valve right out of it's hole. Jammed it back in, good enough to get me home from the airport.

I spend enough money on this car, I don't need to be buying a new air box.
I'll stick with the pop-off valve

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1980 911-SC Targa

Works for me!

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Bob
1982 911SC Targa
PCA - Founders Region

What does Mr. Zimmerman indicate as the careful way to start one's car?

blue
'81 SC Targa
Pop-off enabled ;P

Useful way to avoid major damage:

NEVER hit the gas pedal while starting, nor use the hand-idle!

Jens. Still w/o Pop-valve