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9magazine · Posted: Fri Feb 29, 2008 11:30 am Post subject: Popping from exhaust

Okay what a nice day up here at 7000 feet in the mountains of Northeastern AZ... I have been working on the car most of the day.

I noticed when I take my foot of the gas am getting a popping noise from what I think is the muffler. Timing is set right, and it purrs at 950rpms. Is this the sound of the exhaust manifold going?

Brian
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Brian
9magazine.com
'80 931 5spd - 4bolt wheel

Paul · Posted: Fri Feb 29, 2008 11:33 am Post subject:

I'd check the anti backfire valve on the intake manifold.
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Vince Ponz · Joined: 02 Nov 2002 Posts: 3581 Location: Florida

You might have a pin hole in the line. Sucks air in and therefore the pop.
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77.5 924 modified track car
79 931 Euro stock
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9magazine · Posted: Mon Mar 03, 2008 11:11 pm Post subject:

Its been cold and off again on again snow this weekend. I did find the part listed in the PET, and then went to find out how much it is. Pelican is showing the Vacuum Limiter at $430.00 its a 930 (turbo 911) part number... anyone have a more affordable source? Am hoping its a bad vacuum line!

Brian
9magazine.com
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Brian
9magazine.com
'80 931 5spd - 4bolt wheel

9magazine · Posted: Mon Mar 03, 2008 11:12 pm Post subject:

Its been cold and off again on again snow this weekend. I did find the part listed in the PET, and then went to find out how much it is. Pelican is showing the Vacuum Limiter at $430.00 its a 930 (turbo 911) part number... anyone have a more affordable source? Am hoping its a bad vacuum line!

Brian
9magazine.com
_________________
Brian
9magazine.com
'80 931 5spd - 4bolt wheel

924RACR · Posted: Tue Mar 04, 2008 12:53 am Post subject:

Snow?? Dang, nothing but sun down here in Phoenix!

Popping will often be caused by too-hot exhaust often caused by running lean or too much timing advance - get more fuel in on the shift, which then pops in the overheated exhaust. So check the mixture, maybe it wants a little richer.

Or, as noted, you could have a small leak.
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'79 924 #77 SCCA H Prod racecar
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flosho · Posted: Tue Mar 04, 2008 3:30 am Post subject:

I've always thought that some popping and crackling from the exhaust is normal. If it is caused by hot exhaust and a lean mixture, it would make sense that when you take your foot off the gas after accelerating a bit, on decel the fuel is cut and you will hear some popping.

Many of the tuner cars pop quite a bit on decel, right from the factory, mostly the turbocharged ones.. Caliber srt4, mazdaspeed3, etc.
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Rasta Monsta · Joined: 12 Jul 2006 Posts: 11733 Location: PacNW

Mine used to pop a bit on decel. Check the joint between the head pipe and cat, and between wastegate and cat.
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WeiBe (1987 924S 2.5t) - 931 S3

Smoothie · Posted: Tue Mar 04, 2008 11:22 am Post subject:

Aussie931 · Joined: 28 Aug 2006 Posts: 55 Location: Canberra

I agree with smoothie. The vacuum limiter is unlikely to be suspect. I had the same issue with my 931 and it was due to the mixture being too lean. I replaced all of the vacuum lines - this helped a little however it didn't get sorted completely until I adjusted the mixture. In regards to the decel valve/vacuum limiter, I have removed mine and it has made very little difference to the car's performance. From what I understand, it is fitted more for emission control as it leans off what would otherwise be a rich mixture on decel after heavy acceleration.
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1979 Series 1 924 turbo