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tj924 · Posted: Wed Jul 20, 2005 12:33 am Post subject: Exhaust Backfiring/Popping

I am now getting these little backfiring/popping sounds from the exhaust when I let off the accelerator. This only started a week or so ago when the big rubber boot that connects over the air metering plate popped off the plastic tube that leads to the next rubber boot & caused the car to run really crappy (drove it home about 35 km like that & discovered the problem the next morning). That has now been fixed (rubber boot & plastic tube both pulled off completely, inspected for cracks & reinstalled). Car is running fine (apart from a cold start problem & this backfiring), idles fine, accelerates fine & has new spark plug leads, water pump & a few new vacuum leads as at last weekend (replacing them as time permits).

Any suggestions? Hole in the exhaust or perhaps now running too lean for some reason?
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TJ
Silver '82 924 NA 5-Speed Manual

dpw928 · Joined: 02 Nov 2002 Posts: 1860 Location: owasso, ok 74055

May want to check the advance and retard (if equipped) vacuum lines that run to the distributor. If loose or reversed your retard timing would cause the popping. Also, has the exhaust been modified or blown out? My 928 started the same thing when I installed a rear muffler bypass.

Dennis
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81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

tj924 · Posted: Wed Jul 20, 2005 12:25 pm Post subject:

There are no vacuum lines on the distributor at all, never have been since I've had the car.

The exhaust popping only started after the intake boot had come partially off the plastic tube (one of the brackets had come loose) & the car ran really crappy. Put the intake boot back on & the car runs good again except for this popping sound. Only happens when I take my foot of the gas & the revs are coming back down, doesn't happen at idle or under acceleration.
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TJ
Silver '82 924 NA 5-Speed Manual

Paul · Posted: Wed Jul 20, 2005 12:51 pm Post subject:

Is is a nice burbling noise or sharp cracks?

tj924 · Posted: Wed Jul 20, 2005 1:14 pm Post subject:

Noticable pops that weren't there before. Not loud, but noticable & not that nice burbling sound.
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TJ
Silver '82 924 NA 5-Speed Manual

Paul · Posted: Wed Jul 20, 2005 1:35 pm Post subject:

Does your car have a decel valve? It looks like a big vacuum advance and sits on the intake manifold.

tj924 · Posted: Wed Jul 20, 2005 1:49 pm Post subject:

Paul · Posted: Wed Jul 20, 2005 2:35 pm Post subject:

Make sure it is in working condition....

tj924 · Posted: Wed Jul 20, 2005 2:36 pm Post subject:

Paul · Posted: Wed Jul 20, 2005 2:39 pm Post subject:

Not sure, but you want to make sure the rubber membrane is not leaking air.

tj924 · Posted: Fri Jul 22, 2005 11:56 pm Post subject:

My decel valve only has 2 vacuum lines running to it. I can see a spot for a 3rd small (4mm) line on the bottom of the valve but it is not hooked up. Should this be connected?

OK, so here is what my vacuum lines look like...

Anyone else with an '82 NA able to give me a comparison?
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TJ
Silver '82 924 NA 5-Speed Manual

Gram · Posted: Sat Jul 23, 2005 2:23 am Post subject:

My '82 NA doesn't have any of the braided lines shown in blue..no carbon cannister, decel valve etc. (Continue the black rubber hose to the brake booster for mine). The small hose (I think you show in brown) runs from the throttle body to the distributor (and only one port on each). Car is ex UK..imported here in '85.
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'82 924 NA Euro (hers)
'98 Audi A4 TDI (ours)

gohim · Joined: 02 Nov 2002 Posts: 4459 Location: Rialto, CA

Seems like the advice that others have offered is more complexed than what may actually be worng...

Have you checked the camshaft timing belt to make sure that it has not jumped timing, is correctly tensioned, and has not stripped any teeth?

This could VERY EASILY explain the way that your engine is running.

tj924 · Posted: Sat Jul 23, 2005 8:55 am Post subject:

Paul · Posted: Sat Jul 23, 2005 9:05 am Post subject:

Yes, that hose is the one that activates the valve.