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parker831
Novice
Nov 17, 2012, 12:20 PM
Post #1 of 9
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thumping noise in rear
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I'm hearing a muffled thump in the center rear of my '97 Chevy Lumina (72,000 miles; 3.1 engine) about 2
minutes after I turn off the engine. It comes in a series of 2 thumps.
This started about a week ago and now the thumping is occurring when I'm
in a slow idle--just a series of two thumps. Maybe it's releasing
pressure? Any ideas what this could be?
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Racer 51
User
Nov 17, 2012, 1:24 PM
Post #2 of 9
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Re: thumping noise in rear
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Make sure that one or more components of the exhaust sytem are not loose and viberating aganist the floorboards.
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parker831
Novice
Nov 17, 2012, 2:52 PM
Post #4 of 9
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Re: thumping noise in rear
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Thanks guys. A friend's husband was able to diagnose it for me. He said the trouble is that my gas cap is not releasing gas fumes properly. So the noise I'm hearing is the release of the built-up gas fumes. Whew! Seems like it'll be an easy fix! 
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Hammer Time
Ultimate Carjunky
/ Moderator
Nov 17, 2012, 3:00 PM
Post #5 of 9
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Re: thumping noise in rear
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The gas cap is not supposed to release fumes. They spend a lot of money to prevent that.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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parker831
Novice
Nov 17, 2012, 4:41 PM
Post #6 of 9
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Re: thumping noise in rear
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Hmmmm. Maybe I misunderstood. He said I need a specific type of gas cap. Maybe it's something else about the gas cap??
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Hammer Time
Ultimate Carjunky
/ Moderator
Nov 17, 2012, 4:43 PM
Post #7 of 9
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Re: thumping noise in rear
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If he's telling you to get a vented cap, do not listen to him. It's not even legal to do that. That system has to remain air tight by law.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Discretesignals
Ultimate Carjunky
/ Moderator
Nov 17, 2012, 5:21 PM
Post #8 of 9
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Re: thumping noise in rear
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You sure the thumping noise is from the fuel tank? The tank is vented through the normally open EVAP vent valve, not the gas cap. If the EVAP vent valve is stuck closed or there is a restriction in the canister or lines and the EVAP system is purging the canister, that will put the tank under too much vacuum. The gas cap has a safety valve in it to relieve excess vacuum to prevent the tank for collapsing. Usually an EVAP problem such as stuck closed vent valve will set a trouble code.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Nov 17, 2012, 5:22 PM)
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parker831
Novice
Nov 18, 2012, 11:34 AM
Post #9 of 9
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Re: thumping noise in rear
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Well, he made a hole in the gas cap to vent it and the thumping noise is gone. So maybe as you say, the EVAP vent valve is stuck closed? Is that an easy fix (I hope)? I'll consult a mechanic and see. I did get an error code, but now I can't find it. Thanks for the advice!
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