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Old 03-22-2003   #1
Registered User
Join Date: Mar 2003
Posts: 32

Question gas tank "thump"

I have a 1989 GTA with a "thumping" gas tank. This thump happens intermittently when the engine is on or off. Sometime the thump is loud and hard enought that you can feel the vibration in your seat. The gas tank/fuel pump has been replace, and this problem occurred before and after the fuel tank replacement. This thump seem relatively harmless, but it never used to do this. Any ideas?
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Old 03-31-2003   #2
GTA Member
Join Date: Jan 2002
Location: chicago
Posts: 66

Could be that its the baffles in your muffler contracting.
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Old 03-31-2003   #3
Nova Nut
Join Date: Nov 2002
Location: British Columbia Canada
Posts: 2

Could be a faulty or incorrect gas cap. They have a one way valve that lets air in but won't let fumes out. If the cap isn't venting properly the pump will collapse the tank giving an oil-can effect. After you shut the car off the tank pops back as the air leaks back in.

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Old 05-16-2003   #4
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Join Date: Oct 2002
Location: Westminster, CA
Posts: 7

My car makes that thump too. I think it is the pressure relief valve on the tank. We have a Lumina at work that makes the same thump.
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Old 05-18-2003   #5
The Anvil
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Join Date: Nov 2002
Location: Vancouver, BC
Posts: 159

I think Randy is right. You need a properly working gas cap for our cars, or you'll get the aformentioned 'oil can' effect. For example, when you got to fill up your car at the gas station, you'll note a sound of air being released from the tank as soon as you crack open the gas cap. This is the internal pressure that keeps the tank from 'oil can' scenario.
1989 GTA L98

Gunmetal metallic, L98, all performance options.
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