Fuel Pump Loud at 1/2 Tank or Less

Fuel Pump Loud at 1/2 Tank or Less

When my gas tank is about 1/2 full the fuel pump starts buzzing loudly. Fill the tank and the buzz goes away. Anybody know what's up with that?


1989 928 S4 A/T

Try running without the gas cap to see if that makes it quiet. The fuel tank system is sealed. There is a
one way check valve near the tank that allows the tank to inhale air to compensate for the volume of fuel that is consumed by the engine. There is much more to the system: overflow tank, charcoal canister, vacuum switch at the engine, etc. But when it malfunctions the tank can collapse into itself as the pump sucks it dry . This would explain the increased fuel pump noise as the tank gets 1/2 full .

Jim Bailey
928 International

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