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Releasing Air/Pressure from Gas Tank

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All,

Just curious. I have heard that, if your car/truck doesn't start, one remedy may be to take off your gas cap in order to release the air/pressure from the tank.

What does this do that it may help your car start? does this in some way affect you fuel pump or injector?

Thanks for any info.

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I think that you may be referring to a condition that was not that common in pre-fuel injected vehicles. The fuel tank "breather" would become plugged which would result in the tank developing enough vacuum that the mechanical pump could no longer pull fuel from the tank. The vehicle would typically stall after an extended drive and would restart after sitting for a couple of hours and would consistently and predictably do this. One way to diagnose this was to drive the vehicle with the gas cap off/loose and see if it stalled.

This would likely not occur in a newer fuel injected vehicle vehicle!

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Why not, airjer? Can't the tank become vacuum locked anymore?

Bob

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An electric fuel pump will push fuel with a sealed tank until the tank collapses. A typical pump can deliver pressures at or more than 100psi if need be (fuel pressure is regulated by the regulator to anywhere from 35-65psi). As long as the pump is in fuel it will push it. Mechanical pumps only delivered 8-12 psi of pressure and it had to "suck" fuel from the tank. If the amount of vacuum in the tank was to great the pump could no longer suck.

I have yet to run into a drive-ability issue where tank vacuum lock is the issue!

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I remember a certain model that had fuel tank pressure sensors that would short out, and if you changed the pressure in the tank, it would start, or stall. I dont remember what model it was though.

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