Air-pressure vs. lift pump?

[Whitefire]

At the risk of sounding totaly ignorant I'll throw caution to the wind and just ask. If you had a fuel cap that vented@ say 5lbs. and teed into the vent tube at say 3or4lbs. would that be more pressure than the return side of the pump could handle? If not it seems to be a cheaper more reliable alternative to the lift-pump,no air=normal operation,to much air=vented to atmosphere. Any ideas or am I to far off-base.

[tcelano1]

While we're on the subject of fuel pressures, does anyone here know what the supply line pressure from the fuel tank pump is supposed to be? I'm starting down the road of putting a Duramax into a boat, and haven't gotten as far as getting a service manual yet. BTW, anyone know where I could get one other than the dealer?

Thanks!

[tcelano1]

While we're on the subject of fuel pressures, does anyone here know what the supply line pressure from the fuel tank pump is supposed to be? I'm starting down the road of putting a Duramax into a boat, and haven't gotten as far as getting a service manual yet. BTW, anyone know where I could get one other than the dealer?

Thanks!

[Idle_Chatter]

tcelano1, there is no "fuel tank pump" on a Duramax. The Duramax has only one fuel pump on the engine (that does contain two stages) that pulls fuel all the way from the fuel tank by producing a vacuum on the fuel supply line. Therefore, the fuel line pressure is atmospheric or below and the fuel caps are all designed to vent inward only, breaking vacuum in the tanks.

[tcelano1]

Idle_Chatter

Thanks for the info. Yes, after snooping around my truck, I realized there were no electrical connections back there. That's a nice design, and I've always hated the idea of having an electrical pump immersed in my fuel supply; obviously more so with gasoline...

[jbplock]

Originally posted by tcelano1:
... I've always hated the idea of having an electrical pump immersed in my fuel supply; obviously more so with gasoline... IIRC a gasoline immersed pump is actually safer than one immersed in diesel because the vapor pressure of gasoline is high enough to displace the air in the tank. So with out air to support combustion the risk of fire from a "sparky" pump is much less. However, with diesel fuel the vapor pressure is lower and there is air above the fuel in the tank (which is also why we can get water in the fuel from condensation) so a tank mounted pump is more risky.
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