1977 454engine fuel pressure drops to 0 in 15mts

[1977ChevyC20]

I have a 1977 Chevy truck with a 454 motor and a Holly carburetor, I installed a fuel pressure gauge and notice the fuel pressure ( starting at 6 psi ) slowly and gradually drops to 0 psi in 15 minutes when idling
I have installed a new fuel pump and filter and the same thing happens
I have removed the fuel caps when the fuel pressure is at 0 psi and there is no difference
The truck has duel tanks, I am wondering about the Left, Right tank switch or if there is a air leak ( nick or hole ) in one of the pipes ( if the fuel pump is sucking in air leading to decrease of fuel pressure )
Before i drop the tanks and renew the fuel line at the rear tanks I am wondering if anyone had any ideas
At 0 psi the trucks engine revs up and down a bit and gets slower, I suppose it would eventually stop
I can blow down the inlet pipe near the oem ( stock standard ) mechanical fuel pump and i can hear the bubbles in the left side fuel pump

[Man With A Gun]

A dirty sock in the fuel tank could cause that.

I had that problem one time. Sediment would fall from the sock to the bottom of the tank when there was no fuel suction keeping it on the sock. Then when I cranked it the truck would run for a while until it finally got starved for fuel.

You'd think it would be unlikely for both fuel tanks to have that problem, but the reality is that both tanks have had the same life with the same temperature, humidity and other conditions and were filled in the same manner from the same fueling stations.

Clean out the tanks or replace them. Change the socks at the same time.

[wilkin250r]

Could also be a problem with the fuel line, especially if both tanks seem to exhibit identical symptoms.

I had a similar problem with a 72, the truck would fire and run, but fuel pressure would drop within a few minutes. The problem was a deteriorated section of rubber line running between the tank and the hard line on the frame. The bad section wasn't completely clogged, so if the truck sat for long enough, a trickle of fuel would seep through and fill up the fuel lines, which is what allowed the truck to start, but I couldn't pull fuel fast enough to KEEP it running. When I replaced that section of line, and everything ran perfect.

A dirty sock on the sending unit, deteriorated section of rubber line, corroded/rusty section of steel line, dirt, kink in the fuel line, ANY of these problems can cause what you are seeing, because they would all choke the fuel flow. It sounds to me like you have a clogged fuel line somewhere along the way.

[geezer#99]

Originally Posted by 1977ChevyC20

I have a 1977 Chevy truck with a 454 motor and a Holly carburetor, I installed a fuel pressure gauge and notice the fuel pressure ( starting at 6 psi ) slowly and gradually drops to 0 psi in 15 minutes when idling
I have installed a new fuel pump and filter and the same thing happens
I have removed the fuel caps when the fuel pressure is at 0 psi and there is no difference
The truck has duel tanks, I am wondering about the Left, Right tank switch or if there is a air leak ( nick or hole ) in one of the pipes ( if the fuel pump is sucking in air leading to decrease of fuel pressure )
Before i drop the tanks and renew the fuel line at the rear tanks I am wondering if anyone had any ideas

So with zero pressure does it keep idling or does it quit?
If it keeps running it’s likely just the gauge, especially if it’s a liquid filled gauge. They don’t like heat.

[RotaryRocketeer]

Originally Posted by geezer#99

So with zero pressure does it keep idling or does it quit?
If it keeps running it’s likely just the gauge, especially if it’s a liquid filled gauge. They don’t like heat.

A very important question/point.

[1977ChevyC20]

Originally Posted by RotaryRocketeer

A very important question/point.

With 0 fuel pressure the truck's motor revs up and down a bit, it keeps going although I think it would eventually stall
Funny thing is i can blow down the fuel line easy and I can hear the bubbles in the left side tank, seems all clear, that's why i'm thinking it could be the fuel pump sucking in a bit of air from a crack in the old fuel lines going to the tank

[1977ChevyC20]

Originally Posted by geezer#99

So with zero pressure does it keep idling or does it quit?
If it keeps running it’s likely just the gauge, especially if it’s a liquid filled gauge. They don’t like heat.

With 0 fuel pressure it keeps going but the engine revs up and down a bit, it would probably eventually stop

[geezer#99]

Originally Posted by 1977ChevyC20

With 0 fuel pressure it keeps going but the engine revs up and down a bit, it would probably eventually stop

So does it quit or not.
Can you drive it beyond 15 minutes?
Or are you just letting it sit and idle.
What’s the ambient temperature and what’s the motor temperature?
Do you have a heat shield under the carb?
Are your floats adjusted correctly?

[f.monroe]

Stock manifolds or headers? Is the fuel line on the frame rail near an exhaust pipe or a header? If so shield it and see if it is heating issue. Could be a vapor lock type thing going on.
Frank

[homemade87]

Originally Posted by geezer#99

So with zero pressure does it keep idling or does it quit?
If it keeps running it’s likely just the gauge, especially if it’s a liquid filled gauge. They don’t like heat.

I agree with this . Those oil filled gauges will read lower fuel pressure once they get warm . Dont depend on it if this is what you have . I had one do the same thing and I wound up draining the oil out of it and it worked better . The oil gets heat soaked . If indeed you have 0 pressure that engine will not run .

[Dead Parrot]

Install a new hose from the switch valve to the frame rail. Don't have to drop the tanks to do that. Also from the frame rail to fuel pump. If there is enough extra hose from the tanks, clip just enough off the ends to put fresh rubber on the valve connections. Check the metal line along the frame for evidence of leaks.

Assuming a mechanical fuel pump, check your engine oil for gas. Might be the pump leaking into the motor.