Pressure in the gas tank

[Jeff Berk]

I noticed that my gas tank seems to build up pressure, is this normal?

Also, and possibly related, I had fuel leaking out of my rear SU carb after a few miles last week and the fuel pressure gage recently was showing 7.5 psi after a drive. The carbon canister was deleted by a PO. The return line to the fuel tank was reconnected after ? years so it has not been tested other than to confirm it was not leaking.

My fuel system is currently as follows:

1) Fuel is pumped from the fuel tank, through a fuel filter using an after-market electric fuel pump to the engine compartment. 

2) Fuel is pumped through the mechanical fuel pump and into a DIY fuel rail with three outlets. Two of the outlets flow into a pair of round top S.U. type carburetors.  The third port is connected to a low-pressure fuel-pressure regulator.

3) The fuel-pressure regulator has two active ports and a pressure gage. One port is connected to the fuel rail and the second port is connected to the return fuel line.

[SteveJ]

No, it is not normal to build pressure. Make sure the vent line is not capped off.

[Jeff Berk]

Is the vent line the one on the top of the reservoir tank in the right rear fender?

As long as I'm pulling interior panels out, are there any suggestions on how to get them to stop creaking? Silicone on the plastic to plastic contact seams perhaps?

 

[SteveJ]

First, locate the vent line in the engine bay. Blow some compressed air at low to moderate pressure into the vent line, and have someone listen at the filler with the gas cap off. The helper should hear the woosh. That also allows you to figure out if someone capped off the vent line in the engine bay.

For the plastic panels, you might try some thin weather stripping. https://www.amazon.com/Weather-Stripping-Sliding-Speaker-Dashboard/dp/B07WGTHBSC

Or maybe something like this: https://www.amazon.com/Weather-Stripping-Silicone-Window-Transparent/dp/B07XRVBSYC

Here's some 1 mm weather strip. https://www.amazon.com/uxcell-Thick-Adhesive-Weather-Strip/dp/B07L6JKJD1

[Captain Obvious]

7 hours ago, Jeff Berk said:

I noticed that my gas tank seems to build up pressure, is this normal?

Kinda depends on the year... If your car is new enough that it originally had a carbon canister, then yes... It may be normal for the tank to build some pressure.

[SteveJ]

3 minutes ago, Captain Obvious said:

Kinda depends on the year... If your car is new enough that it originally had a carbon canister, then yes... It may be normal for the tank to build some pressure.

He has a 74. I never noticed pressure building up in the tank in my car. Of course, the carbon canister was not hooked up properly.

[Captain Obvious]

Roger that. I think 75 was the first year for the "check valve" in the vent line. So for 74, there shouldn't be pressure build-up.

74 didn't have one of these, right?

No idea why that's sideways. It's not sideways in native format...

[SteveJ]

I skimmed through the EF section of the 74 FSM. I didn't see that valve, and it wasn't described in the theory of operation on EF-32.

I would not be surprised if the vent line from the tank was capped at the tank or in the engine bay.

[Jeff Berk]

I'll check for the vent line tomorrow morning. The fuel return line was capped off until I reestablished a return flow fuel system a few months back.

Is there some way of adding a signature line or information in the block to the left of messages that includes vehicle information?

I think that image is sideways to permit fuel to drain back into the tank:) 

Edited July 18, 2022 by Jeff Berk

[dutchzcarguy]

Lots of cars build a vacuum in the fueltank.. it's not much as there is a very small hole in the lid of the tank that is just big enough to give the tank some air..

My 300zxtt always sucks a bit air when i open the tank, thats normal.

Never had a pressure or vacuum in the 240z.

[Captain Obvious]

11 hours ago, Jeff Berk said:

I think that image is sideways to permit fuel to drain back into the tank:) 

Haha! That's the best suggestion so far!

[kully 560]

I have a 74 and no check valve, and no pressure at all. it could be a clog return line