Gas tank pressure

[ahshoe]

Got a 55 T-Bird... Been and is working well,but I have a lot of gas tank pressure that builds up. When I release the gas cap I literally can feel and see the tank release the pressure. I think I have a vented gas cap.Says anti surge on it. Now that pressure seems to be creating a concern with the gasket and gasket mounting 6 bolts that hold the sender unit in the top of the gas tank. Cause they are is leaking around the gasket and around the bolts that hold the sender to the tank, but only under this pressure. I replaced the gasket twice but it is still doing it. Is my problem the gas CAP not venting properly? Does the tank even need pressure. By the way I have and had for over 15 years an electric fuel pump on the rear of the car by the tank all working fine and a fuel pressure guage on the carb line near the card that reads about 4.5# + or -

[rotorwrench]

Pressure build up is usually caused by changing temperatures where the heat causes expansion. The way a lot of vented caps are made, they are designed to keep the tank from building a low pressure situation in operation and not so much for a build up of pressure. With a vacuum, it's only a matter of time before the fuel pump can no longer draw fuel. With a pressure build up situation, it actually would help fuel draw until the pressure finally equalizes. If the cap has a ball valve then the ball may load up against the vent port and act as a one way check valve. It will allow air in but won't allow air out.

I don't know how the 1955 T-bird is set up. Maybe an owner of one of these cars can give better information about your cap.

[dmsfrr]

The '55/'57 T-Birds need to have a vented gas cap to prevent any pressure or vacuum problems in the tank. They didn't come with any other vent to equalize with 'outside' air pressure.

FYI, '57 Birds do have a vent tube on the tank but it goes back into the filler neck several inches below the gas cap, not to outside air. (attached photos)
.

[paul2748]

Sounds like your cap is non-vented or is bad. Should be replaced

[Ford blue blood]

Almost all new replacement caps will let air in but no gas fumes out (thank you EPA). Drill a small hole in the rivet that holds it all together and that will let the tank breath.

[KULTULZ]

Sponsored Links (Register now to hide all advertisements)

The cap is either defective and/or wrong application.

A new STANT cap is about six bucks.

[dmsfrr]

I've been told the '55 T-Bird gas caps need to have a recessed center (attached photo) so it will clear the mounting studs of the emblem on the fuel door. Not sure how true that is, mine is flat and it seems to work fine.
Just something to be aware of.

[KULTULZ]

Originally Posted by dmsfrr

I've been told the '55 T-Bird gas caps need to have a recessed center (attached photo) so it will clear the mounting studs of the emblem on the fuel door. Not sure how true that is, mine is flat and it seems to work fine.

Just something to be aware of.

GOOD POINT !

The MPC shows service replacement as B9A 9030-C (1955-59). That crosses to a STANT PN which I had but lost (but can look up again if needed). I am sure they took that into consideration.

Same as don't slam the hood until you are sure ...

EDIT -

OK ... The B9A 9030-C is shown below. Completely different from the original filler cap -

Also, FORD 49/59 MPC showing replacement cap. S is the application ID for the BIRD in the chart.

[ahshoe]

Just replaced it with a Stat #10640... $7.50....We will see what happens on a test drive tonight.

[cokefirst]

Originally Posted by dmsfrr

I've been told the '55 T-Bird gas caps need to have a recessed center (attached photo) so it will clear the mounting studs of the emblem on the fuel door. Not sure how true that is, mine is flat and it seems to work fine.
Just something to be aware of.

You are correct, the factory gas cap on the 1955 was recessed to clear the tangs that hang down from the cross flags and Ford crest emblem. The 1956 did not have those emblems since the gas door was concealed by the spare tire. Therefore, the gas cap on the 1956 models was flat on top without the recess.

[KULTULZ]

Originally Posted by cokefirst

You are correct, the factory gas cap on the 1955 was recessed to clear the tangs that hang down from the cross flags and Ford crest emblem. The 1956 did not have those emblems since the gas door was concealed by the spare tire. Therefore, the gas cap on the 1956 models was flat on top without the recess.

BIRD (original) fuel caps available here - https://concoursparts.com/

- EDITED TO SHOW MORE ATTACHMENT DETAIL

[RobD55]

Originally Posted by ahshoe

Got a 55 T-Bird... Been and is working well,but I have a lot of gas tank pressure that builds up. When I release the gas cap I literally can feel and see the tank release the pressure. I think I have a vented gas cap.Says anti surge on it. Now that pressure seems to be creating a concern with the gasket and gasket mounting 6 bolts that hold the sender unit in the top of the gas tank. Cause they are is leaking around the gasket and around the bolts that hold the sender to the tank, but only under this pressure. I replaced the gasket twice but it is still doing it. Is my problem the gas CAP not venting properly? Does the tank even need pressure. By the way I have and had for over 15 years an electric fuel pump on the rear of the car by the tank all working fine and a fuel pressure guage on the carb line near the card that reads about 4.5# + or -

I had a similar problem here in Western Australia with my 57 Bird, which also had a "vented cap". I ordered a new cap from CASCO and problem went away.

[KULTULZ]

Almost all new replacement caps will let air in but no gas fumes out (thank you EPA).

There is good reason for that.

A vented fuel tank will also allow unlimited atmospheric water vapor to be drawn into the tank fouling ethanol blended gasoline. And no, keeping the tank full will not stop it.

[Tom in TN]

We shouldn't be running booze in our old cars anyway. Just asking for trouble.

[Dobie Gillis]

Originally Posted by Tom in TN

We shouldn't be running booze in our old cars anyway. Just asking for trouble.

Some moonshiners did if the 'shine was in a tank instead of Mason jars. They had the tank plumbed in to the fuel system. If they got into a chase with the revenooers and it looked like they were gonna be caught they opened a valve and burned the cargo in the engine.

[zuburg]

So is one fix to get a vented cap and run non-ethanol gas? I’ll start by drilling a hole in my old cap.

[dmsfrr]

Originally Posted by zuburg

So is one fix to get a vented cap and run non-ethanol gas? I’ll start by drilling a hole in my old cap.

Yes, a very small hole.

https://www.pure-gas.org/

[Crankster]

A fuel tank of the type under discussion (the cap) must allow air in to replace the fuel being drawn by the fuel pump, there is no way around this or a partial vacuum in the tank will result, and this means engine = no worky. This has nothing to do with the EPA. They also need to vent in the other direction. Therein lies the problem. This characteristic was considered unacceptable due to gaseous vapor emissions and a contributor to decreased air quality.

Modern car fuel systems, on the other hand, with fuel infection and carbon vapor recovery canisters and the rest of it are completely sealed as far as letting gasoline vapor emissions out, or near enough. They do not "breathe" in the way the old school primitive fuel tanks will, but they (obviously) have to allow air in. What happens in the old steel tanks - open to the atmosphere for all intents and purposes, is a a kind of bellows effect due to alternating hot and cold temperature swings. Indeed this is what the EPA regulations are designed to prevent. It is greatly minimized by a full tank, because there is far less moisture laden air being drawn in. You can really see this effect on humid days following cool nights , the outside of metal fuel tanks will be covered and dripping with moisture condensation like a glass of cold beer.

The "Check Engine" light will even illuminate if the fuel cap is not secured on the modern systems, "modern" meaning anything made after about 1985 or 1990.

[KULTULZ]

If one decides the best method of venting is to drill a vent through the cap, most likely and depending on where the fuel filler neck is located, one will quickly learn the meaning of ANTI-SLOSH FILLER CAP.

This is not rocket science, but finding a correctly designed service replacement filler cap for this period vehicle might be a challenge.

OK ... I've heard enough, I am out of here ...

[zuburg]

I just ordered the Stant vented cap mentioned earlier. I decided to inspect my old cap more closely. It appears to me it may have been a vented cap, but modified by PO to unvent it. On the top of the cap the words “Pressure” and “Vacuum” are printed. On the inside you can see a small hole right in the middle. On the outside, it appears like someone covered up the middle of the cap with JB Weld or something. See pictures.