Gas pressure, oh my the smell!

[White Spyder]

OK, jokes and double entendres are appreciated, but there is an issue.

Should I be getting pressure build up in the gas tank of my 48 Chrysler? I replace the gasket a week or so ago and all was good. Went for a ride today and the gas fumes were back. Looked at the sending unit cover and I saw bubbling gas along the edges. Turned the gas cap and got a large venting of pressure.

I have re sealed with gasket material and fuel safe sealer but if it is building pressure in the tank I fear that it will breach the seals again.

Thanks in advance for your thoughts. Now let the fun begin!

[Sharps40]

No. Tank should be vented at the cap

[Niel Hoback]

Ditto. a vented cap prevents pressure build-up.

[William Davey]

I bought replacement gas caps at NAPA that were supposed to be vented, they were not.  You'll get a little gas on your lips, but remove the cap and blow on the little hole in the brass looking part of the cap.  If no air passes, it's not vented.  My solution was to drill the guts out of the gas cap. Worked for me.

[knuckleharley]

OK, jokes and double entendres are appreciated, but there is an issue.

Should I be getting pressure build up in the gas tank of my 48 Chrysler? I replace the gasket a week or so ago and all was good. Went for a ride today and the gas fumes were back. Looked at the sending unit cover and I saw bubbling gas along the edges. Turned the gas cap and got a large venting of pressure.

I have re sealed with gasket material and fuel safe sealer but if it is building pressure in the tank I fear that it will breach the seals again.

Thanks in advance for your thoughts. Now let the fun begin!

Buy a vented gas cap.

 

Seriously. Some tanks are vented at the cap from the factory.

 

No gas tank is air tight. If they were the gas would stop flowing the instant all the air had been sucked out by the fuel pump. Look for the little tube running from near the top of the filler neck. That's for venting the tanks. If your tank doesn't have that tube,you need a vented gas cap.

 

According to my 1955 Stant catalog,you need a Stant G-20 gas cap. You can probably find one on ebay,amazon,or at O'Reilly auto parts. If not,look for a interchange list.

Edited June 12, 2016 by knuckleharley

[The Oil Soup]

I believe the cap is vented to let air in the tank as fuel is drawn off and normalize the negative internal pressure.

[DonaldSmith]

Why would gas be under pressure at the sending unit?  

 

Gravity, if the tank is filled up into the filler pipe.  (I had lots of fun getting the hoses and clamps right and tight.)  

 

Gas heating up and expanding, in a non-vented tank, although running the car should offset this.

 

Just my guess.  

[knuckleharley]

Why would gas be under pressure at the sending unit?  

 

Gravity, if the tank is filled up into the filler pipe.  (I had lots of fun getting the hoses and clamps right and tight.)  

 

Gas heating up and expanding, in a non-vented tank, although running the car should offset this.

 

Just my guess.  

Negative pressure because liquids have to have air to flow,and once the carb,fuel pump,and gas lines run dry it will air-lock. I first discovered this while building a panhead chopper in the early 70's. It had a peanut gas tank on it that came with the cap,and I could fire it up and take off down the road,and suddenly the damn thing would just quit running. Leave it sit for a few mintues,and it would fire up and take another short trip down the road before stopping. The second time I did this I unscrewed the gas cap as soon as I got it stopped,and could hear the tank "pop" as air got in it.. Bought a vented gas cap,and things were just peachy keen.

 

Do not drill a hole in your cap. The vented caps have a shield that keeps the gas from splashing back out of the cap top.

[bobostski]

It sound from your first message that there's positive pressure in your tank if it is creating air bubbles. If so, is there a problem wit the fuel pump.

[White Spyder]

Thanks all. The cap was not vented. Interesting it has run fine for the last few years with that cap on it My thought is that because the gasket at the sending unit was old and deteriated that it was self venting there causing the original issue of fumes. When I replaced it recently and took the car out yesterday to a show it sat in the sun (94*) with an almost full tank, the pressure built up enough to push through the new gasket. New gasket made, sealer applied and vented cap acquired.

[Sharps40]

[White Spyder]

Update.

The cap I got was a temporary fix as it was black plastic. I ordered one one line from a MOPAR supply company (they advertise here) that should have been for the year make and model car I have I.E. Vented. When it arrived it turned out not to be vented. They have agreed to a return but had not herd of the need to be vented.

Now I am in search of a vented chrome or stainless cap. Any one have one or ideas Stant only lists a crankcase breather cap on their site

Edited July 7, 2016 by White Spyder

[greg g]

Napa

[William Davey]

The non vented cap I bought was from NAPA and it was the only one listed for my '55 Dodge C-3-B8.  It was definitely not vented, if I drove or parked in the hot sun,  then opened the cap there was a whoosh of gas fumes from the filler neck. Went back to my local NAPA where they have guys that actually know the parts they sell and there was no other vented cap option.  So I drilled the guts out of the center of my cap and it became vented.  

[knuckleharley]

 Now I am in search of a vented chrome or stainless cap. Any one have one or ideas Stant only lists a crankcase breather cap on their site<<

 

Here is probably more than you want to know directly from the 1955 STANT catalog.  You might want to copy and save the application list because who knows what cross-reference you might need later on.

 

Stant G-51 "A" Size For 49-52 Chevrolet,49-52 Ford  LOCKING GAS CAP

   This cap can be used also on same applications as G-50,which are Buick,Chev,Chrys,Plymouth,Dodge,Olds,Pontiac,etc

Duplicates original equipment - prevents gas leakage. High pressure zlne die casting.

Highly polished and chromium plated in accordance with the standards of the automotive industry. SPIRAL CAPIllARY VENT admits proper air flow but prevents loss of fuel. Self-closing positive sealing stainless steel keyhole cover.

============================================================================

EVRSEAL STAINLESS STEEL, SPECIAL RESTRICTED VENT GAS CAP  (NON LOCKING)

Duplicates original equipment of '49, '50 Chev., '49 to '52 Ford. Eccentrically weighted metering pin keeps vent clean. Developed in accordance with Ford and Chevrolet engineering specifications to prevent loss of fuel because of surging.

Stant "Floating Seal" Construction' Stainless Steel Shell - Highly Polished

G-21 - "A" Size - Fits '49 to '52 Chev., '49 to '52 Ford

1955

catalagU8 APPLICATION CHART - STAINLESS STEEL GAS CAPS

Allis Chalmers B, C, WC, WD, WF, M, MU,G..............................•. G-20

Auburn '27 to '37 .........................•......................... G-20

Autocar '47, '48 C-40-75-80 splash apron tanks only " G-25

Buick '35, '36 (all to '55 optional) G-20

Cadillac '35 to '40 except torpedo ....................•........•......... G-20

Case Tractor VAC , .. , , G-25

Chevrolet '35 to '48 , ...................•.... , . , G-20

Chevrolet '49 to '52. ,. , , '" ...•.......•.................. G-21

Chevrolet Trucks '37 to '55 Comm. and Uti! , . , G-20 or G-23

Chrysler '35 to '52 and '53 to '55 Station Wagon , G-20

Chrysler '53, '54 except Station Wagon G-22

Crosley all (bend filler neck cams out to fit) G-20

DeSoto '35 to '52 and '53 to '55 Station Wagon G·20

DeSoto '53, '54 except Station Wagon .........•...................... , G-22

Diamond T '35 to '54 most models G-20 or G-23

Divco Truck '41 to '55 G-20

Dodge '28 to '52 and '53 to '55 Station Wagon G-20

Dodge '53, '54 except Station Wagon G-22

Dodge Truck '28 to '55 except under seat tanks G-20 or G-23

Federal Truck '31 to '54 most models G-20 or G-23

Ford '32 to '48 G-25

Ford '49 to '52 ('53 to '55 except Station Wagon use G-311 G-21

Ford Truck '37 to '50 G-25

Ford Truck '51 to '55 G-20 or G-23

GMC '37 T-14, 18; '38 to '47 except T-60C, T-60D, T-83BG, T-51BG G-20

GMC '48 to '55 Compare with old cap or write Stant

Graham '34 to '41 G-20

Henry J ..............•.......... , G-20

Hudson '30, '31 G-20

Hudson '32 to '36 , '. G-25

Hupmobile '38 to '41 G-20

Indiana Truck '37 to '52 most models G-25

International Truck '37 to '53 D-2-15-30-35-40, K-1 to K-5, KB 1 to KB-5, Lseries

except C. O. E.;Rseries except C. O. E G-20 or G-23

International Truck '41 to '50, K-6 to K-11 : G-25

International Harvester Tractor Model Cub and H & M G-25

Kaiser & Fraser optional G-20

LaSalle '35 to '40 except Torpedo G-20

Lincoln Zephyr '36 to '39 G-25

Lincoln '42 to '48 ..........................•......................... G-25

Mack Jr. '37 ·G-20

Mack '37 EH, EJ G-25

Massey Harris Pony G-20

McCormickFormall, Super C, H, Super M G-25

Mercury to '40 G-25

Mercury '52; '53 early (Station Wagon G-20) G-21

Mercury late '53 to '55 ('55 Station Wagon-G-20) G-22

Nash '26 to '51; Metropolitan '54, '55; Rambler '52 G-20

Nash Lafayette '35 to '39 G-20

Oldsmobile '27 to '40 except Torpedo (all to '55 optional) G-20

Packard '31 to '39 except 120, 115, 110 G-25

Packard '31 to '39120,115,110; '40 6 & Small 8 '" G-20

Pierce Arrow '29 to '38 G-20

Plymouth '29 to '52, and '53 to '55 Station Wagon , " '" '" .. G-20

Plymouth '53, '54 except Station Wagon G-22

Plymouth Trucks to '54 , G-20 or G-23

Pontiac '26 to '40 except '40 Torpedo (all to '55 optional) " G-20

Reo '33 to '36 G-20

Reo Truck '37 to '55 most models G-20 or G-23

Reo Truck '37 to '55 some models G-25

Studebaker all '28 to '39; Champ '40 to '42 (all to '54 optional) G-20

Studebaker Truck '37 to '55 G-20 or G-23

Terraplane '35 to '39 G·25

White Truck 620,631; '37700,800 series; '41 to '48 WA, WB G-25

Willys '474-63 St. Wag. and Pon., 2 WD 3,4 ton, 4 WD, 1 ton G-21

Willys '48 4-463 St. Wag. % ton and Jeepster; 6-663 Sed. St. Wag. 2 WD 3,4 ton,

4 WD 1 ton (4 W. Dr.). G-21

Willys '49 4-463 Jeep, Jeepster, 2 WD.........•.......................... G-21

Willys '49 4 VJ-3, 6 VJ-3-6, 6-643 Jeep Sed. St., St. Wag G-20

Willys '50 4-463 '" , G-20

Willys '50 4-473 WD, 473 HT, 4-473 SW, 473 SD, 4-473 VJ, 4-VJ3, 6-673 SW,

6-673 VJ, 6-663, 6-VJ-36 .. , , , . G-21

Willys '51 to '54 4-473 SW, 4-473 VJ, 473 HT, 6-673 SW.. , " G-21 or G-23

Willys CJ-2A, CJ-3A G-25

Willys Aero Ace, Wing, Lark, all. , G-21

• 6 •

Edited July 8, 2016 by knuckleharley

[medium_jon]

I've wondered about this for some time. Is the 'vented' nature of the caps to:

• allow air into the tank to replace the fuel removed my the pump?

OR

• allow vapor to escape? As in the pressure from rising temps (my favorite formula PV = nrT)

The two I've had on my P20 don't do the second. But I think they have had no problems with the first. 

[Branded]

EPA won't allow gas caps to vent to atmosphere, the caps can allow air in to the tank but not out. I purchased a new cap and what I done was pried the check valve out of the cap with a screw driver, problem solved, no more building pressure inside the tank

Edited August 5, 2017 by Branded

[knuckleharley]

1 hour ago, Branded said:

EPA won't allow gas caps to vent to atmosphere, the caps can allow air in to the tank but not out. I purchased a new cap and what I done was pried the check valve out of the cap with a screw driver, problem solved, no more building pressure inside the tank

It would have been easier to have just bought a non-pressurized gas cap like originally came on your car. The EPA can't ban those caps.

[Branded]

13 hours ago, knuckleharley said:

It would have been easier to have just bought a non-pressurized gas cap like originally came on your car. The EPA can't ban those caps.

Well, it took less than a minute to pop the check valve out of the cap I bought from Napa for $7 bucks, or I could have taken the easy way out and bought the NOS cap on E Bay for $45 buck. I like my idea better ;-)

[medium_jon]

Providing I don't have fuel leaking or fuel odor in the vehicle, does the build up of pressure in the tank do any harm?

[Branded]

10 minutes ago, medium_jon said:

Providing I don't have fuel leaking or fuel odor in the vehicle, does the build up of pressure in the tank do any harm?

I would think the pressure build up is going to possibly push fuel past the needle and seat, could cause flooding problems, diaphragm issues with the  fuel pump, gaskets and seals. 

[knuckleharley]

4 hours ago, Branded said:

Well, it took less than a minute to pop the check valve out of the cap I bought from Napa for $7 bucks, or I could have taken the easy way out and bought the NOS cap on E Bay for $45 buck. I like my idea better ;-)

It would have been ever easier to tell the guy at NAPA that you wanted a non-pressurized cap.

 

I posted a list of Stant gas and radiator caps here from a 1955 catalog a while back,and I'm sure it's still here.

 

Edited August 6, 2017 by knuckleharley

[Branded]

6 hours ago, knuckleharley said:

It would have been ever easier to tell the guy at NAPA that you wanted a non-pressurized cap.

 

I posted a list of Stant gas and radiator caps here from a 1955 catalog a while back,and I'm sure it's still here.

 

That is what I'm trying to explain, yes you can buy a vented cap for our cars at Napa and most other parts stores. The problem is they are vented only to allow air in, the check valves are mandated by the EPA to not allow gas vapors to escape to atmosphere. I'm not nor will I turn this into a whizzing match. I stated how to resolve the issue on the check valve to allow the tank to vent both ways. I'll leave it at that sir.

[knuckleharley]

2 hours ago, Branded said:

That is what I'm trying to explain, yes you can buy a vented cap for our cars at Napa and most other parts stores. The problem is they are vented only to allow air in, the check valves are mandated by the EPA to not allow gas vapors to escape to atmosphere. I'm not nor will I turn this into a whizzing match. I stated how to resolve the issue on the check valve to allow the tank to vent both ways. I'll leave it at that sir.

All I can say is I had no problem using the gas cap I bought using the Stant part number on my 51 Ford,which requires a vented cap. I bought it last year,and it is a locking gas cap.

I bought it from Amazon because I didn't want to make a 50 mile round trip to buy a gas cap,and they are cheaper anyhow,but where I bought it from is irrelevant because I ordered the same parts number.

[Branded]

32 minutes ago, knuckleharley said:

All I can say is I had no problem using the gas cap I bought using the Stant part number on my 51 Ford,which requires a vented cap. I bought it last year,and it is a locking gas cap.

I bought it from Amazon because I didn't want to make a 50 mile round trip to buy a gas cap,and they are cheaper anyhow,but where I bought it from is irrelevant because I ordered the same parts number.

There lies the difference, from what I've read the locking gas caps for some reason vent both ways.