Normal fuel tank pressure - what's the design intent?

[Timothy B.]

Looking for fuel system experts!

Hello tech discussion board after about 15 years!! I used to post and respond allot years ago and have maintained my membership, but the Vette has been in the garage and taking a backseat to my son born in 2010....;-) Now I am trying to get back into working on the car and driving it more often.

Getting the car running more often is a challenge, and especially frustrating are things that I already fixed and replaced before and now having to do again due to time deterioration....uugh! I am sure ya'll know that feeling!

Anyway, one thing I am working on now on my 67 L79 car is heat induced leakage through the carb, and an item often mentioned is making sure the fuel tank is properly vented. Holley tech support mentioned this as something to check. I have an original fuel cap on my 67 that has always been fine and says "vented" on it. I have checked out the one way valve and it seems to behave normal for the valve (based on what others have reported in searches) - when you blow through the small hole it seals, and when you suck on the small hole it allows air into the tank. So that protects the tank from vacuum and makes sense.

When tank pressure builds in the tank it appears the cap is designed to vent only when the pressure builds to a point where it can unseat the large seal of the cap pushing against the bigger spring inside the cap. When you push by hand on the inside body of the cap against the outside of the cap, it deflects at around 30 lbs roughly (rough measure using kitchen scale). The diameter of the pressure area of the cap is about 3 inches, or a surface area of about 7 in^2. That would be about 4.3 psi tank pressure to deflect the 30# spring and vent... Does anyone know if the intent to have the tank build to about 4+ psi naturally before venting? For reference, I see folks post that with a "vented" cap they should not hear venting when they remove the cap, but 4 psi pressure in the tank would definitely create a hiss when you open the cap. I can't find anywhere it reported what the normal and design vent pressure in the tank is supposed to be. Just looking at the cap design, it seems like whats described above is the only way the cap can vent pressure unless I am missing something. If there are any vented fuel cap experts out there please let me know if I have this right or not, and please point me to more details.

[Duke W.]

Re: Fuel system experts! Normal fuel tank pressure?

My '77 P & A catalog, Group 3.208, lists fuel filler cap, 3952708, as applicable to '63 to '69 model years and notes that this cap is "vented".

For '70-'74 cap 3994339 is listed; '75-'76 is 7470051 and this cap must have been for a filler neck with the restrictor that only allows the small diameter unleaded fuel nozzle to be inserted. The note "non-vented" applies to both these caps. The part number for '77 changed and again in '78.

The cap on my SWC is definitely original and is vented both ways. It doesn't have a part number that I can see, but since part number starting with 39 did not appear until sometime later in the sixties. the 3952708 must be a later released part that was applicable back to '63.

What is the fuel filler cap part number in the '67 AIM? I have a vague recollection that it has either a pressure relief or both pressure and vacuum relief valves. With the beginning of evap emission control systems, caps were either non-vented with vacuum and pressure relief valves somewhere in the fuel feed system or built into the cap and my understanding that they open at about +/- two psi.

The significant spring force you feel likely has nothing to do with venting. It's there so the gasket is firmly compressed so liquid fuel cannot spill out. It may also act as a "safety valve" to vent pressure in case the normal vent channel somehow gets plugged.

You may need to apply more pressure to the vent hole to open the valve, assuming it has one.

Duke

[Mark F.]

Re: Fuel system experts! Normal fuel tank pressure?

...What is the fuel filler cap part number in the '67 AIM?... Duke

Duke - Looks like 3843576 in my '67 AIM...dunno if that helps or not?
UPC 8 Sheet B4

[Timothy B.]

Re: Fuel system experts! Normal fuel tank pressure?

Thanks all and nice to hear form you Duke! Been years since I have had the pleasure! I will check it again when home later, but best I could tell the valve inside the cap is only a one way valve to open and relieve vacuum in the tank. It seems to just seal on pressure inside the tank. I will check it again and see if I can determine if at any pressure it will vent.

[Joe L.]

Re: Fuel system experts! Normal fuel tank pressure?

My '77 P & A catalog, Group 3.208, lists fuel filler cap, 3952708, as applicable to '63 to '69 model years and notes that this cap is "vented".

For '70-'74 cap 3994339 is listed; '75-'76 is 7470051 and this cap must have been for a filler neck with the restrictor that only allows the small diameter unleaded fuel nozzle to be inserted. The note "non-vented" applies to both these caps. The part number for '77 changed and again in '78.

The cap on my SWC is definitely original and is vented both ways. It doesn't have a part number that I can see, but since part number starting with 39 did not appear until sometime later in the sixties. the 3952708 must be a later released part that was applicable back to '63.

What is the fuel filler cap part number in the '67 AIM? I have a vague recollection that it has either a pressure relief or both pressure and vacuum relief valves. With the beginning of evap emission control systems, caps were either non-vented with vacuum and pressure relief valves somewhere in the fuel feed system or built into the cap and my understanding that they open at about +/- two psi.

The significant spring force you feel likely has nothing to do with venting. It's there so the gasket is firmly compressed so liquid fuel cannot spill out. It may also act as a "safety valve" to vent pressure in case the normal vent channel somehow gets plugged.

You may need to apply more pressure to the vent hole to open the valve, assuming it has one.

Duke

Duke-----


Early 1963 used fuel cap GM #3820865. As I seem to recall, this cap used a simple, small hole for venting of both pressure and vacuum. Later 1963 through early 1968 used cap GM #3843576. This cap had an integral pressure/vacuum valve.

Sometime during the 1968 model year, the cap was changed to GM #3921689. It replaced the 3843576 in July, 1968 for SERVICE of all 1963-68. It was used in PRODUCTION for only part of the 1968 model year, led a short life and was discontinued and replaced in May, 1969. I know nothing else about this cap.

For 1969, cap GM #3952708 was introduced to PRODUCTION and SERVICE. It replaced the 3921689 in May, 1969 and was used in PRODUCTION for only the 1969 model year but, of course, was SERVICE for all 1963-69 until it's discontinuation in the last several years. It has an integral pressure vacuum valve. Based on physical comparison between the 3843576 and 3952708 the only difference that is apparent is that the former uses a thin, brown, cellulosic gasket whereas the latter uses a thicker neoprene rubber gasket.

In late 1969, a pressure/vacuum valve was added to the Corvette fuel tank. However, while this made the vented fuel cap redundant, the vented fuel cap was used in PRODUCTION through the end of the 1969 model year.

For 1970, cap GM #3974743 was introduced to PRODUCTION. This is a sealed cap with no integral pressure/vacuum valve. It had a "scalloped" perimeter and, except for the "SEALED" embossment and other embossed script, appeared like the earlier caps. Some of these caps have a red plastic placard that advises to "open slowly". This cap may have been used into early 1971.

For 1971, caps GM #3983384, 3994335, and 3994339 were introduced. These are also sealed caps, are so embossed, but have a smooth outer perimeter. The 3994339 was used through 1974.

All of the above referenced caps are large diameter, Corvette-only parts.

For 1975 and later, Corvettes transitioned to the much smaller, "corporate-wide" type caps.

[Leif A.]

Re: Fuel system experts! Normal fuel tank pressure?

Duke-----


Early 1963 used fuel cap GM #3820865. As I seem to recall, this cap used a simple, small hole for venting of both pressure and vacuum. Later 1963 through early 1968 used cap GM #3843576. This cap had an integral pressure/vacuum valve.

Sometime during the 1968 model year, the cap was changed to GM #3921689. It replaced the 3843576 in July, 1968 for SERVICE of all 1963-68. It was used in PRODUCTION for only part of the 1968 model year, led a short life and was discontinued and replaced in May, 1969. I know nothing else about this cap.

For 1969, cap GM #3952708 was introduced to PRODUCTION and SERVICE. It replaced the 3921689 in May, 1969 and was used in PRODUCTION for only the 1969 model year but, of course, was SERVICE for all 1963-69 until it's discontinuation in the last several years. It has an integral pressure vacuum valve. Based on physical comparison between the 3843576 and 3952708 the only difference that is apparent is that the former uses a thin, brown, cellulosic gasket whereas the latter uses a thicker neoprene rubber gasket.

In late 1969, a pressure/vacuum valve was added to the Corvette fuel tank. However, while this made the vented fuel cap redundant, the vented fuel cap was used in PRODUCTION through the end of the 1969 model year.

For 1970, cap GM #3974743 was introduced to PRODUCTION. This is a sealed cap with no integral pressure/vacuum valve. It had a "scalloped" perimeter and, except for the "SEALED" embossment and other embossed script, appeared like the earlier caps. Some of these caps have a red plastic placard that advises to "open slowly". This cap may have been used into early 1971.

For 1971, caps GM #3983384, 3994335, and 3994339 were introduced. These are also sealed caps, are so embossed, but have a smooth outer perimeter. The 3994339 was used through 1974.

All of the above referenced caps are large diameter, Corvette-only parts.

For 1975 and later, Corvettes transitioned to the much smaller, "corporate-wide" type caps.

Like this...

[Duke W.]

Re: Fuel system experts! Normal fuel tank pressure?

Duke - Looks like 3843576 in my '67 AIM...dunno if that helps or not?
UPC 8 Sheet B4

Thanks...that jibes with Joe's post on the history of the C2/3 fuel filler caps, and he said the ...576 has integral vacuum and pressure relief valves. They should both open at about 2-3 differential psi, certainly no more than 4, so we'll have to wait until Timothy gets back with more tests.

My SWC's cap (mid March production) is definitely "vented". By that I mean that air flows freely in either direction from the vent hole on the bottom side, so no vacuum or pressure relief valves, which means it must be the ... 863 cap that Joe said was used on "early" '63s.

A cap with built in pressure relief valves should help keep humid air from entering the tank and contaminating the fuel with condensed moisture. If I had known about this back in the day I would have bought a later replacement cap with the pressure/vacuum relief valves, but at this point since my SWC never sees condensing humidity, it's not a big deal.

Thanks for chiming in with the cap history, Joe.

Duke

[Timothy B.]

Re: Fuel system experts! Normal fuel tank pressure?

I checked what I believe to be my original 67 cap again and the small relief valve only relieves and allows flow in the direction of INTO the tank, thru the small hole in the cap. It would prevent the tank from developing negative pressure. I tested with higher pressures and it does not relieve in the other direction at any pressure - it only seals. Its basically a check valve. The only way I can see that this cap would ever relieve pressure inside the tank is when the larger diameter of the cap seal unseats due to the larger spring inside the cap. From my calculation as shown above in my original post, that would be about 4psi or so.

Has anyone ever seen or know of a tech article on C2 caps that thoroughly discusses how the relief valves are supposed to function?