DUB

This is what Ernie sent me:

I have to give credit on my knowing this to three things.. My old notes on the hard drive, John Hinckley and Noland Adams...

Referring to a note that I had from JohnZ, and also referring to the publication of my friend Noland Adams' restoration guide...

On page 94 of the mid-year book it describes the 63-67 vented gas cap (same cap 63-69) as able to relieve both pressure and vacuum both ways. It only vents internal tank pressure above a certain PSI, so while it's not open to the atmosphere pressure it also opens at a certain vacuum level below atmospheric pressure to allow air to replace the volume of fuel used.

SO if I am reading that correctly...The cap does allow air in...but it ALSO can allow pressure to escape when the pressure is great enough.

AS for cars leaking fuel sitting there. I can not dispute any of those replies. I not here to be combative.

SO with the person who started this thread has that much pressure....something is obviously wrong.

DUB

BLUE1972

Hey - never a problem... it's just info.. I always love your work and pictures.

I double checked 2 sites and my 1941 & 61 service Manuals before posting.

The evolution is from a gas cap manufacturer..

It is scary what we had in the pre 70 cars.

I guess we were lucky that the garages and homes were not sealed that good or there may have been a lot of disasters.

I usually let my 71 cool down before bringing it into the shop for the night. If I don't there is a gas smell later / in the morning - it's from the carb.. in my case.

When I was a kid there were state leased gas stations near the beach and the price was low - so a lot of people filled up there. (no name gas) So full tank of cold gas - hot tar parking lot ,,, and drips. I guess it went with the down draft tubes for the crankcase on pre 65 cars... before PCV.

gleninsandiego

So here is a heads up for 69 owners. I did have a 69 that was a barn find that had been sitting for a number of years. That cap appeared to be original and when I tested it there was no venting in either direction. Not good. So I took a blunt small rod and poked it in the vent hole. After lightly pressing it a few times the vent started working again. But then again I may have just poked a hole in whatever diaphram is in there. LOL

So what is happening with the OP tank?

DUB

DUB

azchris

Thanks for everyones input.

I tested the new fuel cap out today. After my 8 mile drive in 75 degree weather there was no noticeable pressure differential when I pulled the cap off. However, 5 hours later I drove the same 8 miles back in 60 degree weather and after stopping at the gas station there was noticeable pressure release when I twisted off the cap.

My fuel level was about 1/4 tank when I stopped for fuel.


Does anyone else ever have any residual pressure when they pull off their fuel cap?

Thanks,

gleninsandiego

Fascinating.

Have you confirmed you can blow out on the gas cap with your mouth?
As we have seen in the above posts, some caps labeled vented do
not let air pressure out. 69 gas caps are unique.

Glenn in San Diego

7T1vette

I believe that NO fuel tank should ever build pressure internally. That's why ALL tanks have some way to vent pressure. Normally, that is so a 'negative' pressure cannot result as fuel level is depleted. But, it also prevents any building of pressure which could balloon the tank or cause problems with the carb handling excessive fuel pressure.

Before the vapor recovery systems were instituted, most tanks had a simple vent on the top of them. When vapor recovery systems came along, THEY became the 'vent' for the tank and the actual tank vents became unnecessary and were removed. Some folks who rip out their vapor recov systems end up with tank pressure problems...unless they install vented gas caps.

I have no idea how your tank can be building pressure, unless it is doing so from heat transfer from your exhaust system. But, you need a vented cap on the tank (which vents both ways).

flyeri

I never had a positive pressure issue until I made a couple of changes. I have a Holley mechanical pump that makes about 9 psi. It was over powering the inlet valve on my Edlebrock carb. I had a regulator so I added that in-between the pump and carb. No tank pressure at this point. On very hot days the car became hard to start when hot. Lars had a post about a return line coming from the fuel log on a Holly carb that was metered down to allow fuel to continually move through the system. So I made up a similar system and plumbed in a return line (had no return previously). After that on 95 degree days I noticed the pressure but only after filling up and driving a couple hours and stopping for a refill. When I installed the return line in the tank I did not run the line down to the bottom of the tank so as the tank empties there is some aeration of the fuel because of the stream running into it. I do not know if the pressure is caused by the rise in temp of the fuel on a hot day or from the aeration or a combination of both. It is not a tremendous pressure as you have described but it is there on the hot days. I know that offers no solution but there are some check valves that can be put in a vent line that will stay closed until you have a small positive pressure and then vent. The cap will allow air in, the vent line will allow pressure out, and the check valve will keep fumes out of the garage.

7T1vette

You just need a FULLY-vented gas cap. This whole exercise is making the proverbial "mountain out of a molehill"....

Tonio

I had the opposite problem on my '72....vacuum pressure was building up in my tank actually partially collapsed the tank! I posted photos of this in some other thread. Anyway, the PO blocked off the vent ports AND used a non-vented cap. I drilled a tiny hole in the cap. Vacuum problem solved. No smell of gas in the garage.

flyeri

Except that the replacements today only allow air in. If you can supply a source I will buy one and I'm sure the op will also. So far no one has be able to.

gleninsandiego

https://www.corvettecentral.com/c3-6...f%3fcount%3d18

This is what I have on my 69 that vents both ways
Notice the years

Glenn in San Diego

7T1vette

Just drill a very small hole thru the cap. The cap is way above the fuel level. What's your problem????

CanadaGrant

The vented cap has a hole that is about a 1/16 of an inch in the underside. It is vented both ways but has a check valve between the lower and upper portion of the cap. The check valve is there to prevent fuel from pouring out the vent hole in a roll over but not the slow release either way of atmospheric air pressure changes due to temperature.The check valve sometimes sticks. Then you need a new cap. If it's new and it sticks? Then you need another new cap.
The quadrajet on the early C3's has a vent hole that is 4 to 5 times larger than the vent hole in the gas cap and is percolating after use all night in your garage from engine heat. That would be your gas smell. Not the cap. The cap is your ONLY vent on a 69 so if there is a pressure problem, look no further...
Put a new cap on, or drill a small hole from the bottom hole in the cap through the sticking valve. You can also release a valve that is stuck in the closed position by pushing a thin metal probe or pick through the underside hole in the cap to release it. There is no vent hole in the top of the cap because it is two piece and it vents out all around the perimeter of the upper cap from the hole in the underside.
The cap vents BOTH ways through the same hole but has a valve in case of an upset. That's so you don't get a load of gas leaking through the open back window onto the back of your head (or right on it with a convert) if you turn it upside down. Air pressure changes due to temperature are OK but sudden large changes on the cap are not.

If the tank doesn't vent on a 69 (it doesn't) then there is no other vent other than the vented cap.

flyeri

Thanks GW. That is what I was looking for.

azchris

I tried and could not blow through it. So I stuck a paper clip end
through the tiny hole and felt it unstick as I pressed up. After that I
could feel a small spring pushing down against me. Must of been stuck.

Now I can blow a small amount of air through it. I guess these Stant
brand caps while new must be very "old" stock.

Thanks!

gleninsandiego

Now we are talking!!!
As mentioned, the same thing worked for me
So we now have a solution

Paperclip first
and an option to buy listed above



OP, let us know if that worked on your overall problem

Destry

I had a crack in my fuel tank when I bought it a few years ago. I replaced the tank. I never had any trouble starting or driving but I noticed pressure in the tank when refueling. I always had a heavy gas smell. After being parked for some time, I decided to track down the culprit and found a leaking return line hose from the filter to the metal line. I replaced it and went for a short drive and refueled the tank. The next morning (maybe 3 mornings)I had a huge gas puddle underneath. I looked at the carb and it was leaking past the needle valves. It didn’t stopped until I removed the gas cap.

I found this thread and THANK YOU!

Unfortunately the gas has leaked passed the oil rings and filled my crankcase. I have drained the oil and removed the filter. I plan on removing the spark plugs and letting it vent for a few days. Hopefully I didn’t break anything.

My gas cap is old and rusty and doesn’t vent either way. Going to get one that works and vents in and out!!!