Fuel tank pressure
Thread Starter
Le Mans Master
Joined: Aug 2010
Posts: 5,339
Likes: 546
From: Central Texas
Fuel tank pressure
I've been following the posts on built up fuel tank pressures and vented/non-vented fuel caps. Most of these threads are addressing older models than mine.
My '81 makes expanding noises in the tank as the outside temperature rises. I understand that it is supposed to have a non-vented fuel cap. I purchased a new cap just to make sure that wasn't my problem. There is a definite release of pressure when opening the cap to put gas in.
I replaced my worn fuel lines at the pump the other day. The small 1/4" metal line going to the pump had fuel under pressure in it. I had to open the fuel cap to stop the pressure. My car has a connected vapor canister.
Should the tank have this built up pressure in it? Does it seem as if everything is working as it should?
My '81 makes expanding noises in the tank as the outside temperature rises. I understand that it is supposed to have a non-vented fuel cap. I purchased a new cap just to make sure that wasn't my problem. There is a definite release of pressure when opening the cap to put gas in.
I replaced my worn fuel lines at the pump the other day. The small 1/4" metal line going to the pump had fuel under pressure in it. I had to open the fuel cap to stop the pressure. My car has a connected vapor canister.
Should the tank have this built up pressure in it? Does it seem as if everything is working as it should?
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
The vapor canister is an 'open' device. The fumes come in, pass through the charcoal and the remaining air goes out the bottom. The tank should essentially be at atmospheric pressure.
But, if the vent line is blocked...or if the charcoal media is clogged, pressure could be generated. Of course, the system wouldn't be doing its job, but either situation would cause elevated pressure.
If the non-vented gas cap has a pressure relief in it (if so, I don't know about it), that relief pressure would be higher than atmospheric, so pressure could build until the relief valve opened. Someone more knowledgeable than me on vented/non-vented cap design will have to clarify that issue.
P.S. Hey, another possibility is that crankcase pressure could be 'backwashing' into the fuel tank if the PCV purge line was blocked or not routed properly. Otherwise, I'm having trouble figuring out how the tank would even build pressure. With fuel flowing out, it should create a weak vacuum in the tank, if it's sealed.
But, if the vent line is blocked...or if the charcoal media is clogged, pressure could be generated. Of course, the system wouldn't be doing its job, but either situation would cause elevated pressure.
If the non-vented gas cap has a pressure relief in it (if so, I don't know about it), that relief pressure would be higher than atmospheric, so pressure could build until the relief valve opened. Someone more knowledgeable than me on vented/non-vented cap design will have to clarify that issue.
P.S. Hey, another possibility is that crankcase pressure could be 'backwashing' into the fuel tank if the PCV purge line was blocked or not routed properly. Otherwise, I'm having trouble figuring out how the tank would even build pressure. With fuel flowing out, it should create a weak vacuum in the tank, if it's sealed.
Last edited by 7T1vette; Aug 8, 2012 at 11:39 PM.
