Gas tank pressure..
Thread Starter
Instructor
Joined: Nov 2008
Posts: 170
Likes: 0
From: Odessa Tx
Gas tank pressure..
Why is my gas tank full of pressure. When go to fill up and pull the cap a bunch of air comes out of the tank because of pressure build up. Whyat causes this and what can this effect?
Le Mans Master
Joined: Jul 2007
Posts: 5,043
Likes: 8
From: Tampa Florida
It is normal to have some pressure in the tank....it should not be excessive though. Open the gas lid slowly and count how long it takes the pressure to release next time you fill the tank at the pump.
Drifting
Joined: Sep 2003
Posts: 1,363
Likes: 7
From: 1994 LT1 Coupe 6-speed with FX3 & 2000 LS1 Vert 6-Speed with F45 Hunterdon County, NJ
it might also have something to do with the type of driving you do... The tank pressure purge process only occurs during very specific circumstances (check your FSM).
Also, to you tend to keep a full or 1/2 full tank or often run it down to nothing -- less fuel, in your hotter climate could contribute to more vapor and thereby more pressure.
Also, to you tend to keep a full or 1/2 full tank or often run it down to nothing -- less fuel, in your hotter climate could contribute to more vapor and thereby more pressure.
Advanced
Joined: Dec 2008
Posts: 71
Likes: 1
From: TX
I have vacuume not pressure in my tank, my guess is so do you. When fuel is removed and air can't replace it ,it causes a lower pressure condition, ie vacuume. I was told it was normal. I leave my fuel cap a little loose becouse is causes me to knock under heavy loads when the vacuume gets to high.
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
Are you saying a vacuum exists when you open the cap; ie, atmospheric pressure is getting sucked in? Port injection works by moving fuel at 38 to 45 psi. And what isn't squirted through the injectors is returned to the tank. The system is sealed to prevent the nasty stuff from escaping into what we breathe and it's either re-circulated back through a charcoal canister or held in the tank when the engine is shutdown. And when the engine is shutdown, normal atmospheric pressure returns to the engine, which if there wasn't something keeping the fumes in the tank, they would, because the tank pressure is above atmospheric, be drawn towards the canister and potentially back into the engine which would make it difficult to restart. So, if you have a vacuum when you open the cap, the most likely place to look is a check valve that's suppose to keep the stuff in the tank until it's running again. That's usually mounted midrail, on the frame, in the engine compartment. On the other hand, if there's a release of pressure, that's normal given system design,and there's usually a warning on the cas cap or fill area telling you to remove the cap slowly until the pressure has bled off.
Thread Starter
Instructor
Joined: Nov 2008
Posts: 170
Likes: 0
From: Odessa Tx
Well its a bunch of pressure. Not just a little bit. However, thanks to some of the responses talking about the charcoal canister...The guy that had the car before me canned the evaporative emmisions control system (that big box in the left front fender) and the charcoal canister was plugged off. It still is hooked up to the tank but its blugged at the canister. For now I unplugged it and havent got that much pressure again. I will have to get that box from somewhere..
Advanced
Joined: Dec 2008
Posts: 71
Likes: 1
From: TX
yes - air is sucking in- at first i thought it was pressure (the hissing sound when I removed the fule cap) but closer look and it was sucking in. So if more fuel is used then returned and it is a semi closed system, one will get a vacuum, right, not pressure. How do you check the valve?
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
There's no vacuum or negative pressure about it - though air is drawn into the engine with negative pressure, fuel is moved from the tank by pressure (carb or injected) and returned to the tank under pressure - it wouldn't get there otherwise. Pressure helps moves the fumes with negative pressure sucking it out of the canister. You don't say what Year you have but most checkvalves are tested by applying 15 inches of vacuum and seeing if it can hold it for 20 seconds or longer. Also, while under vacuum, you should be able to blow air from the tank side to the canister side, and even though there is a restricted vent, virtually nothing should go through it when vacuum isn't applied.
Advanced
Joined: Dec 2008
Posts: 71
Likes: 1
From: TX
'91 - I have a a/c vacuum pump that shoud work for the test. It might be alittle over kill.
I guess my point is: Take a 2 liter bottle filled with your faviote fluid. close the system except a fitting at the bottom and one at the top. pump fluid out and return less, what happens to the bottle? It collapse.
I guess my point is: Take a 2 liter bottle filled with your faviote fluid. close the system except a fitting at the bottom and one at the top. pump fluid out and return less, what happens to the bottle? It collapse.
Le Mans Master
Joined: Jul 2007
Posts: 5,043
Likes: 8
From: Tampa Florida
I get where your going with your using fuel so the bottle colapses. Fuel/fuel vapor can both expand and contract. This test works too: put some gas in a plastic bottle about a 1/4 full put a cap on it and shake it...the bottle expands.....or you can just set the bottle in the sun and watch what happens. The check valve / charcoal canister is there to prevent excessive vapor build up in the tank and release to the canister while the car is parked or turned off. The vapor canister, if connected and working properly, should pull a minor vacuum on the tank while the car is running to remove the extra fuel vapor that would build up from the fuel recirculation in the tank. Most fuel caps are vented to release pressure if it goes above or below a pressure of 2 lbs.
This has always been my understanding of the system......
This has always been my understanding of the system......
Le Mans Master
Joined: Aug 2000
Posts: 8,838
Likes: 31
From: Australia
yes - air is sucking in- at first i thought it was pressure (the hissing sound when I removed the fule cap) but closer look and it was sucking in. So if more fuel is used then returned and it is a semi closed system, one will get a vacuum, right, not pressure. How do you check the valve?
Maybe it is kaput not allowing air into the tank.
On the cap you should be able to suck from the tank side but no blow back through.
Pro
Joined: Oct 2004
Posts: 628
Likes: 6
A few opinions of my own:
The system is designed to be pressurized - it allows for vapor burn-off via the canister system, and also prevents vapors from entering the atmosphere (note- this is why the emissions test in GA tests the gas cap can hold pressure).
The gas tank will naturally pressurize even with the engine off - just recently I was doing some work on the fuel rails on my '92 and I put the gas cap on overnight (fuel rails still disconnected). The next morning, with nice sunshine warming the garage, I had a very large puddle of gas on the floor via the open fuel rails.
It is well known that gas expands with temperature (hence the advice to fill up your car late evening or early morning, you get more gas for your gallon).
With the engine running - with the fuel being pumped to the rails at 38to 43 psi and returned under pressure, even though there is a net loss of 'gallons of fuel' there is a net gain of 'pressure'. {Edit - I'm not sure I'm correct about this, but I'm happy to discuss}
So.. in my opinion... it is working as designed. I would not be concerned about the hissing noise.. all my vehicles do it.
The system is designed to be pressurized - it allows for vapor burn-off via the canister system, and also prevents vapors from entering the atmosphere (note- this is why the emissions test in GA tests the gas cap can hold pressure).
The gas tank will naturally pressurize even with the engine off - just recently I was doing some work on the fuel rails on my '92 and I put the gas cap on overnight (fuel rails still disconnected). The next morning, with nice sunshine warming the garage, I had a very large puddle of gas on the floor via the open fuel rails.
It is well known that gas expands with temperature (hence the advice to fill up your car late evening or early morning, you get more gas for your gallon).
With the engine running - with the fuel being pumped to the rails at 38to 43 psi and returned under pressure, even though there is a net loss of 'gallons of fuel' there is a net gain of 'pressure'. {Edit - I'm not sure I'm correct about this, but I'm happy to discuss}
So.. in my opinion... it is working as designed. I would not be concerned about the hissing noise.. all my vehicles do it.
Last edited by djxib; Jan 4, 2009 at 09:44 AM.
Melting Slicks
Joined: Feb 2006
Posts: 2,753
Likes: 15
From: backwoods upstate ny
Le Mans Master
Joined: Jan 2008
Posts: 7,030
Likes: 10
From: Edmonton, Alberta
because of the location and design of the C4 filler. (been there) I'm with the others who say you shouldn't have a vacuum effect
when you open the cap.
It seems something is amiss somewhere
Jim
Le Mans Master
Joined: Aug 2000
Posts: 8,838
Likes: 31
From: Australia
This alone would raise the vapour pressure on return to the tank.
