Gas tank pressure - air rushes out when the the gas cap is unscrewed

[rwljpl]

A lot of times especially when I’ve been driving for a while, when I unscrew the gas cap, it lets out a strong rush of air. Air is definitely being pushed out.

My truck has a crate 350 and had no real emissions equipment that I know of. I’ve never dropped the tank on this truck so I don’t know anything about the fuel sending unit on it. My guess is that the tank is getting pressurized because the breather is clogged.

I assume having that much pressure in the system is not good. Can I fix the problem by getting a vented gas cap? Or do I need to drop the tank and replace the breather?

Or is the problem something else that I’m missing?

 

[HotRodPC]

You can use a vented cap if you want but by 87 and TBI IIRC, emissions required the evap system to take care of the fumes. It is normal for it to build some air sometimes. Different factors cause it like the temp, elvevation etc. If it's extremely excessive your Evap system or charcoal canister isn't working right, just vent it with a cap and be done and piss off some more Prius Libtards.

 

[rwljpl]

My truck is an 85 but does not seem to have any emissions equipment
I’m not sure it has an evap system of any kind or charcoal canister
The engine is a crate 350 with a carburetor
I might just go with a vented cap and see what happens

 

[SirRobyn0]

Hotrodpc is exactly right, some pressure sometimes as in a slight amount. Much more than just a quick small amount of pressure is a problem. Yes, a vented cap will stop that from happening but like Hotrod said, it should have a evap system and canister. The evap systems are pretty simple and if it were me I'd want it working. The vented cap is fine short term and better than building excessive pressure, but the evap system makes the system more of closed system, and besides keeping gas vapor from getting out, it helps to keep moisture from getting in.

 

[Blue Ox]

My '85 doesn't have evap or anything else like it and I get a hiss when I open the caps. It's not really prolonged, and I've never bothered to see if it's pressure or vacuum. Original caps marked Diesel Fuel Only and it's done it since day one. I guess it all depends on how much hissing the OP is experiencing.

 

[rwljpl]

Bear with my ignorance....what would it take to install an evap system?

 

[SirRobyn0]

Bear with my ignorance....what would it take to install an evap system?

Well first you should look to see if you have one, and if it's functional. On my 1984 C20 the canister, which looks like a big black coffee can is on the drivers side right next to the radiator. Check to see if that is there and then we can work on seeing if the vac lines are all there hooked up and in good shape. But basically off the canister there should be one line to a vacuum source or a vacuum switch, a large line to the bowl vent on the carb, and a 1/4 line running back to the fuel tank switch and then to the tanks assuming you have duel tanks. Really it's pretty simple.

 

[Camar068]

it helps to keep moisture from getting in.

hmmm.....need to fix mine then

 

[rwljpl]

No canister by the radiator.
Just one tank.

 

[RecklessWOT]

excuse my ignorance, but what is really wrong with having pressure in the system? Is it just the possibility of a ruptured tank/line or will it cause driveability issues or something? I half-assed started removing all the emissions stuff on my truck (I am aware of the "benefits" of having a PCV and EGR etc, I still don't want them because reasons) but never finished tidying everything up so it isn't completely finished, so the truck is in a weird in between stage. One thing I never got around to doing was installing a breather in place of the EVAP system so it's basically just completely closed off, that does cause pressure in my tank and it will hiss/whistle/blow a sizeable amount of air for a good 5 seconds after removing the cap sometimes, once in a while it'll actually blow the cap out out into my hand when I'm unscrewing it. Obviously I know it's not normal and I had planned on doing something to address the issue eventually, but is it really a reason to be concerned? What problems can it cause?

 

[SirRobyn0]

excuse my ignorance, but what is really wrong with having pressure in the system? Is it just the possibility of a ruptured tank/line or will it cause driveability issues or something? I half-assed started removing all the emissions stuff on my truck (I am aware of the "benefits" of having a PCV and EGR etc, I still don't want them because reasons) but never finished tidying everything up so it isn't completely finished, so the truck is in a weird in between stage. One thing I never got around to doing was installing a breather in place of the EVAP system so it's basically just completely closed off, that does cause pressure in my tank and it will hiss/whistle/blow a sizeable amount of air for a good 5 seconds after removing the cap sometimes, once in a while it'll actually blow the cap out out into my hand when I'm unscrewing it. Obviously I know it's not normal and I had planned on doing something to address the issue eventually, but is it really a reason to be concerned? What problems can it cause?

It really depends on how much pressure. A little like a short hiss is not an issue, I suppose if enough pressure were to build it could up the fuel pressure, or over come the needle and seat in a carb vehicle. My Jeep use to pressure build in the summer after a drive. In my case it was so bad that you could hear the tank flexing when you took the cap off, and smell the fuel vapor rush out. It ran ok, but I did fear either a rupture or collapse. Really in my mind the biggest concern is with vacuum which can happen. You get a temp drop at night a vacuum forms and the tank collapses, or it won't start in the morning because the pump can't over come the vacuum in the tank. Often times people don't even realize the vacuum is forming because you don't take the cap off until you get to the gas station and by then it has pressure from all the sloshing. I've seen a few collapses at the shop, but it's not a common occurrence.

I would be concerned about the amount of pressure your building and conversely potential for vacuum. If you have a spare fuel filter for your lawn tractor hanging around you could put it into the end of the vent line under the hood, at least for now.

 

[SirRobyn0]

No canister by the radiator.
Just one tank.

There should be three lines off the gas tank, all three will be a different size, with the smallest being 1/4" This is the vapor / vent line that should go to the canister. You'll need to get under your truck and trace that line, see if it has been capped off or if it goes to a canister and it's in another location.

Hopefully there will be something left of the evap system if not you will have to weigh the time and cost of running lines, getting a canister or just going with the vented cap knowing the potential down falls.

 

[HotRodPC]

My '85 doesn't have evap or anything else like it and I get a hiss when I open the caps. It's not really prolonged, and I've never bothered to see if it's pressure or vacuum. Original caps marked Diesel Fuel Only and it's done it since day one. I guess it all depends on how much hissing the OP is experiencing.

Yours is a 3/4 ton. 1/2 tons had more emissions earlier. For example, you don't have a cat, but a half ton would. A Cali truck has different emmissions too. It might not be until 1 ton when you have less emmissions and I've even seen 1 tons have dual AIR pumps too but not much of anything else. Thank the EPA for all that BS. It just got worse and worse in the 80's this why all the 80s vehicles are denutted gutless wonders.

 

[Blue Ox]

Yours is a 3/4 ton. 1/2 tons had more emissions earlier. For example, you don't have a cat, but a half ton would. A Cali truck has different emmissions too. It might not be until 1 ton when you have less emmissions and I've even seen 1 tons have dual AIR pumps too but not much of anything else. Thank the EPA for all that BS. It just got worse and worse in the 80's this why all the 80s vehicles are denutted gutless wonders.

That is true, except mine is a diesel so it doesn't have evap or cats or anything like that. Although being over 8500 GVWR does get me out of EGR.

I was just making the observation that because of that it's a closed system from the factory and the only pressure control is the caps themselves. And you will get some kind of venting when you unscrew the caps.

Now, the '69 Olds has no evap, but it jas a vent hose from the tank with some kind of plastic filter attachment under the car. When that proved to be unobtainium I had to cap the vent hose and I drilled a tiny hole, like .030" in the steel fuel cap to keep the tank from popping or crushing. It never caught fire, but it's probably not recommended.

 

[newbie17]

The only poster that is correct is SirRobyNO. On the 1987, each tank sending unit has (3) hose connections: pressure, return and tank vent. If you have dual tanks, the left and right tank vent hoses are
next to the fuel tank selector valve on the R/H side on the inside frame rail. The lines "Tee" together and
connect to a steel hard line that runs down the inside of the frame, turns left, then runs along the front crossmember to the charcoal cannister. Here is what happened to me (2) months ago. I removed the bed on my '87 R30 to replace the pumps and sending units on both tanks. Every thing went good. I drove on the left tank for a few days. When I got down to 1/4 tank I switched to the R/H tank. I drove on the right tank for a couple of days then stopped at the gas station to fill-up. When I removed the cap on the right tank a lot of pressure released. At first I thought the cap was defective but I remembered that '87s don't have vented caps. Well crap!
Now I have to find out WTF is wrong. Starting at the charcoal cannister I removed the tank vent line. I applied pressure to check for a blockage. Yep, it was blocked. Next, I removed the 1/4" rubber hose from the steel hard line by the fuel selector valve and checked for blockage back to the cannister-ok-no blockage. Next, I separated the L/H and R/H vent lines at the "Tee" fitting by the selector valve. I checked for blockage from the hose at the "Tee" to the tank sending unit-blocked!!!!! Well, that's not what I wanted to find, but that's my luck! Well, I was not about to remove the bed again and dropping the tank is very difficult when you have a 20 gal. tank because it's longer and that makes reaching the hose connections at the sending unit difficult. (it's easier on 16 gal. tanks because they are shorter). When I had the bed off I wanted to change the (3) hoses that go from the sending unit to the selector valve(they were 34 yrs. old). They are no longer available, so I replaced them with HiQuality hoses and left enough slack to be able to lower the tank down enough to access the connections. After I got the tank down I removed the new sending unit and put it on the bench to check the vent valve. It is mounted to the underside of the mounting flange of the sender. Blocked!!! It is plastic and pressed into the steel line on the sender. BTW-the sender is made in China-imagine that!! I removed the vent valve from my original GM-made in
USA-sender and installed it on the new sender. Installed sender back on tank-installed tank-vents good!!!
I know this a long and boring post but thanks for letting me relate my experience to the forum.