Gas tank pressure

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1975 fj40, new gas tank and lines, stock fuel separator, in-line stock check valve off of separator (works as one way valve) in correct orientation, desmoged but p/o eliminated charcoal canister, bvsv and vsv. Stock carb.
When run even for a short period of time there is substantial positive pressure build up on the gas tank, when the cap is removed it is dramatic, not just a “swoosh.” I’m getting ready to install a aftermarket ac delco canister to deal with gas fumes, was going to attach to air cleaner since switched vacuum ports are missing.
THE PROBLEM
Why won’t the tank vent through the check valve coming off of the separator? I can easily suck gas fumes through the hardline on the down stream side of this check valve (can not blow through it). Does it need the vacuum through the cc? Am I missing something?
 
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Nothing is down stream of the valve, just vents to atm until the CC is hooked up. I just don’t understand why the pressure is not venting out this route. Especially since I can suck fuel vapor so easily through it.
 
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What’s even more interesting is what is pushing air into the tank? Do you mean negative pressure/vacuum when you take the cap off, it’s a suction whoosh?
Not negative, it’s positive pressure - I can actually see the tank relax and feel the breeze when the cap is removed, it is pretty loud. Only other tank connections are the fuel filler, filler vent, fuel supply (to carb) and return to tank.
If it’s hot out the pressure builds without running the truck.
I guess the tank vent tube could be faulty - it is from ccot, but I don’t think I should be able to suck air through the downstream tube if it is really plugged - it’s the first thing I’ll check though
 

Pighead

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Not negative, it’s positive pressure - I can actually see the tank relax and feel the breeze when the cap is removed, it is pretty loud. Only other tank connections are the fuel filler, filler vent, fuel supply (to carb) and return to tank.
If it’s hot out the pressure builds without running the truck.
Mine ('74FJ55) does that too. I figure it's just the fuel pump pressurizing the system and the tank via the fuel return line. My gas cap has a vent and i can here it venting while sitting out on a hot day
 
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Mine ('74FJ55) does that too. I figure it's just the fuel pump pressurizing the system and the tank via the fuel return line. My gas cap has a vent and i can here it venting while sitting out on a hot day
Hmm, I need one of those. I thought the vented fuel caps let air in but not out? Did you drill it? I think you are right about the source of the pressure, I just can’t figure out why it won’t vent through the separator. I don’t imagine the repeated oilcanning is good for the tank seams. Pighead, I would just ignore it except it is an alarming amount of pressure.
 

hobbes

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You don’t need to drill your fuel cap. You should fix the problem. Yes, the factory fuel cap lets air into the system. The EVAP let’s air escape. You’ll probably find the previous owner capped the line when he removed the charcoal canister. Try following the steps in post 5 below. You are essentially looking to find the blockage starting at the exit of the charcoal canister and working backwards until you find the blockage. Since you don’t have a canister, start by inspecting the line that should provide the input to the canister. In my case, I had a mud dauber block the input line to the charcoal canister.

 
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Let it pressure up and take the line off the separator to tank, if it relieves pressure then the separator is blocking the out flow of tank pressure. Why can you suck vapor thru and it will not escape on its own, good ? but it does not change the fact that tank pressure is not getting to that check valve or is getting stopped there, IMO take the separator out and have a look.
 

dmaddox

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Hopefully this helps:

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Let it pressure up and take the line off the separator to tank, if it relieves pressure then the separator is blocking the out flow of tank pressure. Why can you suck vapor thru and it will not escape on its own, good ? but it does not change the fact that tank pressure is not getting to that check valve or is getting stopped there, IMO take the separator out and have a look.
Lots of good recommendations, I’m going to get to this on Wednesday and I’ll post up what I find. Thanks to everyone that that replied to this question, I’m always impressed by the mud folks. I’ll be really angry if it’s the tank though, it will be a real pita to pull that and try and get some money back at this point.
 

hobbes

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If you pull the line off the separator and that relieves the pressure, it might or might not be the separator. That will definitively tell you if the tank is the issue. If no pressure is released, your tank is the issue. Possible, but not likely.

Since I’m lazy, I start at the other end of the system. That way once the pressure is released, you know you are at the right spot since by definition everything upstream is allowing the pressure through. Either way works, it just saves potentially having to re-pressurize the system.

Or go half way…Pull the bottom of the check valve on the B-pillar off the hose. If pressure is release, you know it’s your line to the charcoal canister. Make sure it’s uncapped and then blow it out with compressed air. If no pressure is released, pull the top hose. Work your way back from there.

Good luck and let us know.
 
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The pressure is from the natural evaporation of the gasoline, even when parked your tank will build pressure. I think the check valve needs a vacuum signal before it will release the pressure from the tank. I remember going through similar problems with my EVAP system, it feels like the tank is gonna blow apart, but in reality it's probably <1psi in the tank. I was able to sort out my issues with the VSV and emissions computer under the dash, and it's been working fine since. Without those parts, I'm not sure if just hooking it up to the air cleaner will work, it might not have enough vacuum to overcome the check valve.
 

hobbes

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No vacuum signal needed, but I’m sure it helps the delta and scavenge a bit. Pressure just builds beyond the small amount the check valve will hold. That way you don’t stink up you garage when the vehicle sits. My system works fine and it just vents to the atmosphere post charcoal canister.

In addition to the evaporative process, the warm fuel being returned through the fuel return system and the fuel sloshing around both increase pressure as you drive. Modern fuels are worse.

As for running it straight to the air cleaner, I’m sure guys here do that just fine. Not sure if there additional fire risk with a backfire starting w/o the VSV….
 
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I would think the fuel return would cause a vacuum on the tank, not pressure. The amount of fuel returning has to be less than what's being drawn out of the tank. I had that problem on my aux tank before connecting it to the EVAP system, after a couple hours of driving the tank would have a vacuum that would result in fuel starvation until I cracked the gas cap.
 

Pighead

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I would think the fuel return would cause a vacuum on the tank, not pressure. The amount of fuel returning has to be less than what's being drawn out of the tank. I had that problem on my aux tank before connecting it to the EVAP system, after a couple hours of driving the tank would have a vacuum that would result in fuel starvation until I cracked the gas cap.
I argued with myself for hours about this once or twice. Yes, the fluid volume in the tank will decrease, but the fuel returning is still under pressure from the fuel pump. That pressurizes the air in the tank. The whole system is pressurized except for the float bowl. This is just my beer induced theory.
Whenever i get a vacuum whoosh when removing the fuel cap i check my fuel filter and usually change it.
 

hobbes

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I would think the fuel return would cause a vacuum on the tank, not pressure. The amount of fuel returning has to be less than what's being drawn out of the tank. I had that problem on my aux tank before connecting it to the EVAP system, after a couple hours of driving the tank would have a vacuum that would result in fuel starvation until I cracked the gas cap.
The return fuel has been heated, so more vapors than if no fuel return. But how much all those factors play off each other, I have no idea. I just know if you drive with the EVAP line plugged, your tank makes scary noises as it expands.

Shouldn‘t your fuel cap let air into your aux tank? Is it OEM? Does your fuel return go to either tank? (Asking because I need to hook up my aux tank…)
 

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