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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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…)
Makes sense I guess.

My aux tank was connected as a closed loop system for a few years. Essentially I had a fuel pickup and fuel return, but it wasn't connected to the EVAP system. That's when I noticed a lot of strange behaviors such as the vacuum issue I mentioned above. It would only happen on long road trips and only if I was driving on the highway for a few hours continuously. I'm assuming it was pulling enough fuel to cause a vacuum and starve the carb of fuel, kinda scary when you're on the highway. At first I thought it was vapor lock, but switching to the main tank or opening the cap on the aux tank would solve the problem. I would hear a slight pop from the tank expanding when cracking the gas cap so I'm assuming the aftermarket cap wasn't allowing air to equalize in the tank. It was strange because on hot days the fuel would expand enough to overflow out of the filler neck, so it was almost like a check valve working in the wrong direction.

I finally T'd into the EVAP system and it fixed the vacuum problem, but that's when I started having the pressure issue. After a lot of trial and error and vacuum testing that's when I finally got the VSV and emissions computer working and haven't had the issue anymore. I do remember hooking up vacuum gauges on all the lines and doing a lot of test drives to figure out what was happening, and I think the VSV only opens when I was running at WOT and above a certain RPM. This probably correlates to the vacuum readings shown in the manual.

My aux tank is connected through the 6 port switching valve, so the fuel returns from the tank it pulls from. I did modify my quite a bit to eliminate the aux tank fuel filler neck, I documented the setup in another post somewhere.
 

PabloCruise

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This looks like a good thread to check out...

My 40 is smelling up the garage w/ a gas smell.

When it gets warm in the garage, the fuel tank has a positive pressure inside.

It makes sense that the positive pressure in the tank is not reaching the charcoal cannister to await aspiration.
 

PabloCruise

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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?
I would think the proper function would be that the pressure in the tank should be able to exit the tank through the vapor separator and then the vapors are arrested in the charcoal cannister - understood that your cannister has been removed. I would not think you should need source vacuum to allow the tank to vent. Gasoline will continue to warm, and generate vapors after the cruiser is shut down.
 

PabloCruise

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What is the line downstream of the check valve hooked to? You said positive air buildup in the gas tank, so something is pushing air pressure into the fuel tank?

Gasoline will generate vapors (and pressure), even with the Cruiser shut down if it is warm enough.
 

PabloCruise

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I would just start tracing from the tank and providing pressure with an air hose. If everything checks, possible plug inside the tank?

Chasing things w/ a little bit of air pressure sounds good! Can the outlets for vapor lines take some air being backflowed into the tank? I would remove the gas cap and just flow a pretty low pressure, ~10 PSI...
 

PabloCruise

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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.
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

Question - have you verified that the vapor separator and check valve are indeed connected to the vapor outlets on the tank? I have see people do goofy things, like capping the vent lines, or cross-connecting the vent lines back to each other!
 

PabloCruise

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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
Well there is that, but gasoline will generate vapors (and pressure), even with the Cruiser just sitting there if it is warm enough.

Running fuel up to the carb, and then back to the tank via the return line warms it up plenty!
 

PabloCruise

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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.
A properly functioning gas cap does let air in, but does not let vapors out.
At night, when it gets cold and the thermal mass of gasoline in your tank gets cold, it contracts.
Rather than pull a vacuum in your tank, a cap will let air in. The check valve at the vapor separator is a one-way valve to let pressure out.
The idea is to not let the vapors vent to atmosphere, or fill your garage with gasoline vapors.
 

PabloCruise

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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.
Not worse than dropping a Pig gas tank!!!
 

PabloCruise

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PabloCruise

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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.
You are mixing the use case of aux tank and return line...
The situation you are describing is a little more unique.
Return line going into a single tank will more likely create positive pressure as you are warming gasoline right next to an engine, and then returning to the tank to mix w/ cooler gasoline.
 

PabloCruise

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Makes sense I guess.

My aux tank was connected as a closed loop system for a few years. Essentially I had a fuel pickup and fuel return, but it wasn't connected to the EVAP system. That's when I noticed a lot of strange behaviors such as the vacuum issue I mentioned above. It would only happen on long road trips and only if I was driving on the highway for a few hours continuously. I'm assuming it was pulling enough fuel to cause a vacuum and starve the carb of fuel, kinda scary when you're on the highway. At first I thought it was vapor lock, but switching to the main tank or opening the cap on the aux tank would solve the problem. I would hear a slight pop from the tank expanding when cracking the gas cap so I'm assuming the aftermarket cap wasn't allowing air to equalize in the tank. It was strange because on hot days the fuel would expand enough to overflow out of the filler neck, so it was almost like a check valve working in the wrong direction.

I finally T'd into the EVAP system and it fixed the vacuum problem, but that's when I started having the pressure issue. After a lot of trial and error and vacuum testing that's when I finally got the VSV and emissions computer working and haven't had the issue anymore. I do remember hooking up vacuum gauges on all the lines and doing a lot of test drives to figure out what was happening, and I think the VSV only opens when I was running at WOT and above a certain RPM. This probably correlates to the vacuum readings shown in the manual.

My aux tank is connected through the 6 port switching valve, so the fuel returns from the tank it pulls from. I did modify my quite a bit to eliminate the aux tank fuel filler neck, I documented the setup in another post somewhere.
2 tanks is quite a set up to consider.
I was just looking at a manual for a vehicle that has saddle tanks - one on either side of the driveshaft tunnel.
IIRC, they only pulled from one tank. The other tank could fill from the single inlet, but it fed the main tank.
I will see if I can find that schematic.
 
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Yes.

I don't think so, not according to the algorithm @dmaddox shared from the FSM - above ^^^

I'm not exactly clear on how everything works together, but I know that my pressure issue wasn't resolved until I fixed the rest of the emissions equipment connected to the check valve. The check valve on it's own wasn't really doing anything to relieve the pressure, so I assume something else is happening downstream to suck the vapors out of the tank.


You are mixing the use case of aux tank and return line...
The situation you are describing is a little more unique.
Return line going into a single tank will more likely create positive pressure as you are warming gasoline right next to an engine, and then returning to the tank to mix w/ cooler gasoline.

It's been a few years since I've worked on it, but for some reason I was convinced it was a vacuum. I can't remember exactly how I came to that conclusion. As I mentioned, it only happened on long freeway trips after a couple of hours of continuous driving, so I'd probably burn through 10-15 gallon of fuel before the issue would show up. I don't know if the return line fuel at higher temperature would be enough to offset the vacuum created. Also, if it was positive pressure, I don't think I'd get the fuel starvation problem I was seeing.
 

PabloCruise

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I'm not exactly clear on how everything works together, but I know that my pressure issue wasn't resolved until I fixed the rest of the emissions equipment connected to the check valve. The check valve on it's own wasn't really doing anything to relieve the pressure, so I assume something else is happening downstream to suck the vapors out of the tank.




It's been a few years since I've worked on it, but for some reason I was convinced it was a vacuum. I can't remember exactly how I came to that conclusion. As I mentioned, it only happened on long freeway trips after a couple of hours of continuous driving, so I'd probably burn through 10-15 gallon of fuel before the issue would show up. I don't know if the return line fuel at higher temperature would be enough to offset the vacuum created. Also, if it was positive pressure, I don't think I'd get the fuel starvation problem I was seeing.
I could be wrong on both of the above!

Still need to find that diagram of how BMW set up a dual tank system…
 

PabloCruise

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I could be wrong on both of the above!

Still need to find that diagram of how BMW set up a dual tank system…

Found it, but it is not 2 separate tanks, it is one tank that is a saddle - 2 low spots. So filling and venting are similar to a single tanks set up. I do not see any info on how the BMW (e46) collects and purges vapors.
What is unique is the transfer pipe to pull fuel from the side w/o the pick-up.
It looks like fuel level senders in both sides, so the cluster must have some logic to calculate a level based on both inputs:

Screenshot 2022-10-21 142743.jpg

Screenshot 2022-10-21 142651.jpg

Screenshot 2022-10-21 142904.jpg
 

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