Can a pressurized tank lead to a fuel delivery issue?

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effjay55 said:
So i took off the check valve from the fuel vapor separator and pressure was coming out. Starting to think that maybe this check valve is either faulty, or possibly not really meant for this ‘74 1.5F engine.

I have read that there should be a little bit of backpressure thats why the check valve opens past a certain psi… is this enough pressure that would cause this wooshing im so concerned about?

Thinking maybe just removing the check valve out of the equation, what do you think?

This is the check valve I put in…


Maybe Im wrong to assume itll work for the 74… wish there were psi specs somewhere
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If it’s holding back pressure it’s not going to be much. If it had a pressure specification, there would need to be a port to hook up a gauge. You could pull one of the hoses coming from the tank vent lines, hook up an air pressure gauge to it, then cap off the nipple on the bottom of the expansion tank. Get a reading after running it for 15 minutes or so. You’d have to get with someone who has a rig with the same system in working order to get a baseline and compare to. My guess would be a couple psi at most.
 
RUSH55 said:
If it’s holding back pressure it’s not going to be much. If it had a pressure specification, there would need to be a port to hook up a gauge. You could pull one of the hoses coming from the tank vent lines, hook up an air pressure gauge to it, then cap off the nipple on the bottom of the expansion tank. Get a reading after running it for 15 minutes or so. You’d have to get with someone who has a rig with the same system in working order to get a baseline and compare to. My guess would be a couple psi at most.
Click to expand...

Man maybe the back pressure isn’t as crazy as Im thinking? I mean looking back at the vid I posted would you say thats abnormal?

Also do you think the check valve I got from cityracer is fine? Ive seen other check valves for sale that seem to be applicable for a wide range of LC’s from 69-80s such as this one being sold by vintageteq .

Is running without the check valve an option?

I know nobody with a rig like mine. Time to start making friends I guess..
 
The spec is in the material that @RUSH55 posted. 0.48" Hg. 1" Hg is roughly 0.5 psi so it should open around 0.25 psig. That doesn't sound like much but that is still a substantial volume that has to vent in that system to equalize to 0 psig. And the lower the fuel level, the longer it will take.
 
effjay55 said:
Hey Rush, Well last weekend I did the test you suggest and got some mixed results. I would blow the lowest pressure air into the vent lines one by one and in some cases I could hear a slight gurgling and even some sputtering of some fuel coming out of 1 of the vent lines. But for the most part all of the lines were venting the compressed air fine it seems.

Correct me if I’m wrong but I started thinking there’s probably 4 vent lines because, since the tank is so flat, there’s a possibility one of the 4 corners are inevitably going to be flooded with fuel; that the fuel vapor separator is designed to capture any of this sputtering of fuel and return it back to the tank. So I think the operation of these vent lines are good.

I also did the other tests again (gas cap off) with low pressure air and didn’t hear any gurgling like before.

I did however put everything back together and took it for a test drive , the truck drove great for around 30 minutes and had no misfires, but when I got home and opened the gas cap there was still tons of pressure coming out.

This weekend I plan to try and drive it again , let it build up pressure then i plan on disconnecting the vent line before the check valve to see if it’s just not opening up. What do you think? I have a suspicion it’s the valve. It’s new but maybe the pressure specs are off…?
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Not trying to be inflammatory here, but please make sure the check valve is installed in the correct direction.
I've been there. I found out a check valve to my carb (on my 40) was in facing the wrong direction. the vacuum signal was never going to get where it was supposed to go.
 
WarDamnEagle said:
Not trying to insult your intelligence but make sure the check valve is in the correct orientation. You can also take it off and see if you can blow air through it from the upstream side.
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Okay, I'm glad I'm not the only one thinking this way...
 
RUSH55 said:
If you feel that you’ve eliminated any potential blockage in the venting system up to the charcoal canister, then the next step is to go through everything on the other side of the canister.
There is a Vacuum Switching Valve (VSV) which is operated by an emissions control computer. Two lines appear to go from the canister to the carburetor, one of which goes to what is called the Throttle Position (TP) Diaphragm.

View attachment 3839033

View attachment 3839034

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View attachment 3839036
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I think there is just one line going from the can to the carb @RUSH55 . The other line you see in the diagram is coming from the tank/vapor separator.
 
WarDamnEagle said:
The spec is in the material that @RUSH55 posted. 0.48" Hg. 1" Hg is roughly 0.5 psi so it should open around 0.25 psig. That doesn't sound like much but that is still a substantial volume that has to vent in that system to equalize to 0 psig. And the lower the fuel level, the longer it will take.
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Now is that going to be 0.48” Hg of vacuum coming from the charcoal canister side, or 0.25psi/g positive pressure coming from the tank?
Either way, this check valve is supposed to let pressure out of the tank, and not allow flow of air back into the tank. Why this is important, I don’t know. Maybe someone can explain it to me.
In this system, it’s the cap that’s supposed to vent to allow air in. If the cap does not allow for venting of air in, and the check valve does not allow for venting in, then as the engine runs and pulls fuel from the tank, it can potentially put it in a vacuum.
I noticed from the video that your cap is aftermarket. Does it allow for venting?
 
Lifelong40Fan said:
Amen! Or drill a small pin hole in your cap if you’ve eliminated any blockages in the vent lines.
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In that case you are just venting the fuel vapors to atmosphere.
Best case - your Pig (and garage) smell like gas vapors.
Worst case is you have a fire hazard.
 
effjay55 said:
So i took off the check valve from the fuel vapor separator and pressure was coming out. Starting to think that maybe this check valve is either faulty, or possibly not really meant for this ‘74 1.5F engine.

I have read that there should be a little bit of backpressure thats why the check valve opens past a certain psi… is this enough pressure that would cause this wooshing im so concerned about?

Thinking maybe just removing the check valve out of the equation, what do you think?

This is the check valve I put in…

www.cityracerllc.com

OEM Fuel Vapor Check Valve for '75 to '77 Land Cruiser FJ40 FJ55

OEM fuel vapor check valve for 1/'75 to 9/'77 Toyota Land Cruiser FJ40, FJ55. Part # 90917-10020.
www.cityracerllc.com

Maybe Im wrong to assume itll work for the 74… wish there were psi specs somewhere
Click to expand...
I would bypass it for test purposes if you suspect it is not letting vapors reach the charcoal can.
 
RUSH55 said:
If it’s holding back pressure it’s not going to be much. If it had a pressure specification, there would need to be a port to hook up a gauge. You could pull one of the hoses coming from the tank vent lines, hook up an air pressure gauge to it, then cap off the nipple on the bottom of the expansion tank. Get a reading after running it for 15 minutes or so. You’d have to get with someone who has a rig with the same system in working order to get a baseline and compare to. My guess would be a couple psi at most.
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What expansion tank are you speaking of?
 
PabloCruise said:
What expansion tank are you speaking of?
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Separator is more accurate

IMG_5184.webp
 
effjay55 said:
Man maybe the back pressure isn’t as crazy as Im thinking? I mean looking back at the vid I posted would you say thats abnormal?

Also do you think the check valve I got from cityracer is fine? Ive seen other check valves for sale that seem to be applicable for a wide range of LC’s from 69-80s such as this one being sold by vintageteq .

Is running without the check valve an option?

I know nobody with a rig like mine. Time to start making friends I guess..
Click to expand...
I the worst case is that you are passing vapor constantly to the charcoal cannister.
If your vacuum check valve is in place, it is not like you are going to be venting vapor into the carb upon starting the engine.
I think you want to avoid that scenario, a backfire might be bad in that situation.
 
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PabloCruise said:
I the worst case is that you are passing vapor constantly to the charcoal cannister.
If your vacuum check valve is in place, it is not like you are going to start venting vapor into the carb upon starting.
I think you want to avoid that scenario, a backfire might be bad in that situation.
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That makes sense. In other words it’s only drawing vapors or only has vapors in it after it is running. Safety first.
 
RUSH55 said:
Now is that going to be 0.48” Hg of vacuum coming from the charcoal canister side, or 0.25psi/g positive pressure coming from the tank?
Either way, this check valve is supposed to let pressure out of the tank, and not allow flow of air back into the tank. Why this is important, I don’t know. Maybe someone can explain it to me.
In this system, it’s the cap that’s supposed to vent to allow air in. If the cap does not allow for venting of air in, and the check valve does not allow for venting in, then as the engine runs and pulls fuel from the tank, it can potentially put it in a vacuum.
I noticed from the video that your cap is aftermarket. Does it allow for venting?
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Read the material you attached in Post 49. I interpret Paragraph 5 of the "Fuel Evaporative Emissions Control System" to say that if pressure in the system before the check valve (tank, vapor separator, etc), gets over 0.48" Hg then the check valve will open and allow flow to the charcoal canister. It also says if the pressure goes above 0.60" Hg on the charcoal canister side, the check valve will allow flow back to the tank.

So this is a special two way check valve. No flow until 0.48" pressure in the tank, one way flow from the tank to the charcoal canister between 0.48" and 0.60" Hg, and bi-directional flow above 0.60". You could also state one way back to tank above 0.60" but I'm not sure that's what would really happen. I think in that scenario there would be an open check valve and no flow with pressure building up uniformly on both sides of the check valve. That's my understanding of the intended operation.

Paragraph 7 under "Inspection" confirms that the check valve should allow flow with some resistance in both directions. Pretty sure no one's diaphragm is calibrated to discern the difference of 0.06 psi when blowing into a hose so the check valve should be installed by physical inspection.

None of the above has anything to do with venting the tank when it's under vacuum. That is the job of the fuel cap which should allow air in when the pressure in the tank goes negative.
 
WarDamnEagle said:
Read the material you attached in Post 49. I interpret Paragraph 5 of the "Fuel Evaporative Emissions Control System" to say that if pressure in the system before the check valve (tank, vapor separator, etc), gets over 0.48" Hg then the check valve will open and allow flow to the charcoal canister. It also says if the pressure goes above 0.60" Hg on the charcoal canister side, the check valve will allow flow back to the tank.

So this is a special two way check valve. No flow until 0.48" pressure in the tank, one way flow from the tank to the charcoal canister between 0.48" and 0.60" Hg, and bi-directional flow above 0.60". You could also state one way back to tank above 0.60" but I'm not sure that's what would really happen. I think in that scenario there would be an open check valve and no flow with pressure building up uniformly on both sides of the check valve. That's my understanding of the intended operation.

Paragraph 7 under "Inspection" confirms that the check valve should allow flow with some resistance in both directions. Pretty sure no one's diaphragm is calibrated to discern the difference of 0.06 psi when blowing into a hose so the check valve should be installed by physical inspection.

None of the above has anything to do with venting the tank when it's under vacuum. That is the job of the fuel cap which should allow air in when the pressure in the tank goes negative.
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That is interesting.
AFAIK, there should not be positive pressure in the can, unless that bottom vent is 100% clogged, and it never gets a vacuum to purge.
In that case the can would be FUBAR.
 
WarDamnEagle said:
Read the material you attached in Post 49. I interpret Paragraph 5 of the "Fuel Evaporative Emissions Control System" to say that if pressure in the system before the check valve (tank, vapor separator, etc), gets over 0.48" Hg then the check valve will open and allow flow to the charcoal canister. It also says if the pressure goes above 0.60" Hg on the charcoal canister side, the check valve will allow flow back to the tank.

So this is a special two way check valve. No flow until 0.48" pressure in the tank, one way flow from the tank to the charcoal canister between 0.48" and 0.60" Hg, and bi-directional flow above 0.60". You could also state one way back to tank above 0.60" but I'm not sure that's what would really happen. I think in that scenario there would be an open check valve and no flow with pressure building up uniformly on both sides of the check valve. That's my understanding of the intended operation.

Paragraph 7 under "Inspection" confirms that the check valve should allow flow with some resistance in both directions. Pretty sure no one's diaphragm is calibrated to discern the difference of 0.06 psi when blowing into a hose so the check valve should be installed by physical inspection.

None of the above has anything to do with venting the tank when it's under vacuum. That is the job of the fuel cap which should allow air in when the pressure in the tank goes negative.
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You’re right about that check valve. Must have missed the part of it acting in two directions.

So my question now is does this cap vent?

IMG_5186.webp
 
PabloCruise said:
Cool. Vapor separator.

You gotta keep em separated!

Or, we could go with some old school Vapors! This song always cracks me up!



Click to expand...

When I woke up this morning, I had no idea I’d later be listening to ‘I Think I’m Turning Japanese’. I really did not think so.🤣
 
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