Another pressurized gas tank thread

[ginmtb]

Sorry to create another thread on pressurized gas tanks but I'm stumped on this and need some help. Here is some background info:

- Brand new OEM tank
- Replaced all rubber fuel lines
- Kept all stock venting equipment (check valve, the plastic device that the three vent tubes connect to)
- Checked vent line to charcoal canister and it is clear - air blows through. Check valve has a slight click when air is blown through it assuming that is the valve working.
- Replaced OEM charcoal canister and replaced with Duralast VC120 model - hoses connected correctly, purge line runs to VSV(?) as pictured
-The line off the VSV(?) that goes straight out is connected to the manifold
- Have a stock gas cap but also have experimented with the one pictured below which I believe is a vented cap for both vacuum and pressure if that's what the stamped words on it represent.

With the OEM cap on it builds pretty good pressure, enough that I occasionally will hear the sound of metal popping (like the cap on a sealed jar but on a bigger scale). With the "vented" cap it still builds pressure but not as much.

Anyone have any thoughts as to why it isn't venting properly? I suppose it could be the check valve, which figures that thing is still available but almost $50! I was cleaning up the garage and put some parts away in the 75 boxes and saw the check valve maybe I'll trade and see if that makes a difference.

As always your assistance is appreciated!

New canister and incoming and purge lines

Purge line connected to VSV(?)

Vented gas cap?

OEM gas cap

 

[algeeba]

Remove the check valve.

I bought my '78 28 yrs ago and it never had that check valve. Never new it existed. Two months ago I began replacing my fuel lines due to gas smell in the cabin and decided to order and install that check valve. Dumb. Ever since, I have had pressure build up in the tank and then started smelling gas again about three weeks ago. Removed the darn check valve 4 days ago, no more pressure build up and no more gas smell any more. I'm leaving it that way.

I also have the Toyota gas cap you show and the Duralast charcoal canister.

 

[sadam_husain]

Are you still carb or FI

 

[ginmtb]

Carb - I have a Trollhole in there now.

 

[ginmtb]

Remove the check valve.

I bought my '78 28 yrs ago and it never had that check valve. Never new it existed. Two months ago I began replacing my fuel lines due to gas smell in the cabin and decided to order and install that check valve. Dumb. Ever since, I have had pressure build up in the tank and then started smelling gas again about three weeks ago. Removed the darn check valve 4 days ago, no more pressure build up and no more gas smell any more. I'm leaving it that way.

I also have the Toyota gas cap you show and the Duralast charcoal canister.

I will give that a shot. Thanks for the advice. Easy job too!

 

[78fj40mg]

Try reversing the check valve and see if it works ok without any pressure build up.

 

[ginmtb]

Here is the check valve removed - part number on top and "TO ENGINE" on bottom. So the orientation was correct. Right now I'm running it without the valve and put on the OEM cap. I'll report back in the morning to see what the results are...

 

[Racer65]

That check valve is for '72 to '76 only. Since your truck is a '74, you might still need it. I think its purpose is to keep the fuel in the line as opposed to falling back to the tank (a check valve is a one way valve). Otherwise your fuel pump might need to work harder to draw fuel from the tank.

 

[wscbill]

Here's a diagram from the 1974 Emission Control and Maintenance Schedule manual that shows the canister and check valve connections.

The VSV (or computer or speed sensor) may not be working. If the VSV is stuck shut for whatever reason, the canister can not purge through the intake manifold. In this case, removing the check valve and installing a vented gas cap allows the vapor to vent through the cap instead.

 

[wscbill]

More information from the 1975 Emission Control Manual.

Operation of the VSV.

Details on the gas cap and check valve.

 

[ginmtb]

That check valve is for '72 to '76 only. Since your truck is a '74, you might still need it. I think its purpose is to keep the fuel in the line as opposed to falling back to the tank (a check valve is a one way valve). Otherwise your fuel pump might need to work harder to draw fuel from the tank.

@Racer65 this is for a vent line - so hopefully there isn't fuel in that line! If so I've got bigger issues...

 

[ginmtb]

Here's a diagram from the 1974 Emission Control and Maintenance Schedule manual that shows the canister and check valve connections.

The VSV (or computer or speed sensor) may not be working. If the VSV is stuck shut for whatever reason, the canister can not purge through the intake manifold. In this case, removing the check valve and installing a vented gas cap allows the vapor to vent through the cap instead.

View attachment 1316501

Thank you @wscbill - so if the VSV isn't working there should be pressure in the tank now with the check valve removed. I'm laying in bed but will get up soon to check out. Is there anyway to test the VSV?

 

[ginmtb]

BTW - my 40 is desmogged so seems like I could bypass the VSV and run the purge line directly to the manifold. Thoughts?

 

[ginmtb]

This is an interesting conversation from way back in the day on Pirate from our very own @Pin_Head. I will check out the little computer as well.

 

[Pin_Head]

No kidding? I must be an old MF.

 

[kruisinkid]

if you are desmogged remove everything except charc canister and associated plumbing. the check valve flap could also be "glued" shut from years of vapor sticking in there.

 

[ginmtb]

if you are desmogged remove everything except charc canister and associated plumbing. the check valve flap could also be "glued" shut from years of vapor sticking in there.

Seems like I should at least run the purge line from the charcoal canister to the intake manifold so it's not vented to the atmosphere (and smell up the garage). Or would the charcoal canister basically absorb those?

 

[Racer65]

This is evolving into quite an interesting thread, especially the part about PC board solder. Wow!

 

[algeeba]

My dad's '73 apparently never had that valve either and he's the original owner. I'm not sure if he remembers that he removed it or not. He also vents the canister through a nipple he installed onto the air cleaner after removing the VSV.

Anyways, to test the VSV can a vacuum gauge be installed on the canister side of the VSV, be brought into the cabin, drive until you reach the desired speed and see if there is vacuum?

 

[ginmtb]

I took off the gas cap and no pressure... The plot thickens. I removed the check valve but everything else is the same. Won't be able to get to the computer board until sometime probably tomorrow.