Pressure in gas tank

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TEQ

TEQ

Tigger
Joined
Sep 2, 2005
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43
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598
Location
Durango, CO
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www.kindel.com
When I stopped to fill up with gas tonight (maybe 1/8 tank remaining) when I pulled the gas cap off, it practically blew off. POP. Tons of pressure in the gas tank.

While we were working on the rig today, we capped a few vaccum tubes and other non-connected items, including the 1/3" metal tube that is attached to the firewall to the left of the motor (behind the carb). Apparently this is supposed to connect into the manifold (I don't know if it is intake or exhaust) where the excess fumes will be burned. But because my rig (a '78) has a '75 carb on it, there is no where for this tube to connect up to.

See pictures (unfortunately I do not have a pic of the manifold where these tubes connect).

We capped it, and I think that's why the pressure built-up. I have since un-capped it.

Is my hypothesis correct that I should NOT cap this thing?

Where should I route it? I mean, the way it is now, I'm getting gas fumes in the engine bay, and that may be part of the reason my rig smells like gas...
FJ40 Purchase 058 Large Web view.webp
FJ40 Purchase 061 Large Web view.webp
 
You need to buy a gas tank cap that has a pressure release on it and then cap the tube in the engine bay.
 
OR, properly use the stock venting system which includes hooking up the several lines off of the tank to the fuel seperator / check valve / charcoal canister / then to the intake where the fumes will get burned. This has been discussed before so do a search and it should come up........

Noah
 
on my 76 one of the bigger lines was the return line (capped off) the vent was out back buy the filler neck under the cover. it went back to the seperater and then down threw the floor (metal line) next to the quarter panel were it heads forward towards the engine. i just left it unhooked down along the body panel and have had no problems and its been 4 years know, maybe this summer i will run a rubber line up to a vacum port on the carb and see what happens ?
 
Its the line from the charcoal canister to the carb. It was disconnected and making the cab smell of gas. Eventually TEQ will have everything hooked up correctly. We plugged it to see if the gas smell would go away.
 
I have never seen a charcoal cannister that connects directly to the carb. It doesn't need much vacuum. On my '76, it connects to the VSV which in turn gets manifold vacuum. Get a factory vacuum diagram that shows what goes where.
 
My mistake. Its the line from the charcoal canister that feeds back into the intake system so that the extra fumes get processed. We compared it to my unmolested 78 and confirmed that is where it needs to go.
 
bull said:
Its the line from the charcoal canister to the carb. It was disconnected and making the cab smell of gas. Eventually TEQ will have everything hooked up correctly. We plugged it to see if the gas smell would go away.
Click to expand...
There shouldn't be a line from the charcoal canister to the carb. It must be switched by the vacuum switching valve (the black thing on the driver's side fenderwell with the octopus of hoses, as seen in the photo).

Read this and this

...or just get a perforated gas cap :rolleyes: (sorry '74 UA FJ) or drive around without one :rolleyes:
 
Mr. Toad said:
There shouldn't be a line from the charcoal canister to the carb. It must be switched by the vacuum switching valve (the black thing on the driver's side fenderwell with the octopus of hoses, as seen in the photo).

Read this and this

...or just get a perforated gas cap :rolleyes: (sorry '74 UA FJ) or drive around without one :rolleyes:
Click to expand...

Thank you Mr. Toad. I have bookmarked both of those threads as they were educational for me (e.g. "TP == Throttle Positioner"). I am really hoping my factory manuals (I'll get them in a few weeks) will have detailed illustrations and descriptions of how this is all supposed to be wired up.

Do you have any other sources of documentation that would help me better understand the mechanics and theory of the '78 system.

I am likely going to get a '78 carb (Bull, if your offer still stands...), and will be re-doing everything factory correct...
 
TEQ said:
I am really hoping my factory manuals (I'll get them in a few weeks) will have detailed illustrations and descriptions of how this is all supposed to be wired up.

Do you have any other sources of documentation that would help me better understand the mechanics and theory of the '78 system.
Click to expand...
The emissions manuals from Toyota are great, and they're relatively cheap. They cover all Toyota models in a given year, so there's extra info in them, but for about $30, nothing else comes close (IMHO) to explaining what the system does, why it does it, how to test it, and how to fix it if it's not working. Very understandable, good illustrations, etc.

Some of the emissions system components are pretty important. Some aren't. The carb guru FJ40Jim made some good diagrams showing how to remove the not-so-important ones ("de-smogging"--look in tech). I don't know about your '78, but the fuel tank in my '75 is inside the cab. When my evaporative emissions system went on the fritz, I thought the fumes would kill me. So that's why I think that keeping the tank pressure down is one of the important systems...

Best of luck getting your junk back on track.
 
Last edited:
I have never seen a charcoal cannister that connects directly to the carb. It doesn't need much vacuum. On my '76, it connects to the VSV which in turn gets manifold vacuum. Get a factory vacuum diagram that shows what goes where.
Click to expand...
There shouldn't be a line from the charcoal canister to the carb. It must be switched by the vacuum switching valve (the black thing on the driver's side fenderwell with the octopus of hoses, as seen in the photo).
Click to expand...
On the 1978-newer USA trucks there is a direct connection from canister vent to carb bowl vent. If you don't have an emission FSM, see fig 13.3, p.271 in the Haynes book.

The truck in question should have 1978 evap sytem. The 1975 carb doesn't have the large port on the carb for the outer vent hose. As a temporary fix (until a 1978 carb air horn is installed), you could reduce the hose size down and connect it to one of the vent fittings on the aircleaner.

If there is still significant pressure buildup in the tank, you can also swap the two hoses on the top of the canister, labeled "tank" and "purge". Hooking the tank hose to the purge fitting will allow the tank to breathe easily through the canister.
 
I just confirmed that all the lines on my 78 are in their stock location.


Also I'm guessing that the fuel eventually evaporates from the float bowl vent line attached to the charcoal canister making it harder to start. I was concerned that I had a float bowl issue, but this clears it up.
 
My 40 has been doing that since the day it was brought home from the dealership in 74. There has been no work done to the fuel system or carb whatsoever(the only thing was the air injection system was removed.). It even has the original gas cap. I can remember back in the day when we would be wheeling and you could hear a high pitch whistling kinda sounded like a test of the emergency broadcast system. It was the fuel vapors squeezing past the gas cap seal. From my perspective, it is an inherited trait from Toyota.
 
yeah ive heard you can go to a cragen or napa nad get a vented gass cap.
 
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