fuel tank pressurization

[callerys]

I have a 84 CJ (258) with the Howell system. I upgraded the return line to a larger size a couple of years ago. I just now noticed that I am getting a ton of fumes after driving. When pulling the gas cap I get a huge gush of air coming out of the tank. I believe the return line is causing this as when shutting down I car hear it going back into the tank.

How is the tank normally supposed to vent or do I need to do something else?

 

[Lumpy Grits]

PSI is bled off via the gas cap. IF it's designed to do so.
How long has this been going on?
What was the reason for the 'upgrade' in return line dia?
LG

 

[ksouthw80]

vent?

 

[Posi]

The gush of air you hear when you remove the cap is going in not coming out. A vented cap lets air in, not out.
If you don't already have one I'd purchase a vented fuel cap, if you do have one it may be faulty and time for a new one
Your return line is working as it should by returning unused fuel back to the tank.
The original 5/16 return line is the correct size and shouldn't need to go larger
However, the 5/16" feed line in the kit should be 3/8"
Your CJ came from the factory with a fuel venting system but many get hacked up or go missing

 

[Lumpy Grits]

Vent gas-cap is to depressurize at a certain positive psi. Ours is tested at each smog test
Tank vacuum is controlled by other means depending on year and smog stuff.
BTW-My OEM fuel 'feed' line is 5/16" and the return line is 1/8" or such. Again OEM.
LG

 

[callerys]

I upgraded the return line because 1/8 wasn't large enough and causing high fuel pressure and rich conditions on the FI. I am wondering then if I have a bad gas cap (it's a locking one) because air is coming out not going in. I get lightly sprayed with gas when I take it off.

I believe it's 3/8 now. Can't remember.

 

[Posi]

The OP has Howell fuel injection his feed line is 5/16" and return line is 5/16" as well. The return line size is correct, Howell saves a few bucks and sends a 5/16" feed line when it should be 3/8" similar to the gm system they copied.

Ever seen a young kid drink from a sippy box, it doesn't have a vent cap, as they drinks the box collapses.

 

[Posi]

Howell installation instructions -

"Your bypass return fuel line is either 1/4” or 5/16” hose and routed from the
right rear of the TBI along the left side firewall to the vicinity of the vapor and
fuel vent steel lines. Connect the bypass return to the line formerly used to
return vapor from the Jeep fuel filter to the tank."

 

[Posi]

What is the fuel pressure at the TB (feed side)?
What was your block learn numbers before you swapped return lines?
What are they now?

 

[Torxhead]

I would look at the vent line system from the fuel tank to the charcoal cannister. There is a rollover valve, check valve and filter in the cannister that has to be clear. My '86 came with a 1/4" i.d. fuel return line that did not give me any problems with the Howell system. The California oem style gas cap is vented and inspected by the smog man, but still will hold some vacuum. The vent hoses from the tank to the check valves should be as short as possible not to collect any fuel after a steep hill or off camber situation, as the can block venting. As you drive the tank will build vacuum as fuel is used up. A repair manual should show the vent systems for you. The 15 and 20 gal tank systems are a bit different though.

 

[Lumpy Grits]

Check out the valve in the gas cap-It's only designed to purge positive pressure. Not equalize a vacuum.

LG

 

[Posi]

The vented gas cap is designed to vent small amounts of air “into” the gas tank line. The vented cap has a one way release valve that is pressure activated. The pressure is formed on the outside of the tank from the displacement of fuel on the inside. After the pressure equalizes the valve closes. Air is allowed in, but no fumes are allowed out.

 

[Lumpy Grits]

The vented gas cap is designed to vent small amounts of air “into” the gas tank line. The vented cap has a one way release valve that is pressure activated. The pressure is formed on the outside of the tank from the displacement of fuel on the inside. After the pressure equalizes the valve closes. Air is allowed in, but no fumes are allowed out.

Please direct me to a source for this info.
THX,
LG

 

[Posi]

Check out the valve in the gas cap-It's only designed to purge positive pressure. Not equalize a vacuum.

LG

Not the way it works, back in the 70s and 80s fuel caps (at least on Jeeps) had a 2 way check valve that work with pressure and “vacuum”. The relief valve would open with a pressure of .75-1.5 psi or a vacuum of 1.1” to 1.8”.

The newer caps are one way and work as I mentioned (vacuum). The pressure relief is built into the cap designed to release when the cap is removed.

Google “how fuel cap works”.

 

[Lumpy Grits]

Not the way it works, back in the 70s and 80s fuel caps (at least on Jeeps) had a 2 way check valve that work with pressure and “vacuum”. The relief valve would open with a pressure of .75-1.5 psi or a vacuum of 1.1” to 1.8”.

The newer caps are one way and work as I mentioned (vacuum). The pressure relief is built into the cap designed to release when the cap is removed.

Google “how fuel cap works”.

You are correct about the 2-way deal on our CJ gas caps. Just proved it to myownself.

THX,
LG

 

[Posi]

 

[callerys]

Wondering since I have a new locking gas cap that there isn't an issue with the two way not working. Air is coming out when I remove the cap.

I will have to check the pressure again. I believe is was like 18 or something originally in the feed and it dropped into the safe range under 12. Again that was a while ago. The air thing is something new.

 

[Posi]

What is the part # of your new cap?

 

[Lumpy Grits]

Why don't you just put the old cap back on?
LG

 

[callerys]

Sorry I misspoke. The gas cap is new as in a year or so ago from the stock one. I looked at it and it's some random no name locking cap. Doing some more reading I wonder if I have a problem with the charcoal canister?

Anything thoughts on that or should I just go get another cap? Original one is long gone.

Recommendations on a two-way cap (doesn't have to be locking)