The dreaded overpressure fuel tank! (1 Viewer)

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Everyone above is correct. A previous owner did change out the fuel tank to a pre-74. Last week I removed the fuel tank, cleaned it, rust protected the exterior, and replaced all hoses. Each of the four corner vent hoses runs directly into the driver side evaporator.

There is no return fuel line back from the carburetor. As it does appear that the carburetor was replaced with a weber. That hard line underneath the truck is capped.

This afternoon when I got home the car was sitting in the driveway and gas was pouring out the gas tank. It was maybe 82° here at home and the car had not been moved or started all day.

When I arrived home I removed the evaporator from the rear and blocked off each hole in succession to ensure the evaporator wasn’t clogged. It is not. I also blew air from the vent hose that runs from the back to the front and air came out the front. I also rechecked the canister by blowing air in one nipple on the top air came out the other. I also tried that the opposite direction and it was also clear.

I checked the check valve that sits at the rear and with heavy pressure it does open, I have currently remove that indirect plumbed the vent hose from the evaporator directly to the front without the check valve.

I locked the gas tank cap back on and currently it has not overpressured in leaked. I wonder if it is possible that the check valve is requiring too much pressure to be released therefore forcing the gas after building pressure to take The path of least resistance pushing gas up the filler neck and out the cap.

I also did some reading that stated that if there’s any of the four corner vent lines have any sagging them they may create a P-trap affect begas moving into the curve therefore blocking air From escaping. Again not allowing the air to get out causing the gas to move up the filler neck when pressure is too high.

My current plan is to leave the check valve out. Why do we need it anyway? If pressure is high would we rather at freely move up to the front of the vehicle so we can be expelled? In addition I’m gonna order a new gas cap for the better seal. If anyone knows of one that will work for us 74 55 That has the ability to vent I would really appreciate the tip. I will slowly rule things out and I’ll let everybody know how it goes

Last thing I would ask that for those people who do change to a Weber carburetor where on the tank do they run a return line too? The only connections on this tank are the four vent hoses, two filler neck overflow hoses or filler hose, and the supply line to the carburetor to run the vehicle.
 
Yeah they make fuel pressure regulators with a built in return line so maybe but can't really tell
Is this a fuel pressure regulator? If so it runs from the fuel pump to the carburetor.
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For the fuel return issue, when I ran a Weber on my 1974 FJ40 I just installed an inline fuel filter with a third port. You can get one at your local parts store. See this post for what I did. You should have a fuel return as that helps keep the fuel cool as it circulates. It can also solve delivery pressure issues since it gives the extra fuel delivered a place to go.

The check valve is there to "gently" push back on the fuel vapors so that when you shut off your vehicle in the garage it doesn't continually smell like fuel. The vapors are trapped in the fuel tank and your wife doesn't hate you about the smell. As you drive and the heat/pressure builds, the pressure over comes the check valve and the fumes flow to the charcoal canister. It shouldn’t take much pressure to over come the valve.


My fuel tank pressure issue turned out to be a mud dauber had built a nest and blocked the vapor line the PO had uncapped. This was between the check valve and the charcoal canister. His solution was to drill a hole in the fuel cap. That should not be necessary on a 1974. Took me a bit to find the issue. “Vented” fuel caps vent IN to the tank, not OUT to the atmosphere.
 
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I have a 1971 fj40 that had a center bench fuel separator and a Rear fuel separator thing. I removed/capped off all the fuel evaporation lines and have been experiencing the fuel overflowing issue you described in your first post. Unless I'm gonna be doing a hard day of driving, I don't fill up my gas tank all the way to its max capacity so that it does not begin to overflow/expand due to the vapors when I let it sit. I'll just put 13-14 gallons instead of a full tank at 18 gallons; I have not had to worry about fuel spilling out since.

Also, regardless of the fuel cap being new, you'll still have issues of it spilling out if you overfill. You either have to get the emissions parts working correctly or perhaps drill a bigger hole in the gas cap to allow for the fumes to escape if you decide to fill up all the way. At least in my experience
 
For the fuel return issue, when I ran a Weber on my 1974 FJ40 I just installed an inline fuel filter with a third port. You can get one at your local parts store. See this post for what I did. You should have a fuel return as that helps keep the fuel cool as it circulates. It can also solve delivery pressure issues since it gives the extra fuel delivered a place to go.

The check valve is there to "gently" push back on the fuel vapors so that when you shut off your vehicle in the garage it doesn't continually smell like fuel. The vapors are trapped in the fuel tank and your wife doesn't hate you about the smell. As you drive and the heat/pressure builds, the pressure over comes the check valve and the fumes flow to the charcoal canister. It shouldn’t take much pressure to over come the valve.


My fuel tank pressure issue turned out to be a mud dauber had built a nest and blocked the vapor line the PO had uncapped. This was between the check valve and the charcoal canister. His solution was to drill a hole in the fuel cap. That should not be necessary on a 1974. Took me a bit to find the issue. “Vented” fuel caps vent IN to the tank, not OUT to the atmosphere.
I’m gonna take a look at your post I appreciate you sending it to me and thank you for the advice. Since I recently redid the entire set up from a fuel tank perspective all the hoses are new and everything seems to be flowing. However my gas cap probably isn’t very good so I probably need a new one in addition I probably need to make sure there isn’t any dropper droop within the vent lines so it doesn’t create a P-trap if gas moves up in there. Lastly where do you run your return gas from the front of your truck into your tank? I don’t have any more holes to plug one into. I’m going to read your post thank you
 
This was happening on my 75 pig. After making sure that I could blow through ALL the lines freely, I changed the cap, charcoal canister, and check valve. I'm not sure which one did it, but it has never happened since. Not once. The charcoal canister is not available anymore I believe, so I went with what the 60 series guys were using. Not an exact fit, but it's doing the job and fuel isn't leaking all over. Canister vc120 from auto zone. The vent that runs into the front frame mount is left off as the canister vents to the atmosphere without needing it. I also hear the cap venting on hot days like @Pighead and get a good bit of pressure when removing it.

 
I've only owned a 1974 FJ40. While I'm sure most things are the same, there will be some differences. These things worked great from the factory, they just get "previously owned" over the years. There used to be lots of bad ideas that persisted before the internet.

Here's what I'd do if I had a fuel tank pressure issue:
1. Buy a new OEM fuel cap. Looks like they are still available at $15 (if p/n 77310-60051 is correct). At only $15 there is no reason to go aftermarket (if they are still in stock, which they show).
2. Verify air flows freely from the tank, thru the fuel separator, to the check valve.
3. Bypass the (same) check valve just down stream of the fuel separator for now. (this is a highly suspect piece)
4. Remove the input line to the charcoal canister and verify air flows freely to the charcoal canister from the check valve location.
5. Leave the input hose off the charcoal canister and drive around. See if pressure builds.
6. If the issue is gone, I'd connect the input hose to the charcoal canister then run a fuel hose to a small filter from the output of the charcoal canister. See if pressure builds.
7. If there's no pressure building at this point, I'd go down to my local auto parts store and buy a three port fuel filter with 5/16" (8mm) barbs like in the photo (they are usually on the shelf, I think they were for a Jeep) and see if they can match up an EVAP check valve. If they don't have one, see the further discussion below on the check valve.

The top photo below is as I was installing the fuel filter that gives you a return line. It goes between the fuel pump and the Weber carb. For 1974, the FJ40 (and probably the FJ55) had a hard line from the fuel pump to the carb AND a hard line from the carb back to the fuel tank (this was the factory return). Since the Weber had a soft line input and no return fitting, the POs that put on aftermarket carbs always cut these two hard lines (but the location varied). Later year FJxx fuel pumps had returns built into them, but I think JimC thought the 1974 type setup was always the best since that dealt with the fuel return requirements where it needed to be addressed, at the carb.

I can only tell you on the FJ40, the fuel return line is a metal line that runs parallel to the fuel feed line along the frame. Both lines go thru the floor and into the fuel tank. The second photo below is a FJ40 fuel tank as viewed from above. The return line is the one going into the side of the tank, half way up (the clamp is not slid into place yet).

If you get this far, the only question is whether to install the check valve or not (assuming that was the culprit and you sourced a new one). You do not need it for the truck to run properly. You'll hear arguments for both. I chose to keep mine. It works fine (in my case) and locks in the fuel vapor when the truck is in the garage (my wife has a good nose). You should get a slight whoosh when you open the fuel cap to refuel. Just a couple psi worth of "whoosh", if that makes sense. If you cannot source a generic one at the auto parts store, you can get one at the link in my previous post (for more money).

I just leave the charcoal output vented to the atmosphere thru a small filter for simplicity. If my wife ever complains, I'll try to get the output routed properly.

Off topic, but general Mud recommendation is that you are better off with an OEM carb. Some people have problems getting Webers tuned right. I worked on my awhile, years ago. Mine ran great, so it can be done. But I only got 8 mpg. I'm closer to 14 mpg now with a OEM (and a few other mods). A couple other Weber threads to read are below. The second one with WeberSarge has a ton of information.

(Sorry this got so long winded...)



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I've only owned a 1974 FJ40. While I'm sure most things are the same, there will be some differences. These things worked great from the factory, they just get "previously owned" over the years. There used to be lots of bad ideas that persisted before the internet.

Here's what I'd do if I had a fuel tank pressure issue:
1. Buy a new OEM fuel cap. Looks like they are still available at $15 (if p/n 77310-60051 is correct). At only $15 there is no reason to go aftermarket (if they are still in stock, which they show).
2. Verify air flows freely from the tank, thru the fuel separator, to the check valve.
3. Bypass the (same) check valve just down stream of the fuel separator for now. (this is a highly suspect piece)
4. Remove the input line to the charcoal canister and verify air flows freely to the charcoal canister from the check valve location.
5. Leave the input hose off the charcoal canister and drive around. See if pressure builds.
6. If the issue is gone, I'd connect the input hose to the charcoal canister then run a fuel hose to a small filter from the output of the charcoal canister. See if pressure builds.
7. If there's no pressure building at this point, I'd go down to my local auto parts store and buy a three port fuel filter with 5/16" (8mm) barbs like in the photo (they are usually on the shelf, I think they were for a Jeep) and see if they can match up an EVAP check valve. If they don't have one, see the further discussion below on the check valve.

The top photo below is as I was installing the fuel filter that gives you a return line. It goes between the fuel pump and the Weber carb. For 1974, the FJ40 (and probably the FJ55) had a hard line from the fuel pump to the carb AND a hard line from the carb back to the fuel tank (this was the factory return). Since the Weber had a soft line input and no return fitting, the POs that put on aftermarket carbs always cut these two hard lines (but the location varied). Later year FJxx fuel pumps had returns built into them, but I think JimC thought the 1974 type setup was always the best since that dealt with the fuel return requirements where it needed to be addressed, at the carb.

I can only tell you on the FJ40, the fuel return line is a metal line that runs parallel to the fuel feed line along the frame. Both lines go thru the floor and into the fuel tank. The second photo below is a FJ40 fuel tank as viewed from above. The return line is the one going into the side of the tank, half way up (the clamp is not slid into place yet).

If you get this far, the only question is whether to install the check valve or not (assuming that was the culprit and you sourced a new one). You do not need it for the truck to run properly. You'll hear arguments for both. I chose to keep mine. It works fine (in my case) and locks in the fuel vapor when the truck is in the garage (my wife has a good nose). You should get a slight whoosh when you open the fuel cap to refuel. Just a couple psi worth of "whoosh", if that makes sense. If you cannot source a generic one at the auto parts store, you can get one at the link in my previous post (for more money).

I just leave the charcoal output vented to the atmosphere thru a small filter for simplicity. If my wife ever complains, I'll try to get the output routed properly.

Off topic, but general Mud recommendation is that you are better off with an OEM carb. Some people have problems getting Webers tuned right. I worked on my awhile, years ago. Mine ran great, so it can be done. But I only got 8 mpg. I'm closer to 14 mpg now with a OEM (and a few other mods). A couple other Weber threads to read are below. The second one with WeberSarge has a ton of information.

(Sorry this got so long winded...)



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Thanks for being so detailed. I noticed a few things on mine today that I worked through to try to adjust. First the vent lines coming from the four corners had some sag in them therefore they were collecting gas creating a P-trap effect. I corrected that today. So I’m gonna see how it does over the next couple of days with no check valve and that resolved. I’m also going to order a new gas cap at your recommendation.
 
Drill a small hole on your gas cap or get a vented cap.
 
Drill a small hole on your gas cap or get a vented cap.
A 1974 OEM fuel cap is vented. It vents "in" to the tank, not "out" to the atmosphere.
 
A 1974 OEM fuel cap is vented. It vents "in" to the tank, not "out" to the atmosphere.
In some markets it was still fully vented in and out, and emissions stuff came later.
 
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