New '69 40 owner with fuel (tank pressure??) issue (1 Viewer)

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Aug 4, 2020
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Location
Texas
Hey guys. New poster, but long time thread searcher/reader. Normally a quick search gets me an answer, but I've done a lot of searching and haven't found this specific problem.

I just purchased a '69 FJ40. First owner put in a 2F with 4 speed and PO did a Howell EFI 2 years ago.
Issue I am having is after driving for a short time fuel is pouring out of the fill tube between the cap. Fuel tank is under the seat in stock location but does not appear to be a stock tank. It's similar but all the EVAP lines are missing. Only a fill tube, fuel suction, fuel return, and sending unit. There is a aux tank in the back with switching valves for suction and return. PO stated it's rusty so he stopped using a while ago. I noticed that PO also recently purchased (looked new anyways) unvented fuel cap for primary tank. Since EVAP system has been removed I figured it needed a vented cap and that was the issue. Did a custom venting job on the cap in place to test my theory and it worked......for a time. It seemed like it was fixed with no issues after a couple low speed trips around the hood, but got out for a more extended drive (~10 mintues) and higher speeds (45mph plus) and problem has returned. I also notice that when the problem happens and I remove the fuel cap there is fuel up in the throat/neck of the filler tube.

Any ideas??
 
Tank switch valve is leaking..I bet if you look, there is fuel in the aux tank. I don’t have fuel injection but have 2 tanks. My switch valve leaked and when I parked on a hill would fill the down hill tank and overflow. Took me a while to figure it out...it was a new valve...then I finally splurged, got a replacement. When I took the old/new valve out, found one of the seat o-rings broken. The replacement fixed the issue.
 
Tank switch valve is leaking..I bet if you look, there is fuel in the aux tank. I don’t have fuel injection but have 2 tanks. My switch valve leaked and when I parked on a hill would fill the down hill tank and overflow. Took me a while to figure it out...it was a new valve...then I finally splurged, got a replacement. When I took the old/new valve out, found one of the seat o-rings broken. The replacement fixed the issue.
Thanks I'll check this out. I don't believe it to be the cause since I'm fairly certain the aux tank is empty. No sending unit in Aux tank so I cant check that way, but a little tapping tells me it's empty. Also, there is a noticeable drop in fuel level via the fuel gauge in the main tank when this happens. But I'll bypass that switching valves to eliminate it as a possible. I thought this was the problem in the beginning as well and the aux tank was essentially overfilling the main tank when I saw fuel in the throat of the fill tube. I used a siphon hose and was going to remove some of the fuel. As soon as I inserted the hose it was like I broke some sort of pressure and the fuel drained out of the fill tube and back into the tank.
 
I have a different theory IF you are actually using the rear auxiliary tank. Problem would be:
1. you fill both tanks.
2. you run rear tank first, so front tank is full.
3. your TBI returns fuel to front tank only, which is already full.
4. you'd need the electric switching valve (6 port) that returns fuel to the tank fuel was drawn from.

Lemme know if this is not the case, then I'll give your problem more thought.
 
Ok, after I read your post #3 above, I have a new idea. If you are not going to use the rear tank, then totally remove the switching valve, run fuel line from main tank directly to engine, but route return line from TBI back to the filler neck breather hose ("T'ed" into top breather hose), this way you should not have any air pressure build-up in the tank (if you run the vented gas cap). My theory is by returning gas/pressure back to the filler neck where returned gas can fall back down into the tank, and pressure can escape through the vented cap, you would not be throwing pressure back into the tank ???.
 
Thanks everyone. Attached are some pictures I already have. One of: The Rig when I brought it home on the trailer, Unvented Cap, Custom Vented Cap, Electric fuel pump, switching valves, The picture of the unvented cap was taken when I was driving and noticed the fuel coming out. (wife took the pic) You can see the fuel streaming out. I'm tied up with work today, but will get some more pictures and work done Friday and over the weekend.
Step one for me is going to be verifying aux tank is actually empty and bypassing the switching valves.
It that doesn't seem to fix it then I'll T into the breather hose. (will take some more creative piping work)

Will check back. Thanks again.

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1. When you drill a small hole into the gas cap to vent it, you drill a single small hole (1/2 the size of any of your four holes) into dead center of the cap.
2. I don't think the gas cap is your problem, building up too much pressure in the tank is, so I'd refer you back to my post #5 above.
 
@oilpatchaggie

More meaningful pictures! How about inside the cab, the fuel filler neck, the vent tube feeding from the top of the tank to the neck, etc.
 
@Downey I figured if 1 hole was good 4 was better! In all honesty I wasn't sure and figured if it worked I would look for a production vented cap anyways. I also saw some suggestions of just cutting the rubber o-ring seal on the underside of the cap? Is there one way that seems to be preferred? While it didn't seem to fix the issue I would imagine I still need a vented cap with the set up I described, yes? Will be working over the weekend on your post #5 suggestions.

@73FJ40 Will work on more pictures this afternoon/evening. The ones I posted were just what was on my phone from previous work. I haven't been able to spend any quality time in the garage.
 
Yesterday I was able to shake free and do some rustic plumbing work under the rig. Bypassed both switching valves and used only the tank in the stock location. I have been wrong before so I guess this makes twice. 1 of the valves for sure (maybe both) were leaking by and the aux tank was slap full. Guess my scientific tapping method was flawed. It's been running fine for two days with no fuel coming out of the filler valve. Another curious thing I found was the return line was plumbed to go into the bottom of the tank in the stock location, and the suction was from half way up the tank. (pictures included). I have no idea why that would be unless it is also rusty/dirty and they didn't want to pull off the bottom?? I swapped them to work the way I would expect is correct and will carry an extra fuel filter. I also dropped the aux tank (pictures included) I'll get rid of the fuel and see what inside condition looks like before I decide if I remount with new switching valves. Need to run some more of the fuel out of the main tank and will clean up the plumbing work. Noticed where rubber/pvc lines meet stainless they didn't flare the stainless. I don't know if that completely necessary, but I have some excess PVC coiled up that needs to get straightened out.

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Your rear tank looks to me to be one of the tanks that is full depth all the way back to the rear crossmember, not sloped up towards the frame at the rear of the tank to improve rear departure angle (see my ebay #264758943079, 6th photo). If this is the case I wouldn't be interested in re-installing that tank- - -of course your call.
 

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