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Dec 17, 2005
Well, not exactly...

I can't top the tank off on my '71 FJ40 unless I drive it a few miles before parking it. Otherwise the warm weather expands the gas and it piddles badly in the driveway, marking its territory. If I rock it side to side, the gas tank "burps" a bit and the gas level drops. Is there a vent line I'm overlooking that may be clogged or something? '71s are pretty simple, not like the later 40s with the vapor recovery systems. I don't think I'm overlooking anything.
If yours is anything like my 71 it expands and comes out the filler neck and then dribbles down the side of the body. I use my 40 for hunting, among other things. When it sets along side the forest roads, that are frequently heavily sloped, the fill neck is almost always on the low side. As the day warms up the gas warms and expands and guess what it does? The gas dribbles out the short filler neck.

I don't know of anything you missed.

Yeah, that's exactly what happens. Dribbles out the cap.

I could install the factory cap that positively seals, but two issues. It makes a vacuum and causes fuel starvation. And it isn't locking. In fact, I had to put holes in the skirts of my locking cap to keep it from vacuum locking the fuel system.
My 68 has had the same problem. Most the time from towing it up from the Phoenix area to forest in Northern Arizona. There is a steep climb out of Camp Verde heading east on 260. Since I would top off both tanks before leaving it woadl seep out of the hole for the chain attached to the gas cap. Not sure if 71 still had the hole. Thought the 71/72 had a goofy recovery tank in the rear of the passenger's wheel well. I know 99% of them are probably long gone. I did collect a few of the skid plates that protected them. Figure I used them somehow to make a splash guard for the rear corner since dirt collecting there is the main reason for rust in non-salted areas.

As far as your problem just making sure your passenger's rear wheel is parked on the highest spot should solve the problem. Just having something simple like a block of wood to park it on. Long term effect will be your paint will fade and peel off where it's leaking. Plus if this is happening in the same spot and it leaks on something green it would stay that way.

What Living in the Past said about being aware of how and where you park is the only easy solution to this problem.

I did do something to make it easier to find high places to park. I had a friend make a larger gas tank that went across under both drivers' and passengers' seats. This tank had the fill on the drivers side. Then to accomodate this tank I had him move the tank filler to the drivers side. This doesn't stop the problem of a too short fill neck that causes this dribbling problem. But, because of the LH drive we have here in the USA the roads are sloped to the passenger side. This then means that most places I park are sloping downhill away from the fill neck, whether out along side a forest road or in town.

I know, purists are cringing as they read this, but it works for me.

The stock gas cap is vented. It should allow fuel vapor/pressure to escape without pushing fuel. If it is so full that the fuel level is up into the filler neck past the point the vent hose ties in then it could purge fuel as the pressure escapes. It should not cause a vapor lock/vacuum situation. One solution is to put a T in the vent line and run the 3rd leg of T as a vent. Grab an evap can off something at junkyard or make your own. Hook new vent to one port and run a hose to the air cleaner/intake to pull vapors back into intake. NOT a vacuum source. This will allow tank to vent which will solve the vapor lock issue. You could still use the locked cap but get a new one with no holes. If you put the T in correctly the vapor will escape and any fuel that is trying to purge with it will fall back into tank. The evap can should not collect fuel, only vapor. I have this set up on my 1970 fj40 with a fuel injected motor with return. I have a positive locking cap and it works well. I tied 2 T's into the vent. lower one receives return fuel from injection and higher one hooks to vent. My vent is run to a high point at rear of cage but after running the fuel injected motor and realizing the return causes some fuel mix/splash I added an evap can as mentioned above.
Well, part of the problem is that my driveway is sloped. For security the truck is parked uphill, so we can more easily see if someone is trying to get under the hood.

The gas cap is a GM. I replaced the end of the gas filler with a GM pattern filler, because GM locking gas caps are available and locking caps for a 71 are not.
I will bet you have a pin hole in your tank.

This is exactly how my leak started on my BJ-44, and it is exactly what I thought it was before I took a closer look.

Spend time and watch the leak and where it comes from.


I repaired a fuel tank about a year ago . ( Im a fabricator/welder and very experienced in many types of metal work and fabrications dont try this at home ) This fuel tank was being used by a friend in his show ute (Utility) very old ford XM i think it was but it had a station wagon fuel tank , it had started leaking and he asked me to have a look and see if i could fix it . I wire brushed it with a grinder and wire wheel then I filled it with water till it over flowed it had 17 leaks after i wire wheeled it not just 1 , while it was over flowing i cut its floor out and welded in a new piece i had cut and shaped with a grinder and hammer , the reason it had rusted in the first place was that the water is heavier than fuel , the water sits in the bottom of the tank and rusts out the lowest parts of the tank usually the bottom of the ribs then eventually it leaks . this is my finding of rusted tanks , id imagine if its not leaking from the bottom of ya tank it is a hose or the filler and as you have said when its on a slope im thinking filler tube, put a locking seal cap on it with no holes then add in a breather hose and small inline filter
There is no pin hole in the gas tank. The gas is piddling out the filler tube at the gas cap. The tank is lower, and there'd be gas leaking inside. There is no gas inside, nor is there even a hint of gas fumes inside.

The simplest solution is to try to not let it fill completely up unless driving at least five miles (half a gallon).

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