Need Help Diagnosing Fuel Tank Problem

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Jan 31, 2003
"the whale's vagina", CA
Was out on the trail yesterday and had some problems with my truck running.

Specifics: 73 fj40 w/vortec 350 conversion. Have dual tanks, stock up front and aux in rear, conferr type. Both are plumbed correctly with multiple filters and the setup has worked well for a couple years. I have a tank switch valve which switches between tanks.

The problem is that the engine will die on me when I am switched to the front stock tank. Once I clear the line, via the purge valve on the fuel line, and switch to the rear tank it runs fine. I was able to get home from the trail by transferring the fuel from the front to the rear tank via a siphon tube. I checked the fuel pressure and it is fine, plus the engine ran great the whole way home on the rear tank so therefore it makes me believe that it is obviously a problem with the rear tank setup. The fuel filters are all fairly new, 1 year, so don't think it could be this although it would be possible that a large piece of something got caught in the filter? The front tank is fairly new as well and I have inspected the inside before so don't think it is rust. I inspected all of the lines and nothing is pinched, cut, etc.

The only difference I noticed is that when I take off the front tank gas cap there is a lot of pressure. A large hiss of air happens. This has never been the case. Also, I compared the front and rear tank gas caps and the front one "rattles" when I move it back and forth. They are both vented gas caps and look like they have the exact same plastic mechanism on them. Since the front tank cap is rattling and the rear doesn't I think the gas cap may be broken and not venting? I don't think the inner diapram on the cap should rattle like this. The front stock tank is vented via the stock fuel seperator and I have lines running to the engine charcoal canister.

The above leads me to belive that the problem may be as simple as a faulty gas cap? The main question is will a faulty vented gas cap cause the symptoms I describe above? Anyone with expertise in the is area, any help is appreciated!


what is the condition of the tank?....rust inside causing problems?....thats the problem i had with mine...mine is a 1968 fj40...may not be you problem but something to check anyway....
good luck
Well, to eliminate the cap can't you just run it with them off to see if it makes a difference?

I'd just start checking pressure from the carb all the way to the tank outlet on both systems. Having the pump pressurize the lines while I check each area, blowing through the lines and filters when they're off etc... checking operation of the switches and all that.

Is the cap making a vacuum sound or a pressure sound when removing it?
the tank is new, no rust. the tank has pressure in it, it is definetly releasing pressure when I release it. Engine is FI, not carbureted. Again the main question is will a faulty "vented" cap cause such issues??

AFAIK the venting cap is supposed to let air in when the pump sucks, not let fumes out into the atmosphere. I'd say the cap isn't your problem, it's the pressure.

I don't understand why your tank would be pressurizing, EFI or Carbed, other than slight heat expansion.

I was hoping that if you left the cap off and ran it it would not pressurize and you could determine whether or not the pressure in the tank causes the problem or not.

If it runs the same then I'd say that the cap is not causing it, althouth you may want to replace it, and that the pressurizing of the tank is not the cause of the problem.

If it runs fine then I'd say that yeah it's the pressurizing of it and then you need to figure out the source of that pressure. Again, a new cap may help with that but I still can't see where the pressure is coming from.

Ramble..... You feel me?
Here is some correspondence I just got:

"Second, I think the pressure you have in one tank is not's vacuum. When fuel goes out of the tank, air has to go in to replace the volume lost...otherwise the vehicle will starve when the fuel pump can't suck because of the vacuum in the tank."

this makes sense to me, I believe it is not pressure but VACUUM I am getting in the tank. I believe that the stock vent lines on the stock tank which run to the charcoal canister only allow fumes to vent outward to the canister and be reused and burned in the intake manifold. I don't believe they have any way to allow air INTO the stock tank. Therefore if the cap was the piece that was allowing air to come into the tank to displace the fuel being used and it is now not allowing this and blocking any inward air movement this must be the problem? the vent line system that goes to the charcoal canister I don't think is the issue as I would have major fumes in the cab as I have had in the past when I did not have a proper venting system.

any opinions appreciated.....
Yeah LIKE I SAID... The cap allows air into the tank not out.

Is the wooshing sound a vacuum or pressure releif sound?

What you just stated above is all correct.

I ask again...

WILL IT RUN OK WITH THE CAP OFF?????????????????????????????

Is this thing on? Testing testing.........
Drill A Small Hole In The Gas Cap About A 1/16"-1/8". What It Sounds Like Is You Valve In Your Cap Isn't Working. I've Have Had To Do This Before.... And When Your Switching Tanks With Your Valve If It Will Let You Open It Half Way So Your Sucking From Both Tanks, Then Open It All The Way To The Tank You Need... So Its Like Priming The System... Hope This Helps...
I believe you, 1973Guppie: it's pressure. When you crack the cap, you can feel it blow out on your hand and smell it too, right? Heat will cause your gasoline to vaporize. As you know, gas takes more space than liquid, so the amount of gas inside the tank increases and tank becomes pressurized when it heats up relative to its intial temperature--even as fuel is drawn into the engine for combustion.

But please do not drill a hole in your gas cap and make this another rig that spews excess hydrocarbons into the atmosphere.

Instead, make sure that your charcoal canister (and thus the fuel tank) has vacuum on it when it should have, because you are right about this (mostly):
1973Guppie said:
I believe that the stock vent lines on the stock tank which run to the charcoal canister only allow fumes to vent outward to the canister and be reused and burned in the intake manifold.

but not about this
1973Guppie said:
I don't believe they have any way to allow air INTO the stock tank.
The check valve should flow both ways.

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