Problem- Ocassional (and seemingly random) overflow of rear gas tank.

1. First happend while running on front tank. Rear tank 1/2 full. Though switching valve had failed, been using rear tank with no problems
2. Happened again this week. Running on rear tank this time, front tank empty, rear tank again only 1/2 full.
3. Cannot replicate in my driveway.
4. Took a 40 mile trip running on front tank, no problems.
5. Rear tank is hard to fill with gas, must hold at a funny angle just right, or the pump won't pump. Could this be related?

So, anyone know why a 1/2 full tank is overflowing?

 

I had that same problem a few years ago. turned out to be the selector valve, located about midway inside the drivers side frame rail. it would pump all the gas from my front tank into the back. depending on how much fuel was in the the tanks, it would overflow the rear tank. i could not find a new a one that matched at the dealer or auto stores. went to a boneyard and got one.

 

Quote:

Originally Posted by Handegard

Rear tank is hard to fill with gas, must hold at a funny angle just right, or the pump won't pump. Could this be related?

I suspect the fill tube is installed incorrectly... inside of it is a flexible plastic
tube which I'm guessing is to allow the tank's vapors & air to escape as they're
displaced by liquid fuel. Without it, the nozzles apparently detect back pressure
and shut off as a result.

Pix are from a side tank tube so the shape maybe different than rear tank
plumbing but the ideas are the same:

Quote:

So, anyone know why a 1/2 full tank is overflowing?

I've heard of this before, the other post about the the selector valve is good
info. You might also check to see if your low-pressure fuel return plumbing is
installed correctly.

-chris

 

All very good suggestions.

I need some more information from the original poster though.

Does the truck have evaporative emissions? It should state this on the emissions lable on the radiator support.

If the truck has evaporative emissions, check the line going from the tank to the charcoal canister. Check for plugs in the line. This is how the tank vents with evaporative emissions. The vapors from the tank goes to the charcoal canister where it's stored when the engine is not running, or it's routed from the canister to the PCV valve for burning of the vented vapor during the purge cycle with the engine running.

Also make sure you have the correct gas cap. Either for evaprotive emissions, or non-evaporative emissions. Putting an Evap cap on a non-evap truck will cause a simular problem.

Also make sure that the evaporative emissions system is working correctly, if equipped, not plugged off by previous owners etc... as it can cause a simular result. If parts of the Evap system were removed and /or plugged by a previous owner (if orriginally equipped), repairs to the system, or a non-evap cap will be needed.

Also check the PCV valve, canister purge valves and/or solenoids, etc...

Things that previous owners replace, block, or remove, that don't seem to be related to other components or parts, can sometimes have a unforseen side effect.

 

Edit: Double Post.

 

I've already had my selector valve replaced once, it was overflowing the rear tank. I do suspect it's bad again, but what drives me nuts is that when the rear tank overflowed, it was only 1/2 full. I don't mean it started at 1/2 full, then overflowed, I mean at the moment it was coming out of the tank, it was 1/2 empty both times.

We may have a winner. I took my gas tanks out about a year ago. When I was putting them back in, I do remember something was funny with that vent tube in the rear. I don't remember what, and I was under the gun to get my truck out of the bay at the time, and I've forgotten all about it untill now... I'll have to check into it...

Ahh, more good info. Most of the emissions related stuff was missing or non-functional when I bought the truck. This created many nightmares, and when I replaced the engine, I used a much... simpler approach.

The line from the tanks that used to go to the charcol canisters is currently open in the engine bay, and not sealed. There are no charcol canisters anymore, and non of the associated parts are in the engine bay anymore.

Plus, I don't know about gas caps. Both are aftermarket caps, I replaced them one at a time, and even being a emissions guy, I never would have though about that!


Thanks guys, hopefully we'll get this one knocked out of the park!

 

It's theoreticly possible, since the line was left open, something could have gotten inside and plugged it off.

I would look into different "Vented" gas caps, myself.

 

I was actually just comming back to post about that...

I was thinking it over, and I wanted to ask about the caps...

How can I tell if a cap is evap or non evap? Mine have rubber seals, I assume are evap, but they are just generic Stants or something..

I'd like to get a pair of matching locking caps, I don't have any idea how to tell what ones are sealed...

 

You can't really tell from looking at them. They go by part number.

The Evaporative emissions cap holds pressure, and doesn't normally vent, unless there is a dangerous buildup in pressure then it will release. What I think might be happening to you. Either the truck sits in the sun, and the tank pressurizes, or when you drive the truck there is a vacuum forming in the tank. When the cap safety valve releases, "Blorgh" all over the place. The tank normally vents, releases pressure, through the charcoal canister, and parts of that system are missing, possibly plugged on your truck. etc..

The Non-Evaporative emissions cap, will vent to atmosphere. Letting air in, and letting pressure out.

 

I've seen LMC & RockAuto both listing the proper replacement caps