Extremely High Fuel Tank Pressure

[adill]

Hello all!
The other night my truck stalled out on me and wouldn't start back up for a while. Both tanks were showing empty, my gauge typically reads till about half, finally got to the fuel station and opened my rear tank and diesel exploded all over me. I filled my front tank. As I started driving the rear tank built up pressure again and started pushing fuel out of the brand new cap. (I switched them around to be sure it wasn't just a bad cap). After I parked, fuel kept coming out for about 20 minutes.
What could this be? I am totally stumped and have not idea. Any help would be much appreciated.
Thanks in advance!

 

[madpogue]

Are you sure it's pressure building up, and not just fuel returning to the wrong tank? What happens when you switch the selector to the rear tank?

What year truck?

 

[BDCarrillo]

FSV clogged/jammed/faulty... it's returning fuel from the front tank to the rear. Disassembly instructions are in the tech section

 

[adill]

I can't tell if it is taking anything from the rear tank when I switch it to that or not. Is the fuel tank level gauge anyways connected to that? Cause usually it reads till about half way then starts dropping fast. but when I filled the front tank up it didn't change. Would it be easier to just replace the FSV?

 

[madpogue]

Yes, the FSV determines which tank feeds the gauge. So if the gauge is reading the rear tank properly, but not reading the front tank, then the FSV is highly suspect. Most likely, it's working normally on the rear tank. But when you switch to FRONT, it's switching the SUPPLY to the front tank, but the RETURN is still valved to the REAR tank. So as the truck runs off the front tank, the return fuel it doesn't use overfills the rear tank. That's why I asked what it does when you drive it on the rear tank. If it works normally when running off the rear tank, without overflowing, then that would further suggest this is the case.

Direct OEM replacement FSV is VERY expensive. You can remove it and test the operation by energizing the valve terminals (it's polarity-based), and checking what ports are open/closed by blowing air into them. As said, check the tech section for info on how to open it up and clean it out; this might restore normal ops, and save you a small fortune.

Again, what year truck?

 

[adill]

When I switch to the rear tank, it still was spilling out. Maybe I didn't run it long enough on the rear tank. How expensive is an OEM? And where is the tech section? I've only been on here a few times and don't know where to find it.
Thanks

 

[adill]

It's an 1986

 

[TahoeTom]

Tech articles are at the top of the 6.9/7.3 IDI page.

 

[madpogue]

Diagram - http://userpages.chorus.net/elephant/FuelTankSelector1985.pdf .

Do you have a spare 12V battery, or a jump-start box? If so, you can use it to test the valve after removing it. Connect it to terminals 1 and 2 on the valve, either polarity. Then connect it again with the polarity reversed. Each time you reverse the polarity, you should hear a definite "thwick" sound from the valve, and it should sound the same going both ways.

As this happens, it should switch both the supply and the return. In one position, if you blow into the supply (larger) port that goes to the engine (the ports on the valve are usually marked, or you can take note as you remove the lines), air should come out of either the front or rear supply port, and only one of them. If you blow into the engine return port, air should come out of the corresponding return port (and only the one. If you find that the engine supply blows to the front port but the engine return blows to the rear port, then you've confirmed a bogey "half-lazy" FSV (switching only the supply, not the return). If the port openings are consistent, then do the polarity switch thing and test again. You should find that both supply and return have switched ports. Again, if you only find that one has changed, it's a valve problem.

As you'll read in the write-up in the tech section, a common cause of this is the pickup strainer in either tank (or both) coming apart, and bits of it getting sucked up into the supply and lodging themselves in the valve. This will become evident when you take it apart. But what that also means is that in whichever tank in which this happened, it won't draw fuel below about 1/4 tank (think of a spray bottle with the dip tube thing not quite going to the bottom). Another challenge for another day, but something to be aware of, if you find the truck acting like it's starving for fuel even though the gauge says 1/4.

 

[adill]

Awesome! Thanks for all the help! I will get to it along with changing my injectors and radiator.

 

[OLDBULL8]

Dealer price for FSV is $260 to $300. Auto stores have FSV's for around $80, but do not have the quick connects, the fuel lines have to be adapted. Both are made by Pollack. Auto stores may designate for gasser's, but will work for diesel, just the fuel line connection and mounting is why for gassers.

 

[Wvdirtroad]

Tip on the fsv... The front tank at a minimum needs to come off,including skid and straps. I removed the bed on mine to make everything easier. If you go with anything other than I believe an FSV1 you will need to replace the connector plug, and splice into the nylon/viton lines. If also check to see if your pickup strainer has cracked/ fallen off your senders. I took a piece of rubber hose and v'd off the end to prevent tank suction to the side and that foxed the running low on what's actually a 4-5 gallons left tank too.

If you decide to switch to a newer generation valve I can elaborate more. I have the wiring mapped out for the valve to convert to the newer style. Not necessary, but the Fsv3 was the only carried fsv in my area and the part can be taken back to advance auto anytime it breaks under warranty. All in all I think repaired for abut 120, including the valve, new connector, rubber lines for tanks and some couplers for viton lines.

 

[madpogue]

I pulled the valve from my '85 without touching the front tank. It doesn't have a skid plate, however. I looked at our two '95s, and it'd be close, but still looks like it would be doable with the tank in place. At worst, you might have to remove the skid plate.

OEM replacement strainer is available on Amazon, $30-ish.

 

[Wvdirtroad]

For some reason on my regular cab I just couldn't do it with the tank on. I suppose extended cabs have a longer frame which may carry the fsv far enough forward its not an issue.

 

[madpogue]

We have two regular cabs and an extended cab. FSV is in pretty-much the same place relative to the front tank on all three. The '85 on which I removed the FSV with the tank in place is a regular cab. Just my experience : .