Ghost in the Machine...

[Biggun]

It's in the fuel tank, actually.
I haven't had the truck all that long, but the owner before me recently installed a brand new fuel tank and fuel pump. Yay, I thought; that's a couple of things I won't have to address for quite a while!
Wrong.
Ever since I've had it, the fuel tank periodically expands and contracts with a loud BONG. I've never had a vehicle that did that, but whatever.
Also noticed that the tank pulls a helluva vacuum...but that's good, right?
Anyway, yesterday I noticed a fuel leak under the tank; a slow drip. This morning, it wasn't dripping at all. This afternoon, the drip was double what it was yesterday. I got under there with a scope and a mirror and could find no leak or wetness whatsoever from the bung, nor any of the lines, nor the filler hose or vent tube. The leak seems to be coming from somewhere below the seam of the tank, but I followed the seam and there's nothing. I can't see any of the lower sides or bottom because of the plastic shroud.
Is there an inspection bung on the underside of the tank? I cannot for the life of me figure out where the leak is coming from. The odd banging noises emanating from the fuel tank at all hours of the day and night are suspicious, though...
Anyway, I thought I'd ask here if anyone has come across this. It's going to eat at me all night and day tomorrow until I can get back to it.
Thanks--
CE

 

[alpinecrick]

Kenworth? John Deere? Ski Doo? Or is it safe to assume the vehicle in question is a GMT 400? Pickup? Long or short bed? Suburban? Tahoe? What year? What color is it? (OK, OK, just kidding on the last one).

It shouldn't be pulling a helluva' vacuum. When my tank on my trucks are low on fuel in warm weather sometimes I'll have a slight vacuum when releasing the filler cap. You have a vent problem. Warms up and leaks in the afternoon, contraction and expansion as temps change.

It would be nice if it was the filler cap that is the culprit. Make sure it's the correct one.
Take the cap off and leave it off while sitting and see if it still has the symptoms.

The overflow hose is inconveniently located directly above the filler hose, and ain't easy to access. The filler hose probably needs to disconnected at least at the tank side to check the overflow hose. Make sure your previous owner didn't "improve" the system by blocking off the overflow or vent hose or something......

The vent tube is located just in front of the fuel pump access, you'll have to lower the tank to see if that hasn't been blocked. I don't know how else to do it.

Youtube should have some videos to show what the factory setup is.

 

[Biggun]

Thanks for the info, and my apologies for not introducing my gal: 1998 Chevy Silverado K3500, 350, auto, crew cab, long bed, white.

 

[RichLo]

I had a Blazer with this exact same problem, it turned out to the the EVAP system. The tank is supposed to have a small vacuum on it under normal circumstances but there was a valve in the EVAP system that failed and caused full engine vacuum to pull on the tank.

 

[2001ZR2]

Does it fill slowly? I had a problem with new filler neck on my ZR2.

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Yeah after a month it blocked the fill tube

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[Biggun]

Well, I found the problem. The tank was cinched up tighter’n Dick’s hat band, and it buckled. Made a tiny stress crack where this rib gave way.
Of all the dumb luck...

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[RichLo]

Thats probably not the problem, thats probably a side effect.

That is exactly what happend to my blazer with the EVAP canister problem.

 

[stutaeng]

Thats probably not the problem, thats probably a side effect.

That is exactly what happend to my blazer with the EVAP canister problem.

I'm with RichLo on this one. I was messing with my 06 suburban purge solenoid valve because of a SES light.

I found a video on how to test the valves and decided to test the valve on my '99 4.3 and my '00 K3500/5.7 just for fun.

It turned out the valve on the K3500 was stuck closed. My symptoms were fuel pressure somewhat fluctuates and refueling acting like if tank is full, presumably because of back pressure from vapors.

Depending on temperature, the gas tank has vacuum or pressure, which the EVAP system takes care of.

I put a working valve from another vehicle for the time being and watching to see if symptoms go away.

The EVAP systems are sort of complicated in themselves. Strangely, they seemingly never fail on this generation trucks, I thought. Newer vehicles have more finicky EVAP systems.

Let me see if I can find the video on testing this purge valve...

 

[stutaeng]

https://www.google.com/url?sa=t&sou...BMAZ6BAgIEAE&usg=AOvVaw3XpH2YyPweIDhJqD6KaTBB

 

[Biggun]

UPDATE (...and still not fixed...)

So:

• new tank
• new gas cap
• new purge solenoid

Still pulls a hella-vacuum. Actually, it's worse now that there's not any leaks.

Having said that, there is an oddity in that there are two electrical connections on the fuel pump, but only one is hooked up. I did not see a lead anywhere for the other connection and no indication on the harness of someone having cut it off. I assumed that the other connection is there for a dual tank setup (???) which it doesn't have and never did. If the tank pressure sensor is the other connection, there's the problem...but where'd the harness go...

Also, is the solenoid energized to keep it closed or to open it? Again, I'm assuming it's the latter, but if it's the former, maybe I've got a bad circuit from the PCM?

Thanks, fellas