I thought I would pass this along to anyone interested.....

I removed the '75s gas tank last week to take out the bladder since I could only put in 7-8 gallons of gas and the fuel gauge would go from empty to full. I originally thought the fuel sender might be bad so earlier this year I completely emptied the tank and filled it as far as I could which was about 10 gallons then, less now.

When I took the fuel sender top off the tank I looked inside and could see the bladder bulging out from the sides. The bladder itself is made out of pretty thick pliable stuff and when I removed it I was surprised, but glad, to see that the inside of the tank was clean and shinny, probably as good as the day it left the factory, so the bladder had not had any holes to leak gas between the tank and the bladder.

I read the section in the GM Service Manual (76 Supplement) and it would seem that GM knew of at least one condition that would cause the bladder to collapse: (QUOTE) "There is an equalizer pipe on the tank top which connects to a .035" orifice. The purpose of the equalizer hose is to keep pressure from building up between the tank and bladder and COLLAPSING the bladder." (UNQUOTE)

Then they talk about diagnosing the (QUOTE) "condition of running out of fuel or a limited fill condition, 4 to 6 gallons maximum....." (UNQUOTE) and to check for: 1) Collapsed, kinked, or disconnected vapor hoses at the canister at the frame rail from the fuel tank. 3a-c) Check the vent tube and the orifice for blockage....

That sure seams like the problem most of us are experiencing!

There is a T-connection on the top of the tank joining the two vents. The "equilizer" vent from between the bladder and the tank wall and the other from inside the bladder where the fuel resides. The tank/fuel vent was ok and allowing fuel to be taken out by the fuel pump and replaced by air thus equalizing tank pressure. BUT the "equalizer orifice" vent was blocked and not passing any air. I cleaned it out as they suggested and some "gummy gunk" came out. So that is probably what caused the bladder to collapse.

I won't be reinstalling the bladder, but for those of you who are worried about not having the bladder in the tank and are not getting a maximum fuel fill you might want to try following the procedure that GM recommends for this problem. Nobody currently makes a replacement bladder. Quanta is now making a replacement tank WITHOUT the bladder so if your bladder was leaking the inside of the tank may be rusty or may even have pinholes leaking fuel. Quanta's tanks look pretty good for the price. I looked at one at Carlisle this year.

I am also replacing the fuel sock strainer that is on the end of the pick up tube. The adhesive that binds the end of the sock together was completed gone and the sock was open, so of course there was no "straining" taking place. I'm going to open up the lines at the fuel pump and blow both of them out.

One other lesson learned: I am replacing the rear bumper cover anyhow so I cut that off first and had access to rubber fuel lines and tank cover bolts from the rear. Now that I have the tank, tank cover, and bumper cover off I would hate to have to try and install the bumper cover without removing the tank and tank cover. I don't think I could get my arm or fingers up there to secure the top retainer nuts.

BTW: I did find the build sheet on top of the tank cover and it's about 50% readable and in pretty good shape considering how long it's been there! What info I can make out matches what is on the original window sticker and on the dealer's order.

 

Ken, very interesting find indeed!!! I did'nt know about this "orifice". My problem was'nt a plugged line to the canister because mine was full of holes anyways :lolg: May be my "orifice" was plugged too??? Hell, my water overflow tube at the filler neck had an acorn in it!!! This line(or the vapors in it) would have been venting to the atmosphere. If you don't want that "bladder" material, I'll take it :yesnod: That stuff is great for jackstand pads, mudflaps, anti-squeak pads, faucet washers, etc. :lolg: When I replaced my '74's rear bumper, I too dropped the tank. It's just so much easier to get to those bumper bolts that it is'nt funny :lolg:

 

Paul -

I'm keeping the bladder and I'll use some of it for new anti-squeak pads.

Last time we "chatted" I said I did not have a tank shield. I was looking for a shield on the forward side of the tank, like the '74s. I knew the tank cover was in place....

Now we know for sure that the bumpers were attached to the body BEFORE being put on the frame....... and that that HUGE tank cover was installed over the top of the tank AFTER the tank was installed to the frame otherwise they never would have been able to cut those rubber hose lines EXACTLY to length for a tight neat fit. AND the build sheet was stuck to the top of the tank cover for them to use as checkoff list. I had seen pictures of the cars on the assembly line but have never actually seen a picture of the body being lowered on the car, but had guessed that the soft bumpers were installed later, guess I was wrong.....




[Modified by KenSny, 8:56 AM 1/5/2004]

 

Thanks for the info Ken. I need to check my vent now before I have this problem... :thumbs:

 

The only way to get at it is to drop the tank as the "between the tank wall and bladder" vent is in the fuel filler-sending unit assembly which can only be removed from the top of the tank. Not enough room to do with the tank installed.