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10/31/13 11:35am - Original Message: 'Gas Tank Fuel Qty'
rraider1
Former Member
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Woodland, WA - USA

Vette(s):
1977 T-Top,350,Auto Black Ruby Pearl,Steeroids R&P conversion

Joined: 10/14/2003
Posts: 1152
Some interesting info for discussion. In checking the Corvette specifications for fuel info, they list as follows. These are the specifications you can get that show all the info for your specific year.
68 to 70 20 Gal Aprox
71 to 75 18 Gal Aprox
76 to 77 17 Gal Aprox
78 to 82 24 Gal Aprox
 
75 to 77 have the rubber bladder that should have an extra hose on top of tank to help equalize pressure to keep bladder from collapsing.
78 to 82 used plastic liner.
 
the thing that stood out is 76 to 77 only having 17 gallons approximate.
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10/31/13 11:54am - Reply: 'Re: Gas Tank Fuel Qty'
Adams' Apple Lifetime Member
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Duncanville, TX - USA

Vette(s):
#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas

Joined: 11/8/2003
Posts: 19454
The physical size of the tanks are the same up to '77. The difference in how much fuel they spec has to do with the tube attached to the filler. The longer the tube, the less fuel will go in(safely). Once the fuel gets to the bottom of that tube, it will no longer take any more. If you try to add more, it will simply fill the tube up, and then run over, making you think it's "full". There is still a large area of free air space in the tank at that point. Now...if ya wanna cheat, you can shake/push the car side/side, and slosh the fuel around until the level gets lower, and then add a little more. Not really worth the effort, imo, but I've done it on longer trips. Wink

I think the main reason for doing it this way(with the tube thingy) was to keep the fuel level down below the vent, and vapor return lines, so raw fuel sloshage(a new word I just made up) back into the vent lines would be minimized.

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Joel Adams
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10/31/13 3:04pm - Reply: 'Re: Gas Tank Fuel Qty'
rraider1
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Woodland, WA - USA

Vette(s):
1977 T-Top,350,Auto Black Ruby Pearl,Steeroids R&P conversion

Joined: 10/14/2003
Posts: 1152

That sounds good I was wondering if it was fact their is bladders in the tank. I think the most I have put in my 77 was 15 gallons normally I would never let it get that low but we where on a cruise with C5 and C6 they tend to forget we canít go as far.

 

What did you think about the extra line for equalizing pressure for the tanks with bladders?

If that line is clogged it cause the collapse of the bladders.

 



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Bob R
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10/31/13 3:33pm - Reply: 'Re: Gas Tank Fuel Qty'
Case75
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, - Canada

Vette(s):
1975 Corvette 383 stroker w/Patriot 190 heads, Eagle stroker kit, Comp 279TH7 cam, DUI distributor, Edelbrock 7116 intake, Edelbrock 1806 carb, Hedman Hedders, TH400 w/TCI Breakaway 2400 stall, Transgo 1-2 shift kit.

Joined: 6/19/2012
Posts: 230
Interesting timing for this topic. In the process of installing the 383 in my car; I had planned on installing an Edelbrock fuel pump on mine to provide the 6 psi max to match my Edelbrock carb but up on investigation I realized (or remembered) that my '75 has a return line back to the tank from the AC fuel pump and the Edelbrock does not. Is this line needed to equalize the pressure in the tank due to the bladder? I've decided to stick with the AC pump and install a regulator instead. I replaced all my rubber fuel lines this past summer including the small ones on top of the tank by the way. Greg.

|UPDATED|10/31/2013 1:33:25 PM (AZT)|/UPDATED|


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Build Date: May 7, 1975. 383 w/267 RWHP/310 RWTQ
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10/31/13 4:22pm - Reply: 'Re: Gas Tank Fuel Qty'
rraider1
Former Member
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Woodland, WA - USA

Vette(s):
1977 T-Top,350,Auto Black Ruby Pearl,Steeroids R&P conversion

Joined: 10/14/2003
Posts: 1152

This is what I read and the issue davo76 had.

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....

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.



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Bob R
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10/31/13 10:54pm - Reply: 'Re: Gas Tank Fuel Qty'
daveo76
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Vancouver, WA - USA

Vette(s):
1976 Silver/Firethorn. L48, 4spd. Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.

Joined: 8/25/2005
Posts: 831
Thanks for bringing this up, Bob.  For one thing, your PM reminded me that my tank size is only 17 gal vs. the 19 gal that I had in my mind.  And another - I was wondering when I was looking through the Black Book as to why there was so much variation from year to year.  Now and I know thanks to you and Joel.  And yet another - although now it is something I may need to be concerned about - is the bladder and equalization tube issues.  I hope my bladder has not partially collapsed.  I still don't think it should have run out of gas so soon.  I might consider dropping the tank this winter as it is one of the few major things like that I have not yet done.  (Could check for the tank sticker then too!)  But then again, I think I will do some more investigation before I embark on that journey.  Dropping a tank just sounds like more fun than I can handle!   

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1976 Silver/Firethorn.  L48, 4spd.  Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.
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