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Fuel Tank Capacity


Well-known member
Dec 5, 2001
Cross Roads, Texas, U.S.
'89 dk red coupe
A couple months ago, I started a thread about fuel tank capacity.
My '89 won't take more than about 18.5 gallons even if I run it 'til it stalls; and yes, running it 'til it stalled was done on purpose - to find out the fuel capacity!
Various other owners noted that their cars had a bonafide 20 gal. capacity, while still others had cars like mine, but nobody could tell me why.

I think I've found out, from a ZR-1 website:
"PROBLEM - When you go and fill up your fuel tank even though the gas gage reads empty, your tank takes less than 20 gallons.

CORRECTION - There is what is called a "bladder" inside the fuel tank. This bladder is not pliable but a rigid black plastic. What can and does happen is this plastic liner pulls away from the fuel tank walls causing bulges or bubbles. These bulges can shrink the size of the liner and therefore reduce the capacity of the fuel tank. The only correction is to replace the fuel tank. Thanks to the ZR-1 Registry, Dennis Gratton and Jim Ingle for this information.

I also found something describing what I had originally suspected, but since this also comes from a ZR-1 site, it only addresses later models:
My question regarding this issue is: Is there a way to tell if you have the "rubber extension" on the fuel fill pipe, and is there a way to remove it?

"TITLE - 18 gallon fuel tanks
YEAR(S) - '93-'95

PROBLEM - Near the end of 1993 GM found themselves with a weight problem on the 1993 ZR-1. To meet the same EPA class GM had to come up with a way to cut some weight off the '93 ZR-1. They did this by installing a rubber extension on the fuel filler pipe to limit the fuel capacity to 18 gallons. The fuel level float arm was also adjusted to reflect the 18 gallon capacity. All 1994 and 1995 ZR-1's were suppose to once again have 20 gallon fuel tanks. But some owners may find that their reserve light comes on before other '94 and '95 owners but take the same amount of fuel. The problem seems to be that the fuel level float arm that was adjusted for the '93 may not of been re-adjusted for the '94's and '95's

CORRECTION - The tank sender and fuel pumps can be removed and recalibrated (90 Ohms produces a full gauge reading and 0 Ohms reads empty). A new gasket will be needed to complete the repair.
Tank Capacity

The bladder is an issue that seems to be on the rise for many C4 owners. A common reason that contributes to this problem is the use of gasoline octane additives. They will loosen up the bladder from the tank assembly.

Just a few words that maybe interesting to those who use this product.

Good luck,

The other thing worth noting here is the accuracy of the fuel sending unit in the tank. The signal it sends to the gauge is only as good as the sender adjustment. Plus, if any work was done in the tank or the tank was drained for removal or whatever the reason, the sending unit could have been knocked out of adjustment.
Yeah, I understand about inaccurate senders: that's why I'm more inclined to monitor miles traveled on a tank. What annoys me is only being able to put about 18.5 gal. into an empty tank - by which, I mean the point at which the engine won't run any more. It's not a real big deal, but it just means more frequent fuel stops.
If I'm driving from home to the beach for example, about 370 miles away, and I'm driving shall we say 'aggressively', I have to make one stop for gas when I don't want to make the last few miles on "fumes", when absolute dry takes 18 gal. or so. Since this usually occurs around midnight on a Friday, it's a PITA.

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