C2 Sending Unit Question

[Doug M.]

If this question has been posted/answered before, sorry for the dupe.

About 20 years ago, I replaced my gas tank along with the sending unit (I know, I should have hung on to it). Since then, if I use premium unleaded, it works just fine with no leaks; however, since my car has an L79, it really wants higher octane fuel.

Over the years, I've tried aircraft fuel and high octane additive. In each case, the special/modified fuel has leaked like crazy from the sending unit. One time I tried to use fuel with no ethanol with the same result. Once again, if I use premium unleaded, there are no leaks -- I'm stumped!

Any ideas on what's going on? Thanks in advance!

[Gary B.]

Doug,

I assume when you say you’ve had leaks at the sending unit, you mean at the periphery of the locking ring? And not from where the electrical posts pass thru the metal disc. If at the periphery, then the leak is past the large ( ~2.5” diameter) O-ring. If so, could the ethanol in pump gas be causing the O-ring to swell, making a better seal? That is until the ethanol eventually destroys the O-ring.

Before you installed the O-ring did you soak it in motor oil for 2 hours like the ‘66 AIM says to do? And it is a GM O-ring? Repro O-rings have leak problems according to several people.

Gary

[Doug M.]

Hey Gary, thanks for the response!

The leak is at the periphery of the locking ring. Interesting idea about the ethanol pump gas, but the tank/sending unit has been in the car for 20 years, with no leaks, except when I try something other than pump gas.

The tank and sending unit were replaced by a reputable corvette shop, so I'm hoping they used the correct O-ring and prep'd it properly... Maybe I need to drop the tank and replace the seal, just to be sure?

[Leif A.]

Doug,
I can't address the chemistry as to why your sending unit leaks at the O-ring when you use anything but ethanol gas, but I can tell you that your L79 should operate perfectly fine with 91-93 octane with 10% ethanol. That's all I ever used in my '67 L79 along with hundreds of others. I, suspect, if your L79 is not running/behaving properly with ethanol gasoline that there is an issue with the engines over all tune i.e. carburetor settings/function, distributor tune, timing, etc. I don't know your competency level when it comes to the fine tuning of your car but that's where I would address the ability of your car not running well on ethanol gasoline...it, absolutely, will operate perfectly fine with today's unleaded premium fuel.

[Harry S.]

I'm with Leif on the chemistry question. I do know that the original rubber gasket is thicker than the reproduction gaskets.

Al I can suggest is to look for an NOS gasket and an NOS metal seal ring and reinstall the unit.

Make sure you soak the new rubber gasket in oil for a few hours.

Good luck.

[Doug M.]

Leif and Harry,

Thanks for the responses, very helpful. Will take a look at fine tuning as you suggest. And once again, I will pick up a new o-rind a locking ring.

Thanks again!

[Donald H.]

I have to agree with Leif, I have had three L79's, a 66 and 67 corvettes and a 65 El Camino and they all ran and run fine with pump gas and no leaks.

Don

[Doug M.]

Thanks Don.