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Hagerty
Hagerty Employee

Piston Slap: It's a long, long run to a full gauge reading

I have a 1965 Chevrolet Impala SS, and I have replaced the float in the gas tank twice. I have also checked the gauge (by putting 3 volts to it and it does go to full as intended), but when it is full the gauge only reads half a tank.
https://www.hagerty.com/media/maintenance-and-tech/piston-slap-its-a-long-long-run-to-a-full-gauge-r...
4 REPLIES 4
Land_Ark
Intermediate Driver

I'm actually having a similar issue with my '67. I replaced the whole fuel system a few years ago and the gauge doesn't seem to go above 3/4. But, when I first did it, it did work.
I won't go into all the woes of the fuel system since then, but let's just say I drove it for the first time in a few months on Friday, stopped and got gas, and now my garage reeks of gas fumes. Thus is the life.
TonyT
Technician

If your '67 is still stock, it doesn't have a charcoal canister/charcoal air cleaner filter. It would be a fairly involved procedure to retrofit a canister to it, so I would highly recommend that you investigate installing a throttle-body fuel injection system instead. Most of your fuel odor problem is emanating from the carburetor after you shut the engine off because there is a good chance that your local gas station is selling you fuel with ethanol in it. The fuel vaporizes at a lower temperature than non-ethanol fuel, causing the problem you describe.
Land_Ark
Intermediate Driver

Thanks for the reply. Actually, the majority of the fumes, at least the ones the first night that actually got into the house, were from the gas leaking out the filler neck. I plugged that up (again) and the smell wasn't as bad and is at least contained to the garage. The last time this happened I dropped the tank and re-sealed the sender. And the first time I filled up the new tank the sender popped out the top and leaked gas all over the ground.

As I said, there have been many woes. But, that's just with the gas tank. Don't get me started on the rest of the car after 28 years of devotion. Sajeev has actually helped me out with a few of the others over the years.

TonyT
Technician

Unless this car has been under water for an extended period of time, I would discount the wiring itself. GM cars of this vintage were electrically over-engineered and rarely suffer from wiring harness issues. I would check the bulkhead connector at the firewall (the center bolt can loosen over time) and the connector at the fuel tank. The most common issue I have found is the ceramic resistor on the back of the factory gauge. They will fail after twenty or more years of use. If you can locate someone that can restore the gauge, they will substitute a more modern resistor that should make the needle behave. The same goes for the temperature and oil pressure gauges as well. Year One used to offer a instrument cluster renew service; you might check with them.