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Hagerty
Hagerty Employee

Piston Slap: When a Riviera's sending unit fails to resist

I have a 1966 Buick Riviera. Is there a way you can keep the gas gauge from fluctuating back and forth when you have 1/2 a tank of gas, when you go around corners or stop and start. Sajeev answers: Yes, that's usually a sign of the fuel-sending unit that needs attention.
https://www.hagerty.com/media/maintenance-and-tech/piston-slap-when-a-rivieras-sending-unit-fails-to...
3 REPLIES 3
Scoutdude
Intermediate Driver

No there is not any way to fix minor changes that occur from fuel sloshing around on the GM direct reading gauges. It is a fast response style gauge and it will show that float bobbing up and down as the fuel sloshes back and forth. Now if you are talking about occasional large spikes in one particular range then yes it is likely a bad sender.

You don't see this in cars from this era in other brands because they used thermal gauges that show the bi-metalic's strip temperature in response to the heater's current which is controlled by the sender. That means that it takes longer to make the needle move, and that it will average out the little ups and downs of the fuel sloshing around.
audiobycarmine
Technician

"Installation will be pretty straightforward"; if you're experienced in dropping gas tanks.
I did one of these repairs, and would rather not repeat the experience.

That Riv's dashboard is Primo Cool!!
ZZZPR
Intermediate Driver

If the sender is in good shape then the gauge fluctuations are nothing more than an accurate representation of the fuel level as it sloshes back and forth. There's a clever device called the "Gauge Wizard" that can solve that problem by electronically damping the fluctuations in the resistance signal. It also allows you to accurately calibrate the gauge so that when it says "F" the tank really is full, when it says "E" it really is empty, and so forth for every quarter tank increment. You can purchase it directly from the manufacturer in England (Google it) for a reasonable price. I found the setup to be a little fiddly, but once done it worked like a charm for my car.