Piston Slap: Going full send on a Corvette sending unit?

Steven writes:

While my 1993 Corvette’s gas gauge and (new) sending unit/fuel pump assembly test fine, the gauge consistently reads 6 to 7 gallons low. My question to you is, would bending the float arm down about 25 degrees make the gas gauge more accurate?

Sajeev answers:

Hi Stephen: I think you are on to something. I am hoping that new sending unit’s float arm was bent while in transit, or incorrectly bent from the factory. If that doesn’t work, I wonder if the sending unit is defective.

Steven adds:

Actually this is the third sending unit installed by my mechanic. All of them read low by about 6 to 7 gallons. For some reason the shop refuses to bend the float arm after I made several requests to do so. I take it from your reply that would be an acceptable solution.

Apparently GM has discontinued the original ’93 Corvette sending unit assembly, but the aftermarket fitment was installed after two Delcos. All of them read low!

Sajeev:

Well if it’s the THIRD sending unit, I am less optimistic this is the solution. You said the gas gauge is fine, but I wonder if the wiring has some resistance problem between the tank and the gauge. I would also see if there’s a voltage drop on that purple(?) sending unit wire. Maybe the mechanic already checked that?

Steven:

Thanks for your replies. I assume the mechanic checked all the wires in between for shorts, resistance, etc. The ohms read correctly at full and empty.

Sajeev: 

I would ask the mechanic about the wiring, as it never hurts to rule out that variable from your equation. Once answered, go ahead and bend the sending unit, because either it will work or the sending unit doesn’t provide correct resistance (I think its 0 to 90 ohms) for your factory fuel gauge. If the latter is true and GM mis-spec’d the replacement part, it will never work correctly … no matter how you bend it.

It’s entirely possible that the part is not the right one for your Corvette, and no bending will help. If you still have the original sending unit, there are places where you can get it rebuilt. And the more I think about this scenario, the more I think your original part needs to be reconditioned to actually fix your problem.

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