Unlikely to be a problem...but where is the main engine ground connected? On my '88 it goes to the engine block where the fuel pump used to be mounted on the older vehicles. Wouldn't hurt to run a small jumper wire from the cylinder head that the sending unit screws into, to wherever the main engine ground is connected. If that doesn't help, remove the jumper wire.
My ground goes to a stud on the intake manifold straight to the neg. side of the battery and it also has two wires that ground there , a tan wire and a black wire. I took them off and cleaned it up. The stud had forma-gasket ( the hard kind ) on it , After cleaning it up I replaced it with black RTV gasket maker. ( not sure if this stud goes into a water jacket or not and if so I am not sure if the RTV will work out ok )