Classic/Antique Car Repair: Gas gauge, fishing corks, gas gauge

Question
Greetings,

1984 Pininfarina spider 2000. Within 15 to 20 miles after fill up, my gas guage registers about 1/2 full, after another 100 miles the gauge works properly. The fuel sender unit is new. Help!

Thank you,

Enrico

Answer
Hi Enrico:
Fuel Gauges are notorious for false readings. With the new sending unit, you are getting the wrong level of resisted current being sent to the coil that controls the fuel gauge. This could means you have a weak fuel coil.

However, don't try to replace the coil just yet. Instead, try not filling the tank completely full. If the sending unit float is being submerge too heavily in fuel, this could be causing the problem. Too much fuel in a tank can cause a false reading, because the sending unit is suppose to be tilting the needle towards FULL. But if the float on the sending unit is half submerge in fuel, I can see how it would register 1/2 FULL until the fuel level drop to a point where the float could pop up as normal.

It kind works like the old fishing corks. The cork must float freely on top of the water so you will know when it dips below the water, you've got a bite. However, if the cork is already submerged in the water, you can't tell when a fish bites it. Hope all of this makes sense.