Well when I took out the sender coolant came out like crazy so not sure on air bubble. As far as voltage on that wire I’m pretty sure it’s supposed to have small fluctuating voltage coming through the voltage limiter on the dash, that’s how it works with the gauge and the ground. At least that’s what I found from my research, someone can easily double check me with their set up. Just hook up a meter set to dc on that wire and then turn the key on and you will see voltage jumping around 5 volts going + or - a few volts.