After I fill both port and starboard tanks they read full when underway, but after about 20 minutes the port side (one guage, but have the switch to crossover between port and starboard) starts to fluctuate. It will go from full to half to empty to full then seem like it just turns off (way to the left past the E). Starboard seems to be fine, but havent run the boat enough to test both sides adequately. I know this boat will suck fuel, but I am sure I am not using 50 gallons within 15-20 minutes

Where would be the first place u would look into diagnosing the problem?

Thanks!
Mike

I to have this problem...Gonna see what members say about the fix

Try checking the ground at the sender on the tank. I just started to have the same problem. I rewired my boat and have not grounded the sender properly and this is when it started to happen.

It could be a bad gauge, sender or wiring short. You have 2 tanks, 2 senders and 2 sets of wiring. The easiest way is start swapping stuff and see if the problem follows.

usually bad ground or bad sender....