Only using front tank

03 GMC C&C gas engine, its only drawing fuel from the front tank, they both used to go down evenly. There's a lot of knowledge and experience here, where should I start? Gauge reads as if there both working, for instance the front tank will be half empty and the rear is full, the gauge will read 3/4. thanks for any info.

Can't be much help, I'm afraid, but the one I messed with had bad fuel pump in rear tank. rear tank transfers to front as needed mine had a module mounted on frame rail near frt. tank. glad I determined problem was pump-didn't really try to go any deeper into system to see exactly how it works--

When the front tank drops 10% below the rear than the module is supposed to run the pump until the gauge readings for both tanks are the same.

Since the gauge is reading both tanks, the module is working. The problem is either the relay for the rear pump or the rear pump. Put a test light in place of the rear pump and start the truck. the light should light for about 45 seconds. If the module sees no change in fuel levels, the module will cancel the power to the relay.

I ran into something similar on a buddy's '05 the other night. From what I could see the system is set up with a pump that moves the fuel from the rear tank to the front one when it senses the front tank level drops to a certain point. In his case we can only guess that his problem was that he was working on a hill all day and it fooled the setup to a degree that enough fuel didn't get pumped into the front tank to keep it full, so he ran out. By us messing with it, it finally got enough fuel in the front tank to pickup and start again. From that point on he hasn't had any more problems.

That said, the way we tested the transfer pump was to simply unhook the sending unit on the front tank. It wasn't hard to do on his truck since it had a dump bed, but it may be a bit more of a chore if yours has a standard bed. Regardless, unhooking the sending unit should cause the pump to come on. With his we couldn't hear or feel anything with/at the pump itself, but you could hear the fuel being moved if you put your ear to the fill port on the front tank.

Not sure if this will help you much, but it sounds like the transfer pump might be the problem, and this is the only way I can think to test it.

I worked on one of these once, and even tho the pump put out gas flow, it was insufficient to keep front(draw) tank full.(if I remember correctly.)I had to replace the pump. The fuel gauge reads both senders and the cab controller evens out the reading as I recall. Hope this helps, along with what Tim said. Mark