I'm getting a positive fuel backflow through the tank vapor line to the canister flooding the canister and spilling on the driveway. I'm have been running off my right side tank because the left will is not working for some reason. I removed the tank filler cap off the right side tank but the fuel flow continued then I removed the tank filler cap off the left side tank and heard an audile "whoosh" of pressure release. The fuel flow to the canister stopped but after a bit fuel began pouring out of the left side tank filler neck all over the drive. I cut the engine off. and washed down the drive. I'm out of ideas. I replaced the original 30 yr. old canister with a new one but no help. The truck is entirely original with all the original plumbing and wiring.
My truck is a 1987 GMC Sierra TBi, with 454 engine. I had the same issue, change the fuel selector valve (6 ports), it’s located on the passenger side underneath the chassis near the passenger fuel tank (it maybe covered with a plate) behind the fuel filter. It could be plugged or malfunctioned, the reason it’s doing this it’s siphoning the fuel from one tank to another, and because of the pressure build up it dumps it on the front canister through the return fuel line, In regards to the driver side tank not working, there are some possibilities: you may’ve a ground wire on the sending unit not grounding properly, or the sending unit failed, or the the selector switch in the cabin not working, or the same fuel selector valve (I mentioned that above). Check them all, or have a well experienced electrician check them. I’m no expert, I still have electrical issues with my fuel tanks, but I had the same issue (fuel flooding the engine through the canister), I replaced the valve and fuel filter, and it was fixed. Note: a simple check may help, turn the key to on, don’t start the engine, have someone sit in the cabin, you crawl under the truck, make sure the truck is secured, ask him to cycle the fuel selector switch to right and left tank, listen to clicking sound in the fuel selector valve when cycling between both tanks, Listen to each fuel sender unit pump. This will tell you if the electrical connections are ok, also if all components (fuel selector switch, fuel selector valve, sending unit pumps) are working or not. I hope this solves you problem Take care.
Thank you for your reply Captain J. I have since put my truck in the shop and had the fuel tank selector valve replaced and this solved the problem to a point, it's aggravating because the selector valve that was replaced was suppose to be fairly new.. maybe a year or two old. My left tank fuel pump will not run.. the garage said the pump was bad. I plan to run on the right tank for now. Your suggestion about a bad ground may be the problem because of the way the left fuel pump quit working. I'm not finished yet. My left tank is full of fuel. I plan to syphon the gas out of the left and burn it in the right so I can empty the tank in case I have to drop it to replace the pump. Thanks again..
Your welcome The other thing to check is the fuel tanks, they get rusted inside and the rust will rust the sending unit, it may also damage the fuel selector valve and filters. So I recommend checking the inside of fuel tanks. If they’re rusted don’t waste your time of restoring them, just buy you new ones, make sure they are galvanized. Mine are rusted inside, and I’m buying new tanks, fuel selector valve, I already bought new fuel sending units. Keep us posted Good luck
