Half a tank of gas causes problems

We recently bought a used, one owner, 2001 S10 Chevy pickup, 111,000 mi. Very well maintained, runs perfectly UNTIL the gas tank gets to half full or less. Then it runs roughly, stalls, will restart but not keep running. The gas pedal does not seem to deliver gas consistently at this time.

Very often this is a sign of a saturated charcoal canister. Your gas tank breaths in through a charcoal bed in a canister under the hood, and if this gets saturated (which can in some vehicles happen from “topping off” the tank), the gastank can not breath in. As the gas level drops, a vacuum can develop in the tank that prevents the pump from pumping.

The next time this happens, try removing the gas cap and reinstalling it. If you hear a hugh “whoosh” and the problem disappears, you may have a saturated canister. Or perhaps even a plugged ventline.

You could also have a weak gas pump. The weight of the gas adds “head pressure” to assist a weak pump, but once that weight disappears the pump can no longer maintain sufficient pressure to the injectors. This can be diagnosed with a fuel pump pressure test at the engine.

Thanks, we’ll have it checked out. There seems to be no diagnostic code to address this problem, or is there?