Why does 10 percent ethanol fuel cause 1990 Pontiac 6000SE engine stop at one quarter full tank?

Answer 1

Theory 1: Your fuel pump has a pulsator. The rubber of the pulsator has deteriorated to the point where it leaks a bit. Ethanol has further damaged the pulsator making it leak more so that it leaks more and does not provide enough fuel pressure/volume to run the engine.

Theory 2: Same as theory 1 but you have no pulsator. Instead, there is a rubber hose from the pump to the tank unit pipe that is similarly affected.

Theory 3: Pulsator or hose leaking somewhat causing low fuel pressure especially at low fuel tank level. With ethanol added to the fuel, the engine needs more fuel and thus needs more fuel pressure making the problem more likely to affect engine running.

Theory 4: Over the years crud has built up inside the tank. The ethanol has loosened the crud and it now floats at the top of the tank. When the fuel level drops, flotsam clogs the pickup screen. If this is the case, you will need to clean the tank.