Note that there's a second circuit through the oil pressure switch that powers the fuel pump. The relay typically is turned on for one second when you turn on the ignition switch. That is to insure fuel pressure is up for starting in case it bled down over days or weeks. That relay might also turn on during engine rotation, (cranking or running), but regardless, both circuits are designed to go dead if the engine stalls. That's a safety feature in case a fuel line is ruptured in a crash. The pump would dump raw gas on the ground, creating a major fire hazard.
With a broken fuel line there can't be any fuel pressure, so the injectors stop spraying gas and the engine stalls. For the fuel pump relay and Engine Computer, the signal pulses from the crankshaft position sensor and / or the camshaft position sensor will stop arriving, and that tells the computer to turn the fuel pump relay off. At the same time, there can't be any oil pressure with a stalled engine, so that switch also turns off. With both circuits turned off, the fuel pump turns off.
I can't remember if the fuel pump relay stays on when the engine is running or if it only turns on until the engine starts running. SCGRANTURISMO, you might know that. If it turns off and lets the oil pressure switch keep the fuel pump running, a good suspect for intermittent operation is low oil level. A common symptom is the engine runs fine for as much as five to 15 miles, then it stalls without warning. In the first one of these I learned about, a coworker added a quart of oil and the intermittent stalling stopped occurring. What was happening is the low level resulted in low pressure, presumably from sucking up air intermittently, but while that pressure was low enough to turn the fuel pump switch off, it was not low enough to turn the Oil Pressure warning light on.
Tuesday, January 7th, 2020 AT 1:44 PM
