Fuel Pump Runs Constantly Without Key - Previous Owner 12V Hot Wire Bypass

Fuel Systemintermediate✓ Community Verified

Problem

Fuel pump primes and runs continuously without the key in the ignition. Removing the circuit opening relay, fuel pump relay, and main fuse in the engine bay did not stop the pump from running. Additionally, the car would crank but not stay running when attempting to start.

Fuel pump runs/primes constantly without key in ignitionRemoving circuit opening relay does not stop fuel pumpRemoving fuel pump relay in engine bay does not stop fuel pump from primingRemoving main fuse does not stop fuel pumpCar cranks and starts momentarily but dies immediatelyApproximately 0.5V voltage drop between battery terminal and fuel pump connector in trunk

Root Cause

A previous owner wired a constant 12V hot wire (red/silver wire) directly from the battery positive terminal to the fuel pump connector (black/red wire) in the trunk, completely bypassing the OEM ECU-controlled fuel pump circuit (circuit opening relay and fuel pump relay). This caused the pump to receive constant battery voltage regardless of ignition state, and because it bypassed the ECU-controlled relays, the pump did not receive proper power during cranking (the switched 12V source likely dropped out during crank), preventing the engine from staying running.

Confidence

Moderate confidence — Multiple community members agreed on this solution

View Original Thread →Posted September 2024 on SupraForums.com