* Faulty Fuel Pump Relay: This is the most common cause. The relay acts as a switch, turning the pump on and off as needed. A stuck-closed relay will keep the pump energized constantly. Replacing the relay is inexpensive and often solves the problem.
* Short Circuit in the Wiring Harness: A short in the wiring leading to the fuel pump can constantly send power to it. This could be due to damaged insulation, corroded connectors, or a wire rubbing against a metal part. Inspecting the wiring harness carefully for any signs of damage is crucial.
* Faulty Fuel Pump Control Module (PCM or ECM): In some cases, the Powertrain Control Module (PCM) or Engine Control Module (ECM) – the "computer" of your vehicle – might be faulty and sending the wrong signal to the fuel pump, keeping it running. This is less common than a bad relay or wiring, but possible. Diagnosis typically requires a scan tool to read codes.
* Faulty Inertia Switch: The inertia switch (often located under the dash or in the engine bay) is a safety device that cuts power to the fuel pump in a collision. If it's malfunctioning and stuck in the "on" position, it could continuously power the fuel pump.
* Problem with the Fuel Pump itself: While less likely to cause *constant* running (it would usually be a complete failure or intermittent), a faulty fuel pump could potentially malfunction in a way that keeps it running. This is less probable than the other issues.
Troubleshooting Steps:
1. Check the Fuel Pump Relay: Locate the fuel pump relay (consult your owner's manual or online resources for its location). Visually inspect it for any signs of damage. You can try swapping it with another relay of the same type (often a simple swap with another relay in the fuse box—but be careful to not swap critical relays!) If the pump stops, you've likely found the culprit.
2. Inspect the Wiring Harness: Carefully inspect the wiring from the relay to the fuel pump for any damage, corrosion, or chafing. Pay close attention to connectors.
3. Check the Inertia Switch: Locate the inertia switch and check if it's tripped. Reset it if necessary.
4. Use a Scanner: If the above steps don't solve the problem, you'll likely need a scan tool to check for diagnostic trouble codes (DTCs) from the PCM/ECM. These codes can provide valuable clues about the underlying cause.
Important Note: Running the fuel pump constantly will drain your battery quickly and can potentially damage the pump itself from overheating. Address this issue as soon as possible. If you are not comfortable working with automotive electrical systems, it's best to take your Blazer to a qualified mechanic.
