Here's why and how to troubleshoot:
* Fuel Level Sending Unit's Role: This unit is a float inside the fuel tank. As the fuel level changes, the float moves, altering the resistance in a potentiometer. This changing resistance sends a signal to the instrument cluster, which interprets it as the fuel gauge reading. If the unit is faulty (float stuck, potentiometer broken), the gauge will give an inaccurate reading.
* Fuel Pump's Role: The fuel pump's primary function is to move fuel from the tank to the engine. While a completely failed pump could indirectly affect the gauge (no fuel flow, leading to an empty reading), a malfunctioning pump rarely causes a consistently inaccurate *high* reading like you're experiencing.
Troubleshooting Steps:
1. Check the obvious: Before diving into major repairs, make sure there aren't any simple, easily fixable problems:
* Loose Connections: Inspect the wiring harness connected to the fuel sending unit. Corrosion, loose connections, or broken wires can interfere with the signal.
* Ground Connection: A poor ground connection can cause erratic readings. Check the ground wire for the fuel gauge sender and ensure a clean and solid connection.
2. Test the Fuel Sending Unit (Most Likely Culprit): This usually requires dropping the fuel tank. This is a moderately involved job, requiring safety precautions (disconnecting the battery, working in a well-ventilated area, etc.). *Do not work near an open flame.*
* Visual Inspection (After Tank Removal): Once the tank is down, visually inspect the sending unit for any obvious damage, such as a broken float arm or a corroded potentiometer. Try gently moving the float to see if the resistance changes (you'll need a multimeter for this).
* Multimeter Test: With the sending unit disconnected from the wiring harness, carefully measure the resistance at different points of the float's travel. Compare your readings to the specifications in your repair manual (or find them online for your specific Blazer model). If the resistance doesn't change consistently, the sending unit is faulty.
3. Replacement: If the sending unit is faulty, you'll need to replace it. This typically involves:
* Removing the fuel tank: This is often the most labor-intensive part of the repair.
* Replacing the sending unit: Disconnect the old unit, and install the new one carefully, ensuring the float moves freely.
* Reassembling everything: Carefully replace the tank, reconnect all wiring and fuel lines.
Important Considerations:
* Repair Manual: Get a repair manual specific to your 1999 Chevy Blazer. It will provide detailed instructions, diagrams, and torque specifications for the repair.
* Safety: Working with fuel is dangerous. Take all necessary safety precautions. If you are not comfortable performing this repair yourself, take it to a qualified mechanic.
* Parts: Obtain a new fuel level sending unit from a reputable auto parts store. Ensure it's the correct part for your specific Blazer model and engine.
In short, while a faulty fuel pump *could* theoretically affect the gauge, the consistently high reading strongly suggests a problem with the fuel level sending unit. Focus your troubleshooting efforts there first. Remember safety is paramount when working with fuel systems.
