* Fuel Level Sender Unit: The fuel level sender unit is a float inside the fuel tank that measures the fuel level and sends a signal to the gauge. This is the most likely culprit. It's possible it was damaged during the pump replacement, or it was already failing before you replaced the pump.
* Wiring: Check all the wiring connected to the fuel level sender unit and the gauge itself. Look for broken wires, loose connections, or corrosion. A damaged wire anywhere in this circuit will prevent a correct reading.
* Gauge Cluster: The fuel gauge itself could be faulty. This is less likely, but possible.
* Grounding Issues: A poor ground connection can affect the electrical signals in the fuel gauge circuit. Make sure all grounding points are clean and secure.
* Incorrect Installation of the Fuel Pump (unlikely): Though you replaced the pump, double-check that everything was properly reconnected and installed. A mistake during installation *could* be affecting the fuel level sensor (but would probably manifest in other ways too.)
Troubleshooting Steps:
1. Check the Fuel Level Sender Unit: This is the best place to start. You'll likely need to drop the fuel tank to access it (this involves some mechanical work). Inspect it for damage and verify its connection. If you are not comfortable working with fuel systems, it's best to take it to a mechanic.
2. Test the Wiring: With a multimeter, test the voltage and resistance of the wires going to and from the fuel level sender unit and the gauge. Compare your readings to a wiring diagram for your vehicle. You might find a broken wire or a short.
3. Check the Gauge Cluster: If the other components check out, it might be the gauge itself. This is the hardest to test yourself, and often requires replacement.
Recommendation:
Given that you've already replaced the fuel pump, it's highly unlikely this is related to the pump. Focus your attention on the fuel level sender unit and its wiring. If you're not comfortable working on the fuel system, take your Blazer to a qualified mechanic. They have the tools and experience to diagnose the problem accurately and efficiently. Trying to fix it yourself without the necessary knowledge could be dangerous.
