1. Check the Obvious:
* Loose Gas Cap: A loose or faulty gas cap can throw off the fuel level sensor reading. Ensure your gas cap is tight and in good condition. Sometimes a replacement gas cap is all it takes.
* Debris in the Fuel Tank: While less common, debris can interfere with the float and sending unit. This is less likely to be the problem if the gauge is *intermittent*, but more likely if it's consistently inaccurate *or* consistently reading empty. This requires dropping the fuel tank, which is a significant job.
* Low Fuel: The gauge might be more inaccurate near empty. Try filling up the tank to see if the accuracy improves.
2. Test the Fuel Level Sender Unit (the most likely culprit):
The fuel level sender unit is located inside the fuel tank. It's a float-type mechanism that moves with the fuel level and sends a signal to the instrument cluster. Problems with this unit are the most frequent cause of inaccurate gas gauge readings.
* Resistance Check: This requires disconnecting the wire harness from the sender unit (after disconnecting the battery's negative terminal for safety) and using a multimeter to check the resistance at different "mock" fuel levels. You'll need a service manual or online resources for your specific Jimmy's resistance values at various fuel levels (e.g., empty, 1/4 tank, 1/2 tank, full). This test is crucial in determining if the sender unit itself is faulty. If the resistance values are far off from the specifications, the sender unit needs replacement.
* Visual Inspection (Difficult): Dropping the fuel tank is usually needed for visual inspection. You might be able to see some damage without dropping the tank, but this is very difficult and usually not worthwhile.
3. Check the Wiring:
* Wiring Harness: Inspect the wiring harness connecting the fuel level sender unit to the instrument cluster. Look for any broken, corroded, or loose wires. Repair or replace as needed. A visual inspection is usually enough, but you might need a multimeter to check continuity.
* Instrument Cluster: While less common, a faulty instrument cluster can cause gauge problems. This is a more advanced repair, often requiring professional help.
4. Consider Grounding Issues:
* Poor Ground: A poor ground connection can affect the signal from the sender unit. Check all ground connections related to the fuel system.
5. Professional Help:
If you're not comfortable working with fuel systems or electrical components, take your Jimmy to a qualified mechanic. Working with fuel is dangerous; incorrect handling can cause fires or explosions. Replacing the fuel pump assembly (which the sender is often part of) is a substantial repair job.
In short: Start with the simple checks (gas cap, debris if suspected). The fuel level sender unit is the most likely culprit and requires a resistance check (and likely tank removal for replacement). If the sender is good, move on to wiring and grounding issues. The instrument cluster is the least likely problem. A repair manual for your specific 2000 GMC Jimmy will be invaluable in this process.
