Extra - Truck Trend Garage: 1999 GMC Sierra 4x4 Rough Idle and Incorrect Fuel Gauge
Question: I have a 1999 GMC Sierra 4x4 with a 5.3-liter automatic and 92,000 miles on it. At times when I start it, hot or cold, it acts as if it's not running on all cylinders for a minute or two. Then everything is fine. I changed the fuel filter, hoping it was partially clogged. The second, less important, problem is when I fill the tank, the gauge shows empty and the low-fuel lamp illuminates.
| Picture Shown of a 1999-2006 GMC Sierra
Answer: The inaccurate fuel gauge problem is probably easier to diagnose. The fuel gauge sending unit within the tank may have its float stuck in the down position. But you first need to check the resistance of the sending unit at the connector leading to the tank and eliminate the gauge itself or other related circuits causing the issue. Running rough on startup can be more involved. Is it a full-fledged misfire we're talking about or a less pronounced sloppy idle condition? Feeling the "shake" while accelerating (under a load) would indicate a hardcore misfire with one or more cylinders dead in the water, possibly due to an ignition problem like a cracked plug, bad coil, etc. A rough idle that's not noticeable at higher rpm or under a load, could be something more along the lines of fuel leaking into a cylinder from an injector, a vacuum leak, EGR problem, or a carbon-obstructed throttle body (throttle plate) assembly. There are a couple technical service bulletins that may or may not be relevant. One pertains to faulty material in the fuel-filler neck causing fuel contamination and stuck closed injectors, another is a procedure for throttle body cleaning and setup. It most likely will require a technician to duplicate the condition in the shop, while analyzing fuel pressure, spark, diagnostic trouble codes, and incoming digital data, to fix it right the first time.
If you have a technical question regarding your pickup, SUV, or van, feel free to contact Alex, a master technician with the National Institute for Automotive Service Excellence. Send a letter to him in care of Truck Trend Garage, 831 S. Douglas Street, El Segundo, CA 90245, or e-mail us at trucktrend@sourceinterlink.com. Please include the VIN with your question. Due to the volume of questions received every month, we cannot guarantee that everyone's question will be personally answered or will appear in the magazine.
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