GMT-900 - Bad fuel level sensor??

Mossyoakglock · October 30, 2014

My fuel gauge started acting up about 4-5 days ago.

I filled the truck up on Saturday and the only thing different I did was add about 4-6 oz of Techron fuel system cleaner to the tank. Drove around that day and there were no issues. On either Monday or Tuesday morning I noticed that the fuel gauge went right to 3/4 even though I had a full tank of gas (or slightly under). Over the course of about a minute it slowly went to the full mark. It's done this every morning leaving the house and then every afternoon leaving work so both times it sits for 8+ hours. Yesterday it even did it when I left the gym so it was only sitting for about 45 minutes.

Could this be a bad fuel level sensor? In the video the truck had been running for about 30 seconds and the needle went right to the half mark with no interruptions and stayed there for about 15 seconds before it started moving again.

http://youtu.be/Lo6YJvG1Xx4

Colossus · October 31, 2014

GM has had a long standing issue with their fuel level sensors. It was quite common in the 90s and on the GMT-800s and even some of the 900s to where the sending unit (which includes the sensor) will go bad over time, the contacts and terminals and sensors will build up with sulfur and what not, corrode, or just wear out. Sulfur often coats many of the componants in the fuel system and tank as well. The Techron and BG44k (more or less the same product) are good trustworthy fuel system cleaners, but you will more than likely need to replace the fuel sending unit in the tank. The gauage fluctuating will continue to get worse over time. I've seen some where the needle vibrates wickedly, and others where the needle will bounce back and forth between full and empty, setting the low fuel light and sometimes a check engine light. But look on the bright side, when you do the fuel module in the tank, you'll be doing the pump and filter (located in the tank) as well so at least you'll have some peace of mind and not have to worry too much about the fuel pump going out on you in the middle of nowhere! On the GMT-800s fuel pump failure between 70,000-100,000 miles was rather common. Happened on my 99 at right about 85,000 miles. It was a common topic on this site back in the day.