Fuel Guage Callibration?

[gregory914]

My 97 K1500 Burb's fuel guage reads 1/8 of a tank but it's really empty. I found out the hard way, and I know I probably just took years off the life of my fuel pump.

Today I filled it up and the needle points way to the right just past the "F" for full in to the black. I dropped the tank last year and put a new fuel pump in and everything worked fine for the past 14K miles. The guage doesn't flutter or move eracticly it just doesn't go below the line above where the red starts. And after filling it today, I wondered if there is some kind of an adjustment so it would read more acurately.

It used to read exact and at 50K miles the dealer put a new guage cluster in and the fuel guage started reading to the "F" when I filled it but it wuold go down to the empty line and a little below.This new thing has me stumped.

Any help would be appreciated!!!!

[praisethelowered]

I can't offer any help, only consolation. Mine does the same thing. It was almost half a tank off, but I replaced the sending unit and now it's only 1/8th off (like yours). I've read here on the board that letting the tank drop below half full severely shortens the life span off the sending unit.

[NoEcm]

Had a similar problem on a '96. With a full tank (42 gallons) the gauge read 1/8 inch past full and it stayed there for the first 350 miles. At 1/4 tank I ran out of gas. Here is what I did to fix it.

I went to a local electronics store and purchased a 1/4 watt 5 ohm and a 1/4 watt 100 ohm resistor. They were around 10 cents each.

I unplugged the fuel pump/sending unit connector back by the gas tank. The connector is bolted to the cross member right in front of the tank. On a '96 you'll want to short the purple wire to the black with white stripe wire with your resistor(s). With the 5 ohm resistor (key in the on position) the fuel gauge showed empty. With the 100 ohm resistor the fuel gauge showed full. That showed me that the fuel gauge was good.

Knowing that the gauge was good I drained as much of the gasoline out as I could before dropping the tank. Before you drop the tank you'll want to go to your dealer and order the 2 fuel line "O" rings, the "O" ring that seals the fuel pump/sending unit to the tank and maybe even the locking ring.

After dropping the tank you have to clean the sealer that the factory uses on top of the sending unit. Using some lacquer thinner to soften the sealing compound it took me around an hour to get all the goop off. (You want to get it as clean as possible as you don't want any of that stuff falling back into the tank). After all the sealing compound has been cleaned off your can remove the locking ring and pull the fuel pump/sending unit out of the tank. (Checking with my local GM dealer they say that they don’t put any kind of sealer back on when the replacing the fuel pump/sending unit.)

Upon examining the sending unit I noticed that there was corrosion (or some sort of foreign substance) on the coil windings. I used brake cleaner and a soft toothbrush to clean the coil windings. After cleaning I used a VOM to confirm that the sending unit measured ~8 ohms in the empty position and ~100 ohms in the full position. I even plugged the sending unit back into the harness and checked the gauge with the sending unit in the full, half and empty position.

While I had the fuel tank out I decided to completely drain it. I drained out 2 or so gallons of liquid. 1+ gallons of that liquid from the tank was water.

I reinstalled the tank, poured in 2 gallons of fuel and drove a short distance down to the gas station to start the fuel gauge calibration. Pumped in 8.5 gallons and the fuel gauge showed 1/4 tank. Pumped in 19 gallons and the gauge showed 1/2 tank. Pumped in 29.5 gallons and the gauge showed 3/4 tank. Pumped in 40 gallons and the gauge showed full. Everything was good.

Cost of parts was ~$25 for the 3 "O" rings and the locking ring from the dealer. I also invested $3.00 for a couple of bottles of fuel system drier after filling up the tank.

Hope this helps someone.

[gregory914]

So if I'm understanding this right, the problem was the corrosion/foreign sustance on the coil? I've had my tank out last year for a fuel pump so I think I'll be doing it again this year as my pump was whining like crazy before I ran out of gas and it still showed almost a quarter tank.

THANK YOU for your help and I'll let you know how it works out.

[NoEcm]

The "high" fuel gauge reading was caused by the corrosion on the sending unit coil.

After cleaning the sending unit coil, with the float in the full position it read ~90ohms and around ~6ohms on the empty position (I should have taken some readings before cleaning the coil for comparison though).

Its easy enough to check and clean if you've got the fuel pump out of the car. Once the sending unit is out of the vehicle I would also suggest that you connect it back to the wiring harness and move the float around and see how the gauge reacts before putting it back into the tank.

Also, if you've got the tank out, drain out all the fuel. There's around a gallon or so below the fuel tank baffle that never gets used or drained in normal usage. In my case that liquid below the baffle was water.

[gregory914]

Thanks for your realistic and technical feedback, I understand exactly what you're talking about. I probably won't drop the tank again until next month but I will let you know how it went.

Look back a few pages and you can see a step by step of my "adventues in fuel pumps" dated 10-21-04 on page 21 of this forum titled " Suburban Fuel Pump". Please read the entire thread!

So I know the drill, been there/done that, have the t-shirt all that good stuff. But THANK YOU for your precise technical info . Are you an engineer or a Technician because I personaly love it when people give good data like you did.

I'll keep you posted and let me know what you think of my Fuel Pump Thread.