GMC Repair: Fuel guage reading empty at 1/3 tank, fuel system treatment, gm tool

Question
I just bought an '03 GMC SAFARI conversion van with 40K miles on it.  I found that the gas gauge shows just a hair over the full mark when I fill it, just as it should.  But it descends faster than the fuel in the tank.  When it is around half full it only takes 8-9 gallons and when it shows empty and the low fuel light comes on it only took 18 gallons, leaving 9 gallons left for a reserve in a 27 gallon tank.  The gauge operates smoothly, just goes down too fast.  At first I thought it may be a bad sender, but don't want to drop the tank and replace it to find out its a resistor or gauge.  Any advice would be appreciated.

Answer
Hi
I researched the schematic for your van, and it looks like the unless you can actually test the gauge with the proper GM tool, There is no way to test the gauge,
I will tell you, though, from experience I can count on one hand the number of clusters I have changed for faulty fuel gauges, and I cannot even begin to tell you the number of Tank units I have replaced for your EXACT problem.
You can unplug the tank unit and it should peg one way and if you ground the PURPLE wire it should peg the other way.
Those things I can tell you.
The PCM actually sends the fuel level to the IPC via class II serial data, so there is a VERY slight chance the PCM could be sending false data, but there again, experience dictates telling you the tank units are VERY COMMON problems.
Word of advice after replacing unit, GM has issued a bulletim on a new Fuel additive that you can add and will help protect the unit from further contamination of sulphides.

Here is the EXACT bulletin and part numbers, FYI


Important : The GM Fuel System Treatment PLUS is not recommended for use with diesel fuel. The GM Fuel System Treatment PLUS is specially formulated for use in gasoline and/or E85 fuels.

GM Fuel System Treatment PLUS
GM Fuel System Treatment PLUS, P/N 88861011 (for U.S. ACDelco®, use 88861013) (in Canada, 88861012), is now available for use in gasoline engines.

Added Benefits and Uses
The PLUS portion of GM Fuel System Treatment PLUS is the addition of a filmer additive that, when used regularly, can protect fuel system sending units from the corrosive effects of certain sulfur contaminants found in some of today's gasoline.

Sulfur contamination can disrupt electrical continuity of certain fuel sending units and lead to erratic or false fuel gauge readings. With scheduled usage, GM Fuel System Treatment PLUS protects against the effects of harmful sulfurs in gasoline.

The Four Benefits
Just pour in one bottle of the GM Fuel System Treatment PLUS into the fuel tank when refueling around each oil change interval:

   •  Cleans -- Sulfur corrosion from fuel gauge sending units.

   •  Prevents -- Harmful sulfur components from attacking sensitive fuel system electronics.

   •  Protects -- By coating metallic surfaces of the fuel system.

   •  Removes -- Engine deposits left from use of poor quality fuels.

Parts Information
Part Number
Description

88861011 (for U.S. ACDelco, use 88861013)

(in Canada, 88861012)
GM Fuel System Treatment PLUS
Hope this helps
JUST advice from experience
Good Luck
Pawl