Here is the thinking relative to the test...

  • If 0% of the fuel goes back to the sump and the sump does not loose level, then anything were some of the fuel goes back would only add to the level (until it reaches the overflow). 0% return is the worst case scenario or is an EXTREME load.
  • 100% return is unrealistic during operation... but remember that when you fill the tank it will be filled higher than the sump and fills the sump to the max.
  • Based on the test, the only time it draws from the sump is when the tank is empty.
  • It is not relevant if the tank looses gas as it is consumed. Would it make any difference how the sump works if you left the car run at while filling with gas? (Not recommended)


I've never seen a cross section on the web (Other than the poor example I made). Also, I've never seen anything on the web that explains the units operation. And there are multiple designs some more confusing than others. But if someone has info I'd love to see it.

I encourage anyone interested to ask for a failed one from a GM repair center(or any service center). Tell them to break off the ports if they are concerned that you'll try to use it. From a junk yard they will likely charge you for one, especially if they have to remove it. Removing one is a challenge..on an Astro and other trucks you have to drop the tank at a minimum.

The test may not be perfect but it does generate usable data. I believe that it shows that the sump will operate at a higher fuel level than the tank fuel level.