Pressurized Fuel System

I own a 1998 C2500 Silverado. Recently I started noticing extreme gasoline fumes. Took truck in to my local mechanic and found out the fuel pump module had come loose from the gas tank. My mechanic dropped the fuel tank to access the fuel pump module for repair and discovered that the fuel tank had caved in ( imploded ) the full length of the tank. I’ve been told the fuel system is pressurized. How does the pressurized system work and what could have caused deformation of the fuel tank?

The only way I can see that happening is if the vent valve was closed when the purge valve opened in the EVAP system.

The vent valve is normally open and the purge valve is normally closed. When the engine is started, the purge valve opens to draw the gas vapors into the engine. If the vent valve is closed, the 28" or so of Hg (mercury) gauge vacuum the engine draws can be enough to collapse a plastic gas tank.


What Tester said, there was a malfunction in the EVAP system which allowed engine vacuum to lower the pressure in the fuel tank until it collapsed. A “vent valve” is supposed to open anytime there is negative pressure in the tank, which prevents this from happening…

Excellent comments above. Is the check engine light on? Even if it isn’t on, have the ECM diagnostics codes been read out? That might provide add’l clues.

Put simply, it’s an EVAP system problem that’s preventing the tank from breathing in as the gas is pumped out. If you have a tank with a significant size flat surface, the difference in PSI between the ambient air and the tank air multiplied by the square inches in the surface to the extent that the pump is capable of creating it is quite capable of caving in a fuel tank. Yours is not the first that has caved. It isn’t common, as an absolute failure to breath isn’t common and engineers do a pretty good job of designing in support for larger flat surfaces, but it does happen.

The EPA requires all cars meet strict rules for how much raw gasoline is allowed to be emitted into the air from the fuel system. To implement these rules requires the use of some complicated bits of technology in the car’s EVAP system, often it is computer controlled. Some car manufactures these days use pressurized fuel tank systems, others use partial vacuum tank systems. I think GM is an example the former, and Chrysler is an example of the latter.

I can’t speak for GM products like that due to unfamiliarity but I’ve seen imploded fuel tanks on VWs in the past when the tank venting system screwed up. Engine vacuum would suck the tanks down to about 3 gallons total capacity.

Think of an inflated brown paper bag and then inhaling all of the air out of it. The collapsed tanks would look similar to the bag… :slight_smile: