03-26-2012 12:24 PM
Actual cause depends on the engien fuel system design, if it's on an engine with an old scroll fuel system operating at a low pressure then you likely have a suction leak somewhere, which was why CAT and other manufacturers installed sight glasses on return lines of older series engines.
On newer engines, especially unit injected or high pressure common rail systems apparent air bubbles in the fuel return are fairly common due to cavitation and some boil off due to very localized high temperatures in some parts of the fuel systems. Your fuel's actual makeup can make this appear better or worse. Most newer engines have some form of backpressure valve or pressure regulator on the return side of the system that keeps most of those vapor pockets or bubbles unnoticable, open up the line, remove the pressure and the bubbles appear.
Why the return line is plumbed back to the tank instead of back to the inlet of the fuel filter or pump (yes, have seen that done many times as a "cost saving" measure), so inpart anyway, that that entrained vapor can be vented from the fuel.
Hope that helps, Mike L.