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Message: [QUOTE="dobanion, post: 1542499, member: 32384"] FYI if you haven't seen this document, you can learn everything you need to know about the 6.4 fuel system: [URL="http://www.backglass.org/duncan/ps64_manual/64L_Fuel_Supply_System.pdf"]http://www.backglass.org/duncan/ps64_manual/64L_Fuel_Supply_System.pdf[/URL] I'll be referring to that. I know I have heard references to problems with bubbles in the 6.4 system. Seen some youtube videos. I think I spotted something in the diagrams from above that can explain. When you look at the diagram for the engine mounted fuel filter, you will notice it's got two "air bleed orifices" that are designed to allow bubbles in the fuel to get rejected out of the housing and put back down the return line. One orifice for the fuel coming up at the inlet, and another for the fuel coming back from the fuel cooler. Each of the outlets of these orifices combine and flow back out of the filter down the main engine return fuel line. This is a very good design feature. It's also exactly how the Airdog and FASS claim to reject bubbles. Then, at the HFCM, we find the stupid idea. When the fuel is cold, it opens a "recirculation thermostat" that takes the return fuel from the engine, and recirculate it back up to the engine fuel inlet. The document above says it's fully open below 50F, and slowly closes until you have 80F fuel. So the engine can do a pretty good job of removing bubbles, only to have the HFCM send them right back. Bubbles in fuel are hard on injection pumps and injectors. They also effect the injection pressure, spray pattern, ignition delay (ie timing). Bubbles in fuel is BAD mmmmkay? So, my idea is pretty simple. Remove the return lines from the HFCM, connect them end to end (bypassing the thermal valve), then cap the connections on the HFCM. I found the needed parts to do this for about $30 (two quick disconnect caps and one male/male coupler). Maybe this is a old deal, and I haven't found anything new. [/QUOTE]