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Classic Fuel System
I am looking into making changes to my fuel system and looking for any comments or links on the topic. A little background. It looks as though my coach (1983 with Cummins) originally came with a ConMet water separator and primary fuel filter. These are installed high up in the engine bay, above the injector pump inlet. Somewhere along the line the ConMet was ditched and a disposable one installed. This new style claims to filter the fuel to 20 microns, and the primary filter also filters to 20 microns. Cummins recommends 20 micron.

I am considering moving the filters to be located above the fuel tank, but below the injector pump inlet. I have 3 reasons for considering the move. The first is that air in the system accumulates in the fuel separator because its the highest point. Secondly, the high point puts pressure on the shut off valve and may cause some leak down. Lastly, lowering them will improve the suction from the tank.

My goal is to help improve cold start smoke levels. Reducing air in the system I think may help some. Not looking for miracles, its always going to be a smokey beast naturally. I have checked the fuel lines themselves and haven't found leaks so I believe the air is getting in with the fuel or possibly through the shut down valve.

Anyone think this is a worthwhile venture? Also, anyone see a benefit to having both filters?

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1983 Newell
My Detroit Diesel 8V92 uses a 50 micron primary filter and a 20 micron secondary filter. In a two filter system much of the dissolved asphaltenes (distillation residue having asphalt-like properties) in the diesel are caught by the primary filter, resulting in a secondary filter that is relatively clean and better able to remove fine particles. A two filter system can last longer and result in better debris removal than a single fuel filter system. In your system the first filter is a combination primary filter and water separator. Without a separate water separator, I would be leary of using a single fuel filter.
Michael Day
1992 Newell 43.5' #281

From my experience putting a loop higher than the PT fuel pump is not a good idea.  I understand you aren’t going higher than the inlet to the PT pump.  It can work but often leads to trouble with air being trapped.  I would change the FF105 to a FS1000 which is a 10 micron separator/filter.  It never hurts to filter the fuel to fine levels which protects the close tolerances used.  When the 903 was manufactured, 10 micron filtration was not available.  The PT pump has a gear pump on the rear and has a fairly high lift value, I don’t recall the number but it’s about 8” vacumn.  You should be fine with your plan.

Gordon Jones

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