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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?

Attached Files Thumbnail(s)


1983 Newell
Cummins VT903
Allison 654CR

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

I can't address your positioning question but I'll address the filter comments.  In general the smaller micron filter you use the better.  This is especially true for high pressure fuel systems used by most modern diesel engines.  Surprisingly most diesel fuel is not filtered until the point of retail sale.  Pipeline load racks and a other wholesale distributors rely on sedimentation to ensure clean fuel.  Sedimentation works great when done correctly but relies on us humans doing things right which unfortunately sometimes we don't.

At the point of retail sale most retailers will use a 30 micron filter.  However some will use a 10 micron and surprisingly some don't use any filters.  This varies significantly from state to state depending on their laws and enforcement programs 

Now also keep in mind that the way most filters are rated, the micron rating is basically the average size of what the filter will catch.  Therefore they will let some larger particles through also.

So I would recommend a 10 micron final filter and a 30 micron first stage.  From a warranty stand point you can go smaller, but not larger.

Some newer engines are shipped with 2 micron filters, especially smaller engines with smaller injector orifices.

I'll add another reply to address lubricity.

In general the lubricity of diesel fuel has gotten worse over the years correlating closely with the reduced sulfur limits.  Good lubricity quality of diesel fuel is critical for all diesel engines, and especially for high pressure fuel systems.  To meet lubricity requirements with ultra low sulfer diesel requires the use of additives or biodiesel.  Additives are very expensive so distributors try to add "just enough".  Biodiesel in general has very good lubricity and the addition of 1 to 3 percent biodiesel is all it takes to get good lubricity for the finished blend.

By law diesel fuel must meet a lubricity spec of 520 using an ASTM (American Society of Testing Materials, which is now officially ASTM International) test method.  This spec is like a golf score, the lower the number the better.  Unfortunately this legal limit is worse than what most engine manufacturers want, which is 460.  From the samples we run it is rare for diesel fuel to hit the 460 number unless it is a biodiesel blend.  So in general it is safe to say "legal" diesel fuel may be causing wear at a rate which exceeds the engine manufacturers expectations.

Those of you who are nervous about biodiesel in cold weather, shouldn't be.  Minnesota, one of the coldest states in America requires 5% biodiesel in the winter and 20% in the summer.  Most states don't require labeling of biodiesel blends until they reach 5% or more biodiesel, so you're probably already used it without knowing.

In coming years when you see "premium" diesel at the pump you can expect better lubricity, better cold weather operability, and better cetane than what regular diesel will have . However when you see "premium" diesel advertised today it may or may not be better than regular diesel there are no set specs for it in many areas of the country but hopefully that will change soon.

So as far as longevity of your engine, especially the fuel system, good filtration of the fuel and good lubricity of the fuel are very important, but are frequently given little attention.

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