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Old 05-07-2013, 08:29 PM   #1
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Fuel Filter Befuddlement

On my DP I have a Cummins ISB6.7 CM2250 diesel engine. The owners manual indicates fuel filter change (sic - it does not say "filters") at a 12 month interval for RV applications. It says NOTHING about either a secondary filter or a coalescing filter. On the other hand, Cummins has posted on-line the recommended maintenance for the ISB6.7 engines that the coalescing filter (which I presume is the "secondary filter") should be changed "every 3rd or 4th oil change interval") (ISB6.7 for Medium-Duty Truck (2013) - Cummins Engines) ... but yet it goes on to refer to the owners manual and says you should follow that.

So what do I do ... ignore the coalescing filter and never change it (unless I start seeing a lot of water to drain) ... or change is every 3-4 years no matter what...????? Any insight from those more experienced in these engines will be MOST welcome - as I am quite confused at this point....
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Old 05-08-2013, 10:30 AM   #2
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Since no one has responded to your post, I will provide an answer even though I have a CAT engine, not a Cummins. The reason the filter situation is a bit vague is because the chassis builder, not the engine manufacturer, actually outfits the engine with whatever filters it has. Most DPs have two filters.

Many conscientious owners of DP MHs change both their primary (also known as a coalescing filter) and secondary filters every year. For low mileage applications that is pretty much the recommendation of both CAT and Cummins. Filters cost ~$20-50 each so it's a pretty inexpensive preventive maintenance item.
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Old 05-08-2013, 12:31 PM   #3
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I concur with docj! Changing both filters every year is cheap insurance. At a minimum you should check/drain the water separator every month and change them out yearly.
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Old 05-08-2013, 07:21 PM   #4
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If you look at the Cummins manual, they are talking about the filter they included with the engine (on ISB and ISC models) and they refer to that as "primary". But the chassis manufacturer installs the water separator/filter nearer the tank and they view that as "primary", meaning the first filter in the series.

Do you have a chassis owner manual? It's probably a Freightliner, given your make & year of coach.
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Old 05-08-2013, 07:43 PM   #5
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MN Traveler,
First, a correction on your terms. As Gary notes above. The primary filter is the water separator filter. The secondary filter is often called the fuel filter. See the following page: ISB6.7 for Motorhome (EPA 2010) - Cummins Engines

You will note both filters should be changed every 12 months. The coalescing filter is a totally different filter (not a fuel filter) that sits on top of a Cummins engine. It is a crankcase ventilation filter similar to an air filter that catches impurities from the air blowing off the crankcase and being recycled back into the engine. It typically looks like a small cigar box size filter behind the valve cover.
Does this help?
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Old 05-08-2013, 07:56 PM   #6
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The confusion is the term "Coalescing filter". On my 2000 MH- the coalescing filter is in the Air Dryer and removes oil and contaminants from the air brake system (not a Cummins part). On the newer Cummins engines there is a coalescing filter on the Crankcase Emissions system that captures and returns oil directly to the sump. (eliminated the slobber tube but is a Cummins part)

As Dennis just explained.. in more detail..
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Old 05-08-2013, 08:18 PM   #7
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Our DSDP/Spartan has only one filter but it also collects water and has a sensor built in. Supposed to turn on a dash light if water is detected. After 11 years I've never seen the warning light and yes I have changed the filter. Probably not as often as I should as it's supposed to be changed at every oil change and I haven't done that.
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Old 05-08-2013, 10:20 PM   #8
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This discussion is good - and I do think clears things up (if you are careful about the terminology). I did earlier locate the ISB6.7 for motorhomes manual - which is part of what confused me re. the "coalescing filter", because many other sources refer to the "water separator filter" also as a "coalescing filter". I have the ISB6.7 Owners Manual, which says nothing about either a "secondary filter" OR a "coalescing filter" ... but it DOES refer to the "crankcase ventillation filter" (which is Presume is the cigar-box shaped "coalescing filter" discussed above located on top of the engine? The manual says however that it should be changed every 75,000 miles (or 2500 hours). Unfortunately, other sources ALSO refer to the filter in the air-dryer for the compressed air system also as a "coalescing filter". In general, I understand that a "coalescing filter is any filter that separates water from something else (air or fuel).

BTW - the Freightliner Custom Chassis manuals also say NOTHING about service intervals for a fuel/water separator filter (I spoke with them today, and the rep. I spoke with also could find no recommended service interval). Sigh - the world is not pefect....

Again - thank you all for your comments, I do think they have helped to understand the configuration. My plan then (to try to use less confusing terminology) is to change the primary and secondary fuel filters (one of which is the fuel/water separator filter) on a 12 month cycle along with the oil and oil filter. The crancase ventillation filter (which I guess is also called a "coalescing filter) gets changed every 3-4 years. The air/oil separator filter gets changed on its own schedule (which I do not have in my head right now - sorry). I think that makes sense with what everyone above has commented on.... right?

Thanks again!
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Old 05-09-2013, 12:27 AM   #9
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Good plan- The Fuel/water separator filter is a fuel filter and is changed on the same schedule as the oil change. Usually annual.
My ISB (5.9) also has that type as it's only fuel filter.

Back to coalescing: It means bringing together or combining. Except when applied to a filter it means separating or removing. Do not know why anyone would find that confusing.

Congress will coalesce around the nations problems any day now....
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Old 05-10-2013, 12:53 PM   #10
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My 05 Phaeton with the ISB 5.9 engine has the standard fuel/water unit at the rear.
In addition, there is a second unit that Freightliner calls a strainer to block larger trash from getting down stream.
This strainer is around 2 inches dia by 8 inches long. It is located inside the frame rail very near the rear axle.
A real PIA to change. I swapped the strainer for a Golden Rod replacement filter unit that works fine but cart. replacement means sliding under the MH.
Plans are to re-route the GH to the outside of the frame.
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