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Old 10-27-2019, 02:49 AM   #1
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2003 Cummins w/ 3-Filters... Do I need to change all of them? ...And how often?

This summer I installed a FASS "Titanium Series" Lift pump with 2 filters in my 2004 Itasca "Horizon" 40AD. I already had 2 filters and took out the primary filter when I inserted the FASS "TS" pump in it's place. This left me me with 3 fuel filters:

#1) 20u on the suctions side
#2) 10u on the FASS pressure side
#3) 2u on the remote mounted filter head that originally came with my engine and now feeds the CAPS injection pump.

I am very happy with this setup. My questions are as follows:

1) If I store the coach with a biocide, how often would I/you change out the fuel filters?

Note: I drive about 6K-9K miles/year... if that matters?

2) Is changing fuel filters "time dependent" for us RV owners? ...Or is it more so mileage dependent no matter how much time goes by?

FASS says to change the filters every year, but they are use to giving advice to Dodge and Chevy diesel truck owners and are new to the RV owner needs and wants.

3) Why would changing fuel filters be "time dependent" if I use a biocide?

4) Of my three fuel filters, which one would you think would fill-up or get "restricted" first?

5) Would you recommend changing all 3 filters at the same time? If so, how often?

Note: Yes! I have a fuel pressure gauge that is ported to the 1/8" FASS "TS" fuel port provided on there pump; and I can monitor it from the cockpit. However, FASS says the fuel pressure gauge is of little consequence since the FASS fuel pump is a "constant velocity fuel pump," and as such it will always try to putout 16-18PSI.

In fact, if anything, they tell me the "TS" pump will self-regulate based on the spring pressure in the return fuel line. (Apparently, this is why the return fuel line is critical to the TS-pump maintaining regulated fuel pressure.)

However, they admit, the fuel gauge will drop under load or acceleration; and some interpretation can be drawn from that information in regards to how "restricted" your fuel filters are becoming.

Note: I find my fuel filter gauge (with new filters) will show 15-16 PSI when accelerating; and now that i have 6,000 miles on all 3 fuel filters... the PSI will drop to 14 when accelerating. Is this a sign my fuel filter (or filters) are getting restricted? ...AND IF SO WHICH ONE?

FASS tech supports says you can't determine how clogged your fuel filters are by looking at the fuel pressure on the gauge, because the TS pump is a "Constant Volume Pump." So how can you tell?

Are we back to monitoring engine performance in term of engine spudder/cough, or not being able to driver faster than 35 MPH?" I hope not!

My tendency is to think diesel fuel has mostly "large" particulate mater and "organisms" that the 20-micron filter will catch... so this is the filter that will get "restricted" first. What do you fuel experts think?

Note: The attached diagram is applicable to my 2004 Itasca "Horizon" 40AD with a Freightliner Chassis; the "Pink Marker" indicates an alternative installation for a Spartan Chassis commonly found on Monaco RVs.
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2004 Itasca Horizon ISK-40AD, ISC-350HP Cummins
... and the best of 3 Diesels I have owned thus far!
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Old 10-27-2019, 05:22 AM   #2
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Location: Wolf Creek dam area, Ky. Cardinal Center Camp Ground, Marengo Oh. or on the road.
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I dont really think your asking for advice.
Personally, I think that you have it figured out as you have done your research well.
You have made a very nice update to your fuel delivery system and you understand why you did.
As long as you do not buy a tank of bad, contaminated fuel, and thats rare today.
You could go a few years if you want

On my diesel vehicles that run 5 to 8 thousand miles or so annually I have gone two years and then change all fuel filters.

I have debated in my mind, changing the first filter only on low millage vehicles. But thought that while Im at it change them all need to or not.
The cost for 1 or 3 is nothing when you look at everything involved.
And what ? an other 5 to 15 minutes to change them all while your at it.

Its all very Debatable in my opinion.

And comes down to.
1. Want to
2. Need to
3. Have to, sitting along the road knowing you should have ???
Follow a schedule of some kind and dont make a big issue out of it.
4. Do I need to spend the $$$s should never enter the mind. If it does, One should have never purchased a Diesel engine in the first place..

I have never had an issue with water in my fuel since in the 60s. Caucious selection of stations or luck ??? Dont know really, crap can happen.

The diesel vehicles that run 25 thousand miles plus annually get all filters changed every 25 thousand miles, AT THE MOST and sometimes thats every 6 months or so.

When I look at the cost of overall investment and the cost of repairs???

The issue of changing 1, 2, or 3 filters is a no brainer.


you did contribute and made a great post.
I enjoyed your detailed thought out processed thread.

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Old 10-27-2019, 08:03 AM   #3
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I'm not into annual anything...oil, filters, AHot service. Over the years changing the primary (1st) fuel filter usually solves the problem at around 30K miles. On my "new" rig I got sucked into thinking all the filters had been changed before buying it... date and miles written on the oil, fuel, air, and water filters, all easily visible. After 7k miles since buying, it started to not hold 100% power (an indication of needing a fuel filter) so I put on my spare primary filter. Did not fix so tried to remove the secondary on the side of the engine that obvious had not been changed when the others...shame on me for not making sure. Had to go and buy a strap wrench to get it off as a regular filter wrench would not budge it... problem solved.
This post is my opinion, worth what you paid for it.
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Old 10-27-2019, 01:47 PM   #4
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There is a huge difference in the type/brand of fuel filters you can purchase for a given application. Some are designed for extended life and others are so called standard. I usually ran my F350 filters up around 30K. Most of the literature on the Coaches ranges from 15-20k. Good clean fuel is certainly going to be the key as it always has been to long term filter life. I never have used a biocide yet in my 65 or so years of having Diesels and have never needed it.

My main concern has always been the water separator filter and avoiding moisture in the fuel both in the bulk and vehicle. The final filter is critical to the new extremely high pressure fuel injection systems.

As for me 15-20 thousand miles or three years is my comfort zone for the coach and as already noted 30K on the 6.7
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Old 10-27-2019, 07:04 PM   #5
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Im running the adjustable Fass (https://www.amazon.com/FA-D08-095G-A.../dp/B009LKIPKC) with my stock filters. On my CAPS ISL they dont recommend filters under 10 micron. But, I think this is because they dont want to starve the injection pump. After my Fass install I monitored my fuel pressure. After three years with the same filters, my fuel pressure was still at over 10psi on a hard long pull. Before, I would change both filters every year.

If I had your setup, I would move your fuel pressure sending unit from the Fass port to the filter output on your last filter. This will show you fuel pressure to the injection pump. My filter housing had a capped port on top of the housing where I installed my sender.

I run Opti-Lube XL and a biocide in all my fuel.

2003 Country Coach Intrigue 36'
Cummins ISL 400
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Old 10-28-2019, 03:11 AM   #6
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Ivylog: Are you saying your forgot to change your secondary filter once; and then you had a slight performance concern... Changed the primary. Not the problem. Then changed the 2x old secondary and that was the problem?

You have 2 filters right?

I now have 3 filters: 20, 10 and 2 micron respectively. I have not yet need to change them. But when I do I would like to first have a reference (like PSI) or in your case engine performance degrade. And then I want to experiment with changing out one of my filters at a time, until I arrive at the bad filter. I'm just wondering if that would be my 20u or my 10u or my 2u filter?

With my FASS setup I am seeing a slight drop in my electric cockpit PSI gauge; and I think this is because I have ~8,500 miles on my 3 filters. ...But I bet only one is getting restricted more than the other two, but which one?

TR4: Again your sound reasoning helps me out. Thank you! And yes, I do have a 10mm thread port on top of my #3 filter housing/mount. (This would have been the primary filter when the Coach was shipped, but now it's my #3 fiter after I installed the FASS TS Pump. (Which comes as you know with 2 filters... the first is under suction... the 2nd FASS filter is under pressure.)

Here's the 10mm to 1/8"NPT adapter off amazon you can order. I choose the 1/8" NPT because it has a smaller glass gauge diameter vs. the 1/4"NPT type.


And for those who just want to mount a pressure gauge on top of your filter set up... If you have a FASS pump delivering positive fuel pressure to your CAPS Injection Pump then you should order this oil-filled gauge:


If you have a stock filter setup where your CAPS gear pump is sucking fuel through your filters, then you will want a vacuum gauge, but make sure it's oil-filled to deal with the engine vibrations. This one will cover both negative and positive fuel pressures... so when the day come you upgrade to a FASS TS Pump you will have a pressure gauge ready to go!


CLARIFICATION: I only use a biocide in my fuel when I store my rv for 1 month or more.

Also, years ago, I called Cummins filtration and discovered all this micron rating stuff on the retail end has more to do with marketing. There is a science to it, but one filter manufacture's tests and rating do not necessarily correspond to another filter manufacture's tests and ratings. I.e., a Napa gold will give me a (5u) filter when I tell them I have a Fleetguard 1022 (10u) filter. So which is more accurate if they test and rate their filters differently?

I guess somewhere along the line most people, including myself at one time, think a better filter has smaller micron rating. NOT SO! ESPECIALLY WHEN IT COMES TO A CUMMINS ENGINE AS TR4 points out. (Which to me is just another indicator the CAPS gear driven suction pump is not the right application for a diesel pusher!!!)

==> I am so mad at Cummins for not owning up to the poor design they gave us when it comes to fuel delivery! ...So thank god for FASS!!!

To clarify, when I list micron ratings for each of my filters I am referring to Fleetguard (Cummins Filtration) brand filters for my #1 & #2. And as for that 2-micron #3 FASS filter... I have no idea if that would be considered a 5-micron accorting to Fleetguard test & ratings system? ...But I know it's small... very small.

So back to the subject at hand: Do you think a 2-micron FASS filter will clog up sooner then my 10 or 20 micron Fleetguard... just because its 2u and smaller filters always clogg up before larger micron filters? ... I really do not know? ...What's the norm here?

My guess is that there is no norm; and Milford_47 has probably stated it best:
You should change your filters based on whatever makes you feel "comfortable." ...Or until your engine starts to loose performance, like Ivylog says happened to him.

I am willing to accept Ivylog's example -- now that I have a FASS TS Pump! But that's only because I have upgraded to a FASS lift pump.

Back to fuel filters: I think it's possible fuel filters don't actually clog-up. Rather, they create fuel restriction (even when new); and that restriction builds up over time and with use. (Paper deteriorates too so let's not forget this. And Algae does grow!) And then we change them because someone told us to. But is this really necessary if you store your RV with bicide and you have a FASS fuel pump?

Story time: I experienced "restriction" in my fuel delivery system first hand -- when my 3-lift pump bolts got loose; and I found myself changing filters first at 9,000 miles; and then at 5,000 miles.

The problem was that my filter restriction was increasing with time and use, and one day that restriction reduced the vacuum pressure (suction) in my fuel line to the point where my engine would start and then it would quit 1 minute or 2 minutes later.

And over time I came to realize those partially clogged/restricted filters where not the real problem at all. The problem was those 3 loose lift pump bolts that let air to be sucked into the fuel line, between the lift pump, which was working properly, and the lift pump manifold. That's what happens when a gasket breaks its seal. So when you hear people say their Lift Pump Failed, I'll bet your mechanic told you that and 8x out of 10 all you really had to do was to tighten those 3 lift pump bolts by accessing the top of your engine... in the bedroom.

Anyway, this resulted in fuel starvation to my CAPS pump several times. And that's why I upgraded to a FASS TS Lift Pump... because I wanted to protect my CAPS injection pump from fuel starvation!!!

Moral of the story: The sooner you upgrade to a FASS fuel pump the longer can probably go between fuel filter changes -- and the likelihood you will ever starve your CAPS injection pump is no longer a problem!
2004 Itasca Horizon ISK-40AD, ISC-350HP Cummins
... and the best of 3 Diesels I have owned thus far!
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Old 10-28-2019, 03:17 PM   #7
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“Ivylog: Are you saying your forgot to change your secondary filter once; and then you had a slight performance concern... Changed the primary. Not the problem. Then changed the 2x old secondary and that was the problem?”
Fairy new to me DP and it looked like all the filters had been changed (date and miles written on them). Did not slide under to check the fuel filter on the engine ASSUMING all had been changed. Normally the first filter is the one that plugs up first so after changing it there was no improvement so that’s when I discovered the 2nd (was actually rusty) had not been changed...no telling how long although the rig only had 40K miles when I bought it and the problem started at 47K.

This post is my opinion, worth what you paid for it.
Our New ride...08 HR Navigator 45. A nice upgrade after 14 years with a 04 Dynasty 42.
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