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FASS fuel air separation systems can provide particularly impressive improvements in the efficiency of semi diesel engines.

5 Questions View all

How long between changes?

I installed my FASS January of 2013 on CAT C15.

Manual says 30-50,000 miles for Class 8 trucks.

My experience:

21,302 miles clogged up - would not start (not below freezing - perhaps some bad fuel - not sure) Replaced all 3 filters

18936 - poor fuel mileage - changed all 3

39319 - mpg down - changed all 3

23527 - clogged up - stopped engine - replaced all 3

Is this normal? Looks like I need to change filters at 20,000 miles.

If clogged up, I have started at FASS fuel/water separator, then FASS filter, then main CAT filter. This last time, the seal (o ring?) blew off the CAT filter as I removed it due to the pressure.

Am I working in the wrong direction? Should I start with the CAT filter and work backward?

My filter cost: FASS water separator $ 20.90

FASS fuel filter $23.75

CAT fuel filter $35.42


Cost to change all 3 $80.07

Change at 30000 miles = 4/year = $320.28

Change at 20000 miles = 6/year = $480.42

I still estimate 1/2 mpg boost when FASS is working good, so estimated fuel savings of approximately $4616 annually still makes it work even with 6 changes, if my math is correct.

I buy fuel at TA when I can, which is 90% of time. I treat with Howes when extremely cold, and quarterly during warm months just to be safe.

Anything else I should be looking at?


Answer this question I have this problem too

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The rest of the story. . . most recently, truck suddenly started running pretty rough. Pull off interstate off ramp and engine died as I slowed. I thought fuel filters?? so I started with FASS fuel/water separator, changed it, tried to restart, nothing. Next changed FASS filter, nothing. Next changed CAT filter, nothing. Kept trying to manually pump up, loud and could not tell if FASS pump working, but I thought it was? Finally got engine started with starting fluid, and it has run good since. But did find that FASS pump not working. Checked all connections and fuses and looked good. I called Eldon who did install almost exactly one year ago. He told me he had installed a "continuous on" relay capacitor and to use a test wire between the 2 large lugs on it. I did and the pump worked, so that solves that problem. But Eldon did not think that the pump not working would stop the running engine.


Jerry, I got your comment and broke open the filters just now at a truck stop using a hack saw and metal cutters. Did not see any amount of anything like sand or algae, but you see the dark color. Eldon said the filters start off white, so there is something there. But to me, it does not see like near enough to completely clog the series of filters. I will try to post pictures so you can let me know what you think?


Having problem with pictures.


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4 Answers

i usually change my filters around 25000 miles but i've never had them clog up. maybe you have a different issue? have you cut the filters apart to check what's inside when they clogged up and killed the engine?

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Well, this is only one I can get to upload. This is the FASS fuel filter. The water separator and CAT fuel filter look identical in color.

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I change the Fass fuel filter at 25K and clean the Fass f/w separator, change the engine fuel filter at 25K, then after another 25K, I change all 3. Never had FASS to plug, but if it did, the FASS is supposed to bypass, if there is excess pressure on your Cat fuel filter, then the problem could be the FASS spring and ball in the FASS return line, it should hold approx 19#. FASS will send you another spring and ball if you will call them.. Very simple to install..

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This cold winter seems to be proving a problem with the FASS filters, I spoke to my mechanic about it today and they are recomending to go with a larger micron rating 6-8 instead of the 3 that the fass fuel filter has. Maybe they need to make the 10 micron filter again. I was using that last year and had no problems with fuel but they stopped making it in the summer. In 7 years I have never had gelled fuel, 3 times since Dec I have had this problem and only after i have used a fuel aditive due to extreme cold weather!! Anybody else had this experience or is it just a coincidence that I have had this problem. The engine filter seem to be ok no sign of gelling, its just the FASS filters. I have been lucky the one time I had this problem on the rd i changed the FASS filters and everything ran good, the other twice it has happened at the yard a couple hrs in a warm garage changed the filters and everything worked ok.

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Reviving this a little.... go with one of the cross reference filters that they show. I am partial to Donaldson, but Baldwin would be a good choice also. You can get different micron ratings to match what you need on the FASS. The two on the FASS, not as critical to filter down so low. Leave the final filter at recommended micron and go larger on the FASS.


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Will will be eternally grateful.
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