Join Date: Jul 2010
Location: University Place, WA
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Primary and Secondary fuel filter question
I've searched a bit and couldn't find any consensus on this.
I have a Yanmar 4JH4AE engine. The Yanmar has a spin-on 10 micron secondary filter on the engine. As installed by Catalina, it has a Racor 500FG filter as a Primary filter using a 30 micron filter. Yanmar recommends replacing the secondary filter every 250 hours or yearly.
There has been some discussion among C400 owners as to whether a 10 micron primary filter would be better than the 30 micron primary filter. This raised a question in my mind and since I'm completely ignorant about diesel engines, I thought I would ask here.
It is possible to put a vacuum gauge on the Racor filters to determine when the filter element is getting clogged enough to warrant replacing. The idea is that when the vacuum pressure reaches 7-10 mmHg, the filter element is providing enough resistance to fuel flow that it should be replaced. This allows you to only replace the filter when it actually needs replacing rather than doing it every 250 hours or yearly.
That sounds fine and dandy, but if the Racor is only filtering down to 30 microns, that doesn't tell you much about the state of the secondary 10 micron filter. This means that you would still be faced with the task of replacing the secondary filter yearly or every 250 hours since you don't know how many particles under 30 microns passed through the primary filter.
So, what about putting a 10 micron filter on the Primary? This would mean that your primary filter is doing most of the filtration work. If you had a 10 micron filter as primary and a vacuum gauge to tell you when to change it, in theory you would only be changing the primary when it needed it. You could change the secondary as well at the same time, but likely it would really need it then since the primary was doing most of the filtration anyway with the secondary acting only as backup.
Really two questions.
1. Is there any advantage to using a 10 micron primary instead of a 30 micron primary?
2. What actual utility is there in using a vacuum gauge to tell when the primary filter needs replacing?
S/V Legacy, Catalina 400 MkII, Hull #328
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