fuel/water separating filter question

[samba123]

I have a mercury 115 outboard and the mercury OEM fuel/water separating filter for it is a 25 micron filter. Previous year I used a Sierra filter that was a 10 micron filter. Do you think I am ok with using the mercury 25 micron filter? What do you guys use?

 

[zurk]

just use the mfgs recommended filter and replace it annually. also use chevron 94 gas.

 

[Gong Show]

Makes no sense to have your primary and secondary filters the same micron.
First filter at 25, second at 10.

 

[the butcher]

Makes no sense to have your primary and secondary filters the same micron.
First filter at 25, second at 10.

Gong show, can you elaborate on this? I assume the inline fuel filter on outboards is 10 micron? the inline filter on outboards is small..wouldnt it be better to also have the water/fuel separating filter to also be 10 microns since its a significantly larger filter and can capture and hold more? so the only thing that should get into the inline filter is any sediment from within the fuel line post first filter and should normally be fairly clean since the majority of the work has been done by the first(fuel/water) filter? Perhaps my logic is off...

 

[Gong Show]

If both filters are the same, the second filter will do very little and the first filter will colour up quickly. If the first filter is 25 micron and the second is 10, then both filters are doing work. You are replacing both annually, or at least at the same time, with this system you will not try and cheat and not replace the secondary.

 

[Foxsea]

This micron discussion misses an important point. There are two ways of using the micron rating in fuel filters: absolute and nominal. Cheaper filters will describe their products using the nominal definition. That means lots of bigger particles can pass. A 10 micron nominal is probably worse than a 20 micron absolute filter.

Mobile_The_Micron_Rating.pdf

 

[the butcher]

This micron discussion misses an important point. There are two ways of using the micron rating in fuel filters: absolute and nominal. Cheaper filters will describe their products using the nominal definition. That means lots of bigger particles can pass. A 10 micron nominal is probably worse than a 20 micron absolute filter.

Mobile_The_Micron_Rating.pdf

your attachment didnt come through so I cant see any info on the article you attached. How do we know whether the micron rating for a fuel filter is nominal or absolute. I usually only see the number(10,25 etc) and then micron. dont recall ever seeing absolute or nominal.

 

[Foxsea]

your attachment didnt come through so I cant see any info on the article you attached. How do we know whether the micron rating for a fuel filter is nominal or absolute. I usually only see the number(10,25 etc) and then micron. dont recall ever seeing absolute or nominal.

Sorry, just copy and paste. It's from Racor. This paragraph was most interesting:

"Racor’s 2 micron filter medium should only be used in final or secondary filters where the fuel is first filtered by a primary filter. The primary filter for a 2 micron final filter should use a 10 micron medium. The exception in using a 2 micron filter in place of a primary filter is to obtain high-efficiency water separation, and is usually used in marine applications where the fuel supply may be cleaner but also may contain water more often. If the installation can allow the use of a filter large enough, then a 2 micron filter can serve in a system as the only filter in that system."

It's suggested that usually a quality prefilter and then a finer, secondary filter are used. Then it notes, in marine environments, a larger, 2 micron filter for water separation could be used.

 

[Foxsea]

After doing more reading, the industry consensus is that a 10 micron primary and a 2 micron secondary is the way to go for injected engines. Use high quality filters and change both on regular schedules. Keep spares on board. Racor stills sounds like one of the best.

 

[the butcher]

does anyone know what the major outboard manufacturers typically use as the secondary inline under the cowling fuel filter? 2 or 10 microns?

 

[sly_karma]

Evinrude is 10 micron

 

[sly_karma]

Most EFI systems have another filter inside the cowling, on the return line from injectors. Those need periodic inspection and replacement as well.

 

[Gong Show]

Yeah, I'm into diesels.
Guess they are more forgiving than the new efi gas outboards.
10 micron has done the job for over thirty years.