Diesel filters/systems. Getting my head round it all...

mogmog2

mogmog2

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This is our first boat with an inboard but I'm pretty competent with engines & mechanicals. I'm ordering filters and been generally trying to learn about boat diesel fuel systems.

There is a (Cav type?) pre filter with metal bowl and the filter on the engine, which is an MD11C.

If the Cav-type filters are such a pain to change (many posts on here etc) - hanging upside-down in the cockpit in any kind of sea, I can imagine. It seems a sensible solution to split the water separating function back to a separate bowl & fit a spin-on head/filter? There must be a reason that I have missed?

Secondly, I've picked up about priming but don't know what's involved - some heads have priming pumps, some people have fitted priming doodahs etc etc. If the existing system doesn't have this, should it - does it make life easier, or will the existing layout be set up so it doesn't need one?
Many thanks
 
You get better with CAV filters with practice. Curses, that statement now means it'll take me 30 minutes the next time I change one.

You can get spin on filters but they tend to be a bit more expensive. Another solution is parallel CAV filters with changeover switches.

After the primary filter is the lift pump - on my engine Yanmar 4JH, which self-bleeds, this is sufficient for priming after a CAV filter change in over 90% of cases. It is a nuisance and tedious to bleed through a full CAV filters worth of air if doing it by hand - a real pain in the neck if at sea and you should really be on deck seeing what's going on.

Which is why some people have an additional pump, often electric, for priming.
 
The mounting position, or rather the height of the prefilter mounting affects priming ease enormously.
Roughly speaking, the prefilter should be at a similar height to the mid level of the fuel tank so if the tank is full it'll bleed itself. Below about half full it gets more difficult, so I tend to keep my tanks full-ish for an easier life.

I have CAV type prefilters too. I ditched the glass bowls beneath the filters for just the shallow aluminium base, this makes reassembly with new filters much easier (and therefore quicker).

Yes I know the deep bowls are to collect water, but separately I polish my fuel to remove all water, which the CAV filters aren't good at anyway so there's nothing lost in reality.

Edit: There's another reason to keep your CAV's. The filters are dirt cheap and available everywhere. Once you get into specialist spin ons, it won't be so easy and considerably more expensive. Your choice I guess
 
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mogmog2 said:
It seems a sensible solution to split the water separating function back to a separate bowl & fit a spin-on head/filter? There must be a reason that I have missed?
Click to expand...

If you mean a reason not to do it, then no, not really. The availability of very very cheap CAV filters (~£2 each) is about the only semi-good reason to install a CAV head, otherwise it's mostly just habit and inertia. Note that the people in yards speccing them generally aren't the ones who will have to change them.

Pete
 
mogmog2 said:
This is our first boat with an inboard but I'm pretty competent with engines & mechanicals. I'm ordering filters and been generally trying to learn about boat diesel fuel systems.

There is a (Cav type?) pre filter with metal bowl and the filter on the engine, which is an MD11C.

!!!!!!!!Plus a strainer in the lift pump if it has a centre bolt in the cover later pumps without dont have strainer!!!!!


If the Cav-type filters are such a pain to change (many posts on here etc) - hanging upside-down in the cockpit in any kind of sea, I can imagine. It seems a sensible solution to split the water separating function back to a separate bowl & fit a spin-on head/filter? There must be a reason that I have missed?

Secondly, I've picked up about priming but don't know what's involved - some heads have priming pumps, some people have fitted priming doodahs etc etc. If the existing system doesn't have this, should it - does it make life easier, or will the existing layout be set up so it doesn't need one?
Many thanks
Click to expand...
 
ldsn Thanks, that clarifies the bleeding situation.

Trundlebug, from the photos I have to hand, the primary filter is probably higher than mid-point, so that means tickling it through with the lift pump I presume in all but the fullest tank situations.

Our filter has a deep aluminium bowl, which presumably shares the same hassle-factor as the glass bowls?
Out of interest, why aren't the CAV filters good at polishing fuel? ( I was speculating about retiring the fitted set up to a basic fuel polishing system at home, if I replaced it).

Prv, thanks. I agree wholeheartedly with the people who spec and the peope work on the stuff afterwards.


Spin on's seem to be about £10 against the can sort at £2 - a big increase, but not a lot of money though. I'll just order the filters I need now, and leave the plans till I've experienced life with a CAV.

Scottie, thanks for that heads up - if there's a strainer in the lift pump, that's after at least one filter - does it ever catch anything then as it would seem a bit redundant?

And thanks for helping. Got a better picture now.
 
mogmog2 said:
Our filter has a deep aluminium bowl, which presumably shares the same hassle-factor as the glass bowls?
Out of interest, why aren't the CAV filters good at polishing fuel? ( I was speculating about retiring the fitted set up to a basic fuel polishing system at home, if I replaced it).
Click to expand...

I favour using a glass bowl - just makes it easier to shine a torch through and see what the fuel looks like at sea. Without the bowl you lose one of the advantages of CAV filters - and maybe they are antiquated in this day and age.

The CAV filters are generally not that fine. The idea is to take out the worst, and the water, before it reaches the secondary filter. IIRC typical figures are for a CAV filter to remove most impurities >10 microns in diameter and a secondary filter >2 microns. They can be used for a fuel polisher but you have to accept the limitations. I would argue that fuel polishing is to remove the worst of the gunk to stop the fuel system clogging up and is not to make the fuel perfect, but I can accept that others have different view points. The clear thing is to understand the limitations of the filtering you're doing.
 
pvb said:
I'm not sure what that tells you, the fuel in the bowl has already been filtered.
Click to expand...

So what's your point? Glass bowls are useless? Or you're just trying to be a bit provocative as well as pedantic whilst pointing out what most people know already.
 
lpdsn said:
So what's your point? Glass bowls are useless? Or you're just trying to be a bit provocative as well as pedantic whilst pointing out what most people know already.
Click to expand...

No, I just wondered what you can tell from looking at a bowl of filtered fuel. Maybe you can explain, preferably without the insults!
 
You can tell if there is water in the fuel, as it accumulates in the bowl.

You can also tell if anything is getting past the filter. CAV filters aren't perfect by any means or there may be something else wrong with the way it is fitted - never had the latter but it could happen.

I've even spotted bug beginning to grow on the water fuel interface when the engine hasn't been used for a while, which is a wake up call to check the rest of the system out.

Seems to me a good enough reason to have a glass bowl, but as I said happy to accept that others prefer different arrangements.

If you don't want 'insults' don't wade in aggressively and I won't give out to you at all.
 
mogmog2 said:
ldsn Thanks, that clarifies the bleeding situation.

Trundlebug, from the photos I have to hand, the primary filter is probably higher than mid-point, so that means tickling it through with the lift pump I presume in all but the fullest tank situations.

Our filter has a deep aluminium bowl, which presumably shares the same hassle-factor as the glass bowls?
Out of interest, why aren't the CAV filters good at polishing fuel? ( I was speculating about retiring the fitted set up to a basic fuel polishing system at home, if I replaced it).

Prv, thanks. I agree wholeheartedly with the people who spec and the peope work on the stuff afterwards.


Spin on's seem to be about £10 against the can sort at £2 - a big increase, but not a lot of money though. I'll just order the filters I need now, and leave the plans till I've experienced life with a CAV.

Scottie, thanks for that heads up - if there's a strainer in the lift pump, that's after at least one filter - does it ever catch anything then as it would seem a bit redundant?

It is often overlooked and can accumulate a real paddy field

And thanks for helping. Got a better picture now.
Click to expand...
 
Well, I don't think I did "wade in aggressively" but that's your perception, so nothing I could say would change it. For what it's worth, I've never seen any water in a filter bowl on any of the boats I've owned. Perhaps I've been lucky.
 
pvb said:
Well, I don't think I did "wade in aggressively" but that's your perception, so nothing I could say would change it. For what it's worth, I've never seen any water in a filter bowl on any of the boats I've owned. Perhaps I've been lucky.
Click to expand...

Perhaps your fuel system does not draw from the very bottom of the tank?
 
pvb said:
For what it's worth, I've never seen any water in a filter bowl on any of the boats I've owned. Perhaps I've been lucky.
Click to expand...

Had it happen once. Crew who didn't know his own strength stripped the thread of the filler cap. Not happened again since that was sorted.
 
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