Fuel prefilter and filter on VP KAD 300

[About Time]

Hi,

I have noticed something strange .... When I bought my boat few years ago the boat could do 40 knots (+) ... now it seems as 36 knots is max. Really not a big issue but still strange.
No smoke from any of the engines. They idle the same and function is exactly same. The KAD 300 has a max. rpm of abt. 4,200 as per manual... now I can do only 3,650 rpm on both engines. The last 1/2" on the throttle seems not of any use.
The fuel filter (21624740) is 20 micron and when I bought the boat few years ago I remember that there was a prefilter (racor) of 30 micron which makes sense. Prefilters should be less restricted or same as to main filter I should think. Purpose is to clean 95% of the bad stuff out and leave the main filter to take out the rest.
Now (and for the last few years) Volvo has changed from offering 5, 10 and 30 micron to only 10 micron for the KAD 300. I have therefore used the 10 micron the last few years ....
It actually means my prefilter (racor) is more restricted than the main filter which I think is wrong....
Could this cause the lack of abt. 500 rpm on both engines ? and will it hurt my engines that the fuel supply is actually restricted/ cleaned by the racor (only) leaving the main filter of no use ???
I have read a few articles about cars that fuel flow can be restricted by filters but ......
Hope a wise captain can spread some light Happy season to all

 

[madabouttheboat]

Engine mounted filter will be les than 10 micron, so the 10 micron pre filter will have no effect at all, unless it is partially blocked.

 

[madabouttheboat]

When were the injectors last checked/serviced? How many hours on the engine? have the valve clearance checks been kept up to date (every 200 hours)? How old are the turbos? Have the aftercoolers been cleaned out ever?

All or any of the above could account for a drop in performance.

I assume your hull, sterndrives and props are spotlessly clean.

 

[About Time]

Thanks your quick response.
The filter 21624740 (engine mounted) is 20 micron OEM Volvo Penta. I always replace filters and pre-filters every year (70-100 hours of use). Every 4 years I have my tanks checked and cleaned for any minor debris. When I replace the prefilter they are only brown - never black (real dirty).
The engines have 1,070 hours on them and valves are checked every 200 hours. Done last year at 1,015 hours.
Turbos were replaced in 2022.
The temp on both enigines is between 82 degree (slow) and 89 degree (crusing at 25-30 knots) so I have never been thinking about the aftercoolers. I will try to check these.
Also I have never done any service on injectors but both engines idle stable at 650 rpm and act exacly same and both stop at ard. 3,650 rpm.....
Can you tell me how I will know if the injectors require service.
Hull, stern drive and props are clean

 

[madabouttheboat]

Thanks your quick response.
The filter 21624740 (engine mounted) is 20 micron OEM Volvo Penta. I always replace filters and pre-filters every year (70-100 hours of use). Every 4 years I have my tanks checked and cleaned for any minor debris. When I replace the prefilter they are only brown - never black (real dirty).
The engines have 1,070 hours on them and valves are checked every 200 hours. Done last year at 1,015 hours.
Turbos were replaced in 2022.
The temp on both enigines is between 82 degree (slow) and 89 degree (crusing at 25-30 knots) so I have never been thinking about the aftercoolers. I will try to check these.
Also I have never done any service on injectors but both engines idle stable at 650 rpm and act exacly same and both stop at ard. 3,650 rpm.....
Can you tell me how I will know if the injectors require service.
Hull, stern drive and props are clean

Where did you get the information that the engine mounted filter is 20 micron? When I spoke with Volvo about this a few years back they assured me that all their engine mounted filters were in the single figure microns. I would be very surprised to find they are 20 micron, when Volvo themselves will often supply a 10 micron element for the pre-filter.

I think, from memory and I may be mistaken, that injectors are due a service/overhaul/replace at 1000 hours.

The other option, of course, is that your boat has grown heavier during your ownership and the prop sizes are now wrong for the boats weight. If all else is good, then last resort might be a change of props to get the RPM back. It may not get your top speed back but at least your engines won't be labouring. Alternatively, try putting the boat on a crash diet!

 

[About Time]

I did a little research on the Volvo main fuel filter and it seems 20 micron from a phone call to a Volvo shop ...... Also I see aftermarket filters for 21624740 is 20 micron: https://www.napaonline.com/en/p/SME187942
Anyway I will get my injectors checked as it is about time. Then not worry anymore why pre-filter is less micron that main filter but indeed strange in my eyes.

 

[petem]

On my KAD44's I have...

Engine filter 21492771
Prefilter Racor 2010 30 microns

 

[About Time]

Petem, that makes so much more sense. I can conclude that you do not order the VP's prefilter but probably buy a standard racor 30 micron. Thanks for sharing info.

 

[petem]

Petem, that makes so much more sense. I can conclude that you do not order the VP's prefilter but probably buy a standard racor 30 micron. Thanks for sharing info.

He's an independent mechanic so just orders the standard Racor filter as you say.

 

[stelican]

Thanks your quick response.
The filter 21624740 (engine mounted) is 20 micron OEM Volvo Penta. I always replace filters and pre-filters every year (70-100 hours of use). Every 4 years I have my tanks checked and cleaned for any minor debris. When I replace the prefilter they are only brown - never black (real dirty).
The engines have 1,070 hours on them and valves are checked every 200 hours. Done last year at 1,015 hours.
Turbos were replaced in 2022.
The temp on both enigines is between 82 degree (slow) and 89 degree (crusing at 25-30 knots) so I have never been thinking about the aftercoolers. I will try to check these.
Also I have never done any service on injectors but both engines idle stable at 650 rpm and act exacly same and both stop at ard. 3,650 rpm.....
Can you tell me how I will know if the injectors require service.
Hull, stern drive and props are clean

I am pretty sure the intercoolers are on the seawater circuit than does not have temp gauge but I guess you know that.

 

[kashurst]

The power and torque maxes out about 3600ish from memory so unless your boat is very clean underneath with very shiny props, not full of junk and boaty kit etc it will struggle to get that last few hundred revs.

Changing the primary fuel filters to 30 micron might help. Change your air filters too. The air filters on KADS tend to bung up quicker because of the compressor belts.

 

[Portofino]

If you can’t reach the rated rpm at WOT basically your boats overloaded .
I can’t see restricted filtration being the issue from what you have written .

My MAN engine filters ( two on each engine ) are 2 microns and the racor pre filters 20 s .
This boats rated WOT is 2150 so we are 80 over a tiny under 1% which is correct .
2003 engines with a 1000 + hrs done Sept after a June haul out 2023 so twenty yrs old .
Yes the boats got “ cruising stores “ etc etc


Valve ( tappets ) adjustment are critical with marine diesels , Uber critical more than you think a significant amount of Hp can be lost if they a out of spec .
You say it’s been done but did you do it ? The KAD 300 necessitates ( I had a boat with them btw ) moving the fuel lines , fraught with potential rabbit holes and whole heaps of pain - and this can be off putting to some engineers out to make a quick buck !

Also constant running overloading even down at lower rpms say your cruise @ 3200/3400 means elevated EGTs which in tern have a disastrous chronic knock on effect of distorted ( often referred to a mushrooming ) the exhaust valves .
This means they never 100 - seal and you loose power .It’s insidious meaning you don’t notice in everyday use .
Until you try a WOT and find it can’t reach after eliminating the std mech reasons like filtration / turbos etc which by the sound of it you have .

Your boat should reach 1 to 1/2 % over rated rpm .
If it’s gotten lardy with cruising stores then as others have suggested re prop it go down a size or so ?

 

[About Time]

Valve tappets have been done by a mech every 200 hours and I have been present so guess these are fine. Also I change airfilter every season... max. 100 hours. Turbos are changed 200 hours ago on both engines so they should be fine. Guess I will clean the oilcooler, intercooler and change to racor 30 at the end of the season and see what happens. Thanks to all.