Fuel Filter Reduce WOT?

Would a plugged secondary fuel filter make you lose 300 rpm from normal WOT?
I changed the Racor 75900 primary with no rpm change.

My limited experience with plugged filters is that the performance continues to degrade quickly. IOW, it gets worse, the lower rpm level doesn't remain constant. Certainly hurts nothing to change it and cost is pretty minimal.

When was the last time you cleaned or inspected the hull and running gear?

Ted

it could.

Pgitug said:

Would a plugged secondary fuel filter make you lose 300 rpm from normal WOT?
I changed the Racor 75900 primary with no rpm change.

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Also you can kick up some debri in the process of changing the primary that the secondary could have picked up.

I have seen several instances where reduced fuel flow reduced top rpm (in other people's engines).
I have vacuum gauges after my Racors and pressure gauge after the lift pumps. It makes it easy to tell when filters are getting dirty. Always with enough advance warning to plan the filter changes.
If you have dual primary filters that can be switched running, put a 2 micron in one and a 30 in the other. Run on the 2 until it starts to plug, switch to the 30. Do this a couple times and you'll have super clean fuel and tanks. I buy clean fuel and I run on 2 micron primaries all the time. Use biocides and conditioners.
I carry 2000 gallons and it only took one 2 micron change.

With Racor pressure gauges, what RPM should the engine be running when you check the gauge?

hmason said:

With Racor pressure gauges, what RPM should the engine be running when you check the gauge?

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I like the gauges that have have drag indicators. They will show the highest vacuum that you have achieved. That way you can check the gauge with the engine off before or after to have run the engine.

"and you'll have super clean fuel and tanks"

Clean fuel will not strip decades of grunge from inside a fuel tank , unless the boat spends much of its time in quite rough conditions .

hmason said:

With Racor pressure gauges, what RPM should the engine be running when you check the gauge?

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WOT should be 2800 but all I get is 2450.

Pgitug said:

WOT should be 2800 but all I get is 2450.

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In addition to above suggestions, a few more preliminary things to consider:

1. Is this a new happening?
2. If 1 is yes, what did you do "last" in the way of maintenance or just messing around?
3. When was last full RPM under load achieved?
4. If 1 and 2 are yes, retrace steps in 2.
5. Have you verified RPM with a photo tach?
6. Yes, buy a Racor vacuum gauge and install it.

You could also have a fouled/damaged propellor, or the wrong pitch propellor if it has been this way since you owned the boat.

Sunchaser asks all the right questions. If this is a new phenomenon then look to something like a fuel filter. If you are just noticing now, then it could be a bunch of things including being overpropped by a bit. FWIW, I don't hit max rpm either, I am shy by about 200rpm. This was done at the sea trial. Not sure why, I could be over propped slightly.

dhays said:

Sunchaser asks all the right questions. If this is a new phenomenon then look to something like a fuel filter. If you are just noticing now, then it could be a bunch of things including being overpropped by a bit. FWIW, I don't hit max rpm either, I am shy by about 200rpm. This was done at the sea trial. Not sure why, I could be over propped slightly.

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At the sea trial we hit 2725. So I know that a proper pitched prop would read 2850. But now I am at 2400. A few weeks ago I was 2450. So I am guessing it's the secondary fuel filter since I changed the primary Racors.
Today I did change the secondary and flushed a few larger dirt particles out of the old filter. So next time out I will see if we have gained some ground.

Well I can tell you that when I had my ex boat with the new Cummins 6BTA, I once had a reduced rpm situation due to some filter clogging.
It was a very subtle rpm reduction, but non the less due to a filter that needed changing. (which I did later that day after we anchored up for the night).
So that certainly CAN be a cause.
I did not have a vacuum gage on that system so can't help you out with a number.

Photo tach readings would be useful?

Pgitug said:

At the sea trial we hit 2725. So I know that a proper pitched prop would read 2850. But now I am at 2400. A few weeks ago I was 2450. So I am guessing it's the secondary fuel filter since I changed the primary Racors.
Today I did change the secondary and flushed a few larger dirt particles out of the old filter. So next time out I will see if we have gained some ground.

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Let us know what you find out. Also, if you don't already get a vacuum gauge on your Racors.

I have a gauge between my Racor 75900 pair. It has a drag needle that has never gotten to the red.

Pgitug said:

I have a gauge between my Racor 75900 pair. It has a drag needle that has never gotten to the red.

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Great. I would look at replacing the filters though if the gauge ever gets to about 7mmHg.

Pgitug said:

I have a gauge between my Racor 75900 pair. It has a drag needle that has never gotten to the red.

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You may want to talk to a Cummins dealer regarding the safe limit as the gauge warning has no relationship to the vacuum pump limits. For my "C" series Cummins my dealer said the Cummins lift pump had a significantly lower vacuum tolerance before the pump would fail. Don't remember the number, but once I see the needle move on a full tank of fuel, it's time to change the cartridge. It's changed annually regardless. Compared to fuel, the cartridge costs nothing (<$10). The "B" series is more vacuum tolerant, but it would be wise to know Cummin's limit.

Ted

The one time I couldn't reach WOT, the first thing I checked was the Racor. Switching to the second Racor resulted in immediate solution.

Pgitug said:

Would a plugged secondary fuel filter make you lose 300 rpm from normal WOT?
I changed the Racor 75900 primary with no rpm change.

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It most certainly could.
Remove the secondary and cut it open, or just bang it on a paper towel upside down to see what kind of crud is in it.
If it is dirty, you need to improve your primary filtration. The secondary is a "last chance" filter that ideally will never see any high degree of contamination.
Seaboard Marine's website has articles you need to read, also some very good information on boatdiesel.com.

I replaced my Racor primary fuel filters and still was 300 rpm from WOT. I then changed out my on engine secondary fuel filter and got back the 300 rpm I had lost. ??

Pgitug said:

I replaced my Racor primary fuel filters and still was 300 rpm from WOT. I then changed out my on engine secondary fuel filter and got back the 300 rpm I had lost. ??

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Pgitug,
You missed my point in previous message.
If there is significant debris in the secondary filter, your primary filtration is not up to the task.

kapnd said:

Pgitug,

You missed my point in previous message.

If there is significant debris in the secondary filter, your primary filtration is not up to the task.

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Not necessarily. I don't recall if Pgitug mentioned it, but how many times were the primary filters changed without the secondary being changed? We know from his earlier post that it was at least once. It is very possible to pass debri to the secondary in the process of changing the primaries. If this is done several times, it could add up to something significant. Plus, the specs for his engines if I recall, call for 10micron primary and 2 micron secondary. Again, I could be wrong on that. This would mean that even if the primaries are doing their job very well, that eventually the secondary could start to get plugged.

It sounds to me that the secondary was simply not changed often enough, but then I tend to change filters "too often".

boatpoker said:

it could.

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Of course it could. Any restriction in the fuel supply could reduce WOT RPM. Of course other things can reduce it as well but replacing the filter is cheap and easy and is probably in the engine's maintenance schedule anyway.

O C Diver said:

...

When was the last time you cleaned or inspected the hull and running gear?

Ted

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Can't see how a fouled hull can affect engine speed. Boat speed yes, but not engine RPM.

markpierce said:

Can't see how a fouled hull can affect engine speed. Boat speed yes, but not engine RPM.

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A dirty hull is harder to push through the water, obviously. That in turn can create an overpropped condition, whereby the drag is just too great for the motor/prop combo, and RPM's can drop.... Don't ask me how I know... Ha.

David Hughes said:

A dirty hull is harder to push through the water, obviously. That in turn can create an overpropped condition, whereby the drag is just too great for the motor/prop combo, and RPM's can drop.... Don't ask me how I know... Ha.

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This is true and the warning is in my boat's owner's manual.

My diesel engine doesn't slow when there is an increased load such as headwinds and waves. The engine controls operate a governor, and the governor controls fuel flow while maintaining a constant RPM while boat speed may vary. Gasoline engines don't act like this.

markpierce said:

My diesel engine doesn't slow when there is an increased load such as headwinds and waves. The engine controls operate a governor, and the governor controls fuel flow while maintaining a constant RPM while boat speed may vary. Gasoline engines don't act like this.

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But it will slow when your engine can no longer make additional torque at a given rpm. That's what happens when dirty running gear prevents an engine from reaching rated rpms- at that rpm, the total load on the engine from drag (air and water) cannot be overcome by thrust from the propeller.