Exhaust water volume discharge -Perkins 4108

[windsmaker]

Does the volume of exhaust water going overboard with a Perkins 4108, change with increased rpm?
I expect that at higher rpm, there would have increased water exhaust volume.
However, I am seeing on my perkins 4108 that at low rpm, (1K) I have higher water exhaust volume than at higher ( 2K) rpm.
The engine runs cooler at the low rpm. The exhaust water volume at low rpm is good so the raw water pump is working ok.
Why would the water volume decrease with increased rpm's
Any ideas what can cause reduced volume at higher rpm ? (..Clog in the exhaust?)

[nhschneider]

It doesn't sound like a clog to me but I suppose it could be. I did see one situation where the inner lining of an exhaust hose had separated and when the exhaust volume increased, it would restrict the exhaust flow to the point where the engine stalled.

I presume you've checked the impeller and belt tension (if it's a belt-driven raw water pump).

Fair winds and calm seas.

[windsmaker]

hi,
I checked the lines, and they appear clear..
The water volume being discharge typically pulsed out with the exhaust. At engine idle at say 800 rmp, the volume is "about" 1 gal every 2 minute. Is this about right?

Any one know what this discharge water volume should be for different rpm's for the
Perkins 4108?
if we can check the discharge volume we can determine if there is a clog or the raw impeller is failing.

I'll check the discharge and report when I get down to the boat this weekend.
thanks

[Lepke]

Whoever made your raw pump should have rpm/volume/pressure curves.
Some engines have a valve to bypass some of the water directly overboard. You only need enough water to help muffle and cool the exhaust. My mains are setup this way. Too much water in the exhaust will create excessive back pressure and hurt economy.
A pump with a worn impeller can't push as well against increasing back pressure.
My 4108 is a generator engine and above the waterline. At idle, the water is more noticeable than @1800 rpm when it's more spray. You could have a marine buildup somewhere that still allows enough water thru at low rpms.

[Muckle Flugga]

Originally Posted by windsmaker

hi,
I checked the lines, and they appear clear..
The water volume being discharge typically pulsed out with the exhaust. At engine idle at say 800 rmp, the volume is "about" 1 gal every 2 minute. Is this about right?

Any one know what this discharge water volume should be for different rpm's for the
Perkins 4108?
if we can check the discharge volume we can determine if there is a clog or the raw impeller is failing.

I'll check the discharge and report when I get down to the boat this weekend.
thanks

It absolutely does change with rev increase in a positive correlation. What is going on? My guess is that your raw water impeller is cracked at the bases of several vanes, and as you increase revs this causes the vanes to bend back too far and become out of contact with the squeezing asymmetric crescent surface in the pump housing (the cam), which is making it LESS efficient as it increases in rotational speed.