Low flow of cooling water

[lnikl]

I am looking for your experience and ideas.
I have a 21 hp Westerbeke 3 cyl diesel (1983), freshwater cooling. Within the past few months, I have replaced the mixing elbow (6 months) and the impeller (3 months). I have low flow of cooling water coming out and as the engine warms up, I get steam. On the last trip out i had to dial back the speed as the engine temp was rising a little.
I pulled the end caps off of the heat exchanger to see if there was stuff plugging the flow. There is some scale on the tubules as can be seen in the attached photos. The other side of the heat exchanger looks about the same. I wonder if there is more scale in the middle. There were two bits of pencil zinc in one end and I pulled those out. That didn't help.
I find that when I first start the engine, I get ok flow (but only just ok). As the engine warms up and when I crank up the rpms, the flow seems to be less. When the engine is hot, even when I dial back the RPM, the flow doesn't seem to come back.
I am wondering if any of you have some ideas or experience with similar sort of problem. Should I try a phosphoric acid flush of the heat exchanger?

 

[Skipper]

I use a .17 caliber rifle cleaning rod and brush. The .22 is too big for mine but it may fit yours. -Caliper a tube.
Omit the center piece of the rod to shorten it.
Although I would also double check the impeller. Buy spares which you'll eventually need anyway, and test.
Also what type of raw pump do you have? Sherwood are notorious.
And check the intake thru hull and strainer for clogs.

 

[Stu Jackson]

Good advice.

Check the faceplate of the pump, if not smooth you'll get reduced flow. Also check the hoses, if old they could be restricted.

Analyze every individual component of the system, step-by-step, item by item.

 

[Johnb]

I have had problems with crud and scale that packed up into the outlet from the exhaust manifold and would get worse the longer the engine ran. I guess it dropped down when there was no flow and then the cycle would repeat at next start up.

The advise to go through the whole system is good. If you thoroughly restore the system you will get a maintenance holiday for that system. It also helps prevent way more costly problems.

I had my heat exchanger professionally cleaned/rebuilt for $170 and it was well worth it. For that they not only cleaned the bundle but re machined all the sealing surfaces.

 

[lnikl]

Good advice.

Check the faceplate of the pump, if not smooth you'll get reduced flow. Also check the hoses, if old they could be restricted.

Analyze every individual component of the system, step-by-step, item by item.

It was indeed the hose leading from the outlet of the heat exchanger to the mixing elbow. It was one size too small (no idea how the previous owner was able to attach it) and i suppose as it aged, it would sag when warm. I replaced it with the correct size of hose and the flow is now excellent.