curious about cleaning the cooling system on an outboard

I recently aquired a boat ('59 P14) that was "in great shape" he said. I'm in the process of replacing the rotted transom and rotted portion of the floor. I was able to get both motors ('99 Johnson 25 and '88 Evinrude 9.9) running, but the 9.9 kicker wouldn't pee. Tried to remove the lower unit to change the impeller, which was when I found that some idiot JB Welded the 6 bolts in place and I snapped 4. Anyway, got that fixed and replaced the water pump in its entirety and am ready to test fire it again. Been watching videos about tearing the cooling system apart to clean it and got to wondering:
What would happen if I started the motor in a bucket of water and added a coolant system flush, like say a Prestone used in cars, and let that circulate through the system? Even wonder about CLR, assuming it's safe for rubber products like the impellar.
Anyone ever try it?

Whatever juice you decide to use make sure the juice level is above the water pump before starting. CLR is pretty dilute...just flush with lots of water if not in operation soon.

NRWAB said:

I recently aquired a boat ('59 P14) that was "in great shape" he said. I'm in the process of replacing the rotted transom and rotted portion of the floor. I was able to get both motors ('99 Johnson 25 and '88 Evinrude 9.9) running, but the 9.9 kicker wouldn't pee. Tried to remove the lower unit to change the impeller, which was when I found that some idiot JB Welded the 6 bolts in place and I snapped 4. Anyway, got that fixed and replaced the water pump in its entirety and am ready to test fire it again. Been watching videos about tearing the cooling system apart to clean it and got to wondering:
What would happen if I started the motor in a bucket of water and added a coolant system flush, like say a Prestone used in cars, and let that circulate through the system? Even wonder about CLR, assuming it's safe for rubber products like the impellar.
Anyone ever try it?

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Honestly, that sounds like a waste of time and a needless way to pollute (unless of course you are going to be disposing of the stuff with care).

Instead I'd focus on the items that really impact water flow:
1) impeller (done)
2) Thermostat (I'd suggest you change)
...items 3 and beyond are really for those who don't want to let sleeping dogs lie
3) take powerhead off and inspect the upper grommet at top of water pipe (I would only do this if my water flow was lacking)
4) remove port side cover of powerhead and inspect/clean water passage-ways (can recall where this cover is on the 9.9 but I'm sure it's on port side for your 25)
5) Take head off, inspect water ways, etc, and re-install (I'd only do this if I had lower compression or water restrictions that couldn't be solved otherwise

Side note: for 3 and beyond beware of seized bolts. On my 1993 30hp, I don't know what the heck was done to that motor (we are a very long way from salt water) but 1/2 the bolts broke even with I was super patient with them. Like I said, sometimes letting the sleeping dog lie isn't such a bad thing.

bigtime_mcalpine said:

3) take powerhead off and inspect the upper grommet at top of water pipe

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This is a very common problem on the 9.9/15HP engines. I have a '79 9.9 and was chasing the same low flow conditions as the OP. When I removed the powerhead I found the grommet completely deformed and blocking 99% of the water passage.