Water Flow Diagram for Johnson 6 hp

1968 6hp Johnson CD-25A cooling system water flows

I could not find a water flow diagram that really shows the water as it flows through the engine, other than an undetailed drawing in the service manual (see it below), therefore I decided to post here the water flows as I saw it from disassembling the cylinder head and the thermostat cover. I'd like for others more experienced than I to confirm if I am correct.

From what I could see, I surmise that the water

1) Continuously comes in through the main water flow shown below on the engine block, then goes into the thermostat housing hole at the top then down the tube hole on the other end of the thermostat housing and out the exhaust. There is a restriction in the thermostat housing cap, so that the other continuous flow hole #2 below, receives more water pressure.

2) Water also continuously flows to the cylinders through the engine block water outlet at 7 o’clock on the top cylinder. That water fills up the cylinder and can only enter the cylinder head by the 3 small holes and the one bigger hole at 2:30 o’clock on the top cylinder. The large rectangular exhaust hole at 6 o’clock of the bottom cylinder, must have a restriction so that the cylinders do fill up with water before exiting. The big exhaust hole at 5 o’clock of the bottom cylinder is for the cylinder head water to exit to exhaust.

3) The thermostat housing cup that houses the thermostat, has two holes that allow additional water to exit and cool the engine, when the thermostat opens. Until the thermostat opens, the cup fills with water from below from the water coming from the water outlet at 7 o’clock of the top cylinder. When the water heats up, the thermostat opens. (I surmise it is this way, because the thermostat mechanism is below in the cup, and not above it, so there has to be water below to expand the thermostat). Once the thermostat opens, it increases the flow of water to cool the engine. (QUESTION – why would the water not flow down when the thermostat opens, rather than flow out the top and into the tube? ANSWER - According to the service manual diagram, it flows up and out to the exhaust)

Last picture:

That dam on the cover circled in green is there to restrict the flow of water going continuously to exhaust and to increase the amount of water flow that comes from the the water outlet in the 7 o’clock hole of the top cylinder.

Here is the Service Manual water flow diagram:

Here's an update on the cylinder block water flows, the small hole between the two cylinders on the right, that I said might be a drain hole, is likely a circulation hole for water to pass between the cylinders, here's the updated picture:

Crud collected in the thermostat cove because the motor was not taken care of properly. Put it all back together properly with a new thermostat. Lightly surface the head on a glass table with 220 grit in a figure 8 pattern until it all shines the same. Do not use sealer on the head gasket just pull it down slow in at least three passes in a center out pattern to spec. I use permatex aviation gasket maker on all other gaskets just brush a light coat on both sides of the gasket. Do not try and torque the 1/4 20 bolts to spec nice and snug is all thats necessary. If you would have run the motor in a barrel with saltaway in it the head and thermostat cover would have been alot cleaner. Flush the motor after each use with tap water until the head gets hot then disconnect the fuel line and let it run out of gas. Even fresh water has alot of silt and minerals in it that builds up over time if you do not properly flush the motor. Leave the motor in the running position to allow it to fully drain after each use

kimcrwbr1 said:

Crud collected in the thermostat cove because the motor was not taken care of properly.

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You ain't kidding. Though the crud on the thermostat cove must have been some crud likely leftover inside the tube, since I had removed the head and scraped out all the water passages spotless. The passages looked like sheet rock, totally clogged, never sen a working engine that bad. Next time when I work on an engine this clogged, after cleaning it all up, I'll hook a garden hose to the engine inlet tube and flush it out for a while.

The reason why I made this thread was so others could see where to fully clean, as I did not know that water came in through the cylinder block at the 7 o'clock mark on the top cylinder. When you clean these deeper recesses of the engines, one is not sure if they reached the back. In the case of this engine the recesses on either side of the top cylinder are identical except the one at 7 o'clock has a water hole at the far back and the one at 5 o'clock does not. One has to scrape the back till the aluminum shines to know if there is or is not a water outlet hole there and it is not so easy to see when a small water hole is covered in "sheet rock"

Thanks for all the information.

Water flow diagram for Johnson outboard 6hp 6 hp
Water flow diagram for Johnson outboard 6 hp
Cooling system diagram for 1968 Johnson 6hp 6 hp

Does the thermostat need to open before you see water come out of the pee hole?

Nothing to do with the thermostat.----Water comes out the exhaust relief about 3 seconds after motor starts.------Thermostats are often stuck open or missing on these motors.

racerone said:

Nothing to do with the thermostat.----Water comes out the exhaust relief about 3 seconds after motor starts.------Thermostats are often stuck open or missing on these motors.

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Okay, thanks for the information. I’m helping a friend with this motor. I’ve never worked on a 6hp, my evenrude 15 has a pee tube where you can see the water come out of. If the water on the 6hp comes out of the exhaust then you can’t see. Correct. When I started the motor (in a barrel) I couldn’t see any water coming out and it concerned me so I turned it off. If I put in a new thermostat and replace the water pump would I see water coming out.

Thanks for all the info. It gives a lot of good information and a good starting point.

At the back of the exhaust housing ( above the waterline ) is the exhaust relief.----On a good working motor you can clearly see the water pouring out of there at idle.

I currently have two 6hp Evinrudes. A ‘76, (no pee tube), & a ‘77, (with pee tube). Both of them, along with the previous half dozen 6hp models I’ve had spit copious amounts of water out the exhaust relief even at idle.
Here‘S what it looks like. This is a ‘74 model