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1989 Johnson 20hp thermostat / temp. switch question(s)

Jupiter Dinghy

Jupiter Dinghy

Regular Contributor
So I took the 20 HP (J20CRCEM) for a test ride after a carb rebuild and a water pump replacement (for the record this is a motor I bought recently and that spent the last 3 to 4 years unused, so i really don't much about its history).
In the barrel, I had a nice full stream coming out of the pee hole, and water coming out of the exhaust housing, so I thought it's cooling good, let's give it a run, because it needs to run badly.... (lots of that black gooey stuff coming out of it through the gear case hole, which is understandable after this long sitting period).
Got to the ramp, started right up, with water coming out the pee hole , so off I go to give it some full throttle action.
It ran flawlessly for about 2, maybe 3 minutes, and then started misfiring. I rolled the throttle down, and was able to keep it idling, but as soon as I would try opening the throttle some more it would start misfiring again.
I notices the water stream from the pee hole had become just a drip, but water was still coming out of the exhaust holes, however, it got much hotter to the touch. Very hot in fact.
So I keep idling along after making a U-turn to head back to the ramp, and after running idle a while, I notice the pee hole stream was back as it should be, and the water temp had come back down to warm, but not "hot".
Back at the ramp, I take the hood off, and touch the head: Burning hot. Couldn't keep a finger on it at all.
So I'm thinking the motor fooled me into thinking it was cooling fine in the test tank, but under load, it cools for only a couple of minutes and then starts overheating.... sounds like the thermostat right?
So back home I take the head cover off and sure enough the thermostat, could hardly be seen beneath the crusty stuff that covered it entirely. It's definitely done for.

First question: I see on the exploded view that there is also a "temperature switch" encased in the head. Is there a way to test it, or should I replace it along with the thermostat anyway? And if so how do I remove it from there? I don't want to pull on the wire as I think it will break...?

Second question: Can I start the motor with the cover and thermostat removed just to confirm that water is flowing fine to the thermostat housing?

Third question: On the pic below, you will see the 2 holes located in the front of the exhaust housing, and exposing the drive shaft to water.
Are these holes supposed to be open like that? Are they part of the cooling system? Thanks.



 
You ask:
"First question: I see on the exploded view that there is also a "temperature switch" encased in the head. Is there a way to test it, or should I replace it along with the thermostat anyway? And if so how do I remove it from there? I don't want to pull on the wire as I think it will break...?

Second question: Can I start the motor with the cover and thermostat removed just to confirm that water is flowing fine to the thermostat housing?

Third question: On the pic below, you will see the 2 holes located in the front of the exhaust housing, and exposing the drive shaft to water.
Are these holes supposed to be open like that? Are they part of the cooling system? Thanks."

1 - You need to remove the water plate from the cylinder head in order to remove that temperature switch. Test with ohm meter and heat.

2 - Yes, you can do that but it's a waste of time.

3 - Those holes are not entry holes, they are exit holes that prevent water from being forced upwards to the driveshaft splines and lower crankcase/crankshaft seal.

Summary: The water pump impeller has failed, taken a set due to sitting so long. Replace the water pump assembly... AND DO tighten/torque "all" of the power-head bolts. As it stands, those bolts have loosened and will allow water to eventually enter the cylinders if it hasn't already.

Is that engine equipped with a warning horn system? If so, does the horn beep when you turn the key to "ON"?
If there is no warning system, the temperature switch serves no purpose.

Getting that hot..... check all of the wiring over closely for melting, shorting out to the block.
 
Last edited:
Thanks Joe.
To clarify: I installed a brand new water pump/impeller and tested it before reassembly, working great, and I have removed the head cover plate (this is how I could see the encrusted thermostat), but I was just wondering if I was supposed to pry the temp. switch out by grabbing it on the sides or what, as it seems to be in there pretty tight.
I will go over the head bolts torque, thanks for that, as well as the exit holes explanation.
 
Joe: When you say #2 is a waste of time, wouldn't that tell me there is no restriction / blockage between the water tube and the head?...
 
Jupiter Dinghy said:
Joe: When you say #2 is a waste of time, wouldn't that tell me there is no restriction / blockage between the water tube and the head?...
Click to expand...

Yes, it would tell you that... it just would not tell you anything about the cooling capabilities of the water pump. Not something you would want to leave that way.
 
So started it up with the mufffs on and water running out the stat housing, no problem. (Dirty at first, then clear.)
I think it might be wise to replace the head gasket while I'm there with the cover already off, so I will.
Does the gasket need anything on before assembly? How about the bolts threads?
 
Lightly surface the head on a glass table with 22 grit paper swirling in a figure 8 pattern. Then torque the head down in at least three steps using a center out sequence.

9 10
5 6
1 2
4 3
8 7
 
kimcrwbr1 said:
Lightly surface the head on a glass table with 22 grit paper swirling in a figure 8 pattern. Then torque the head down in at least three steps using a center out sequence.
Click to expand...

Will do, thanks. I have a piece of marble left over from a counter top which will do just fine for that.
No need to coat the head bolts with anything?
 
If you put anything on the bolts use 15% less on torque value. Nothing os necessary you will likely never pull the head again if done properly.
 
Thanks to both. I'll just clean the threads on the wire brush wheel and torque to specs (16ft-pd).
Did OMC use different brands for the gasket? BRP is the brand right? (I'll be sure to check the package)
 
Circling back to my first question about the temp. switch, what is it doing exactly? Is it supposed to activate a beeper at a certain temp?
Cause I sure never heard anything, and I'm not sure if this motor is equipped with a sound alarm....?
 
It provides ground for the warning horn in the controls. With the key on short the wire going to the switch to ground and the horn should sound?
 
kimcrwbr1 said:
It provides ground for the warning horn in the controls. With the key on short the wire going to the switch to ground and the horn should sound?
Click to expand...
I can try that when I get home, but I have no battery attached to that thing (manual recoil start), so where does it get power to sound a horn?
 
One less to " worry about "------- But then what purpose does this wire serve ?-----Are we sure this wire " does nothing " as has been stated ?
 
racerone said:
One less to " worry about "------- But then what purpose does this wire serve ?-----Are we sure this wire " does nothing " as has been stated ?
Click to expand...
I have as many question marks in my head, and then some! lol
Unless it was installed on every motor at the factory (regardless of elec. start or manual) so they wouldn't have to cast 2 different heads (one with the housing for it and one without it?) but knowing that only the ones with elec. start could benefit from it?... Sounds strange though....
 
So I took the head off a little bit ago, and didn't see any trace of overheating anywhere, just some carbon build up on piston tops as expected, what is the best way to get them cleaned up (if necessary)?

Just one thing that got me intrigued... what are these rubber things stuck between the outer cylinder walls (see arrows)?



The top one can be seen better here:

 
These rubber things are dams / guides that direct water around the block for proper cooling.----They are not all the same and need to be in the proper place to work !----They are hose but water does not flow thru the hoses.
 
racerone said:
These rubber things are dams / guides that direct water around the block for proper cooling.----They are not all the same and need to be in the proper place to work !----They are hose but water does not flow thru the hoses.
Click to expand...

Thanks racer.... I was wondering why the one on the bottom right has a metal core inside the rubber and the other 2 don't?
Well I won't touch them then.
Should I scrape the carbon on piston tops or leave them alone?
 
You can do a decarb once you get it running. Get a can of seafoam spray and on the water bring it up to temp and at high idle spray the seafoam in the carb until it starts smoking real good and quickly shut it down. Let sit for 1/2 hour or so then start it and run at WOT for about 15 minutes.
 
Your engine came with the powerpack under the flywheel and is subject to damage in a overheat condition. If you have ignition problems and need to replace the PP do the CDI conversion. You can test the slow circuit by shorting the temp switch wire to the PP to ground while running.
 
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