Cool Telltale with Steamy Exhaust Relief - 1994 Johnson 20HP

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renn90

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Typical for this motor?

Just sorting out my recently purchased 1994 Johnson 20 HP. the water coming out of the telltale is cool to warm and the water/exhaust coming out of the exhaust relief is very hot (port at the rear-top of midsection, above the water line). It is too hot to touch for more than 5 seconds or so. To memory, all my other outboards were approximately the same temp from each exit point. (Ran today in a cold water PA river).

The power head feels only warm to the touch, except for one spot (top of head, on the port side). The hot spot won't burn a hand but it is pretty hot.

I had the head cover off to change the t-stat, and all passages were wide open. The head gasket appears to be fairly new (blue color, and clean).
 
Wouldn’t hurt to put a new impeller in. I had a similar motor once that seemed to pump water fine but it was hot and steamy. Part of the impeller had broken off and was partially restricting flow.


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Good point Weldor,

I should have stated that the impeller was changed (not new - used a decent one out of my parts motor). Will be changing all wear parts with new after I sort everything. Better to get all parts needed on one order to keep shipping costs under control.

Mostly wondering if steam out of the exhaust relief, and a hot spot on the head is to be expected. I'm going to check temps at various powerhead locations with my thermal imager this weekend.
 
Your engine operates on a thermostat at idle and just off-idle.
It should be running around 135f give or take. That temp means that you can place your hand on the top of the cylinder head for about 5-6 seconds before it becomes uncomfortable. When on plane the engine is by-passing the thermostat and that water will be a bit cooler. See for yourself.
The overboard indicator is just that....it indicates that your pump is pumping. It is not dependant on the thermostat for flow therefore it will be pumping cooler water. The water you are feeling from the exhaust relief is after the thermostat hence the difference. In fact the water you are feeling is after passing through the water jacket on the rear of the cylinder head. Should be nice and warm!
Change a pump if you have had it in there for 3 years or longer but I wouldn't change it because of what you are seeing.
 
Pappy said:
the water you are feeling is after passing through the water jacket on the rear of the cylinder head. Should be nice and warm!

Thanks Pappy,

What so you think about the steam? Haven't had that on any outboard.
I looked up the running temps from a manual and will check them if i can get on the river this weekend. I'm thinking at worst there is an area that is running a bit hot (at the end of the water circulation circuit). If the temp at the head exceeds 160 dF at 5000 RPM, I'll remove the head and check the diverters and jacket condition.
 
Normally, in our Southern waters, the temp drops to around 100f or so at that RPM. This is measured on the top of the cylinder head. You can find hot spots around the spark plug areas but those areas are not indicative of true running temp. After a hard run it is fairly normal to see some steam for a short time. The exhaust housing gets quite warm from the hot exhaust gases but will cool fairly quickly. Steam much more evident in colder waters.
 
Resolved:
The issue was due to an under-performing water pump assembly. Water volume was not sufficient.

This is what happened:
* I had used what appeared to be a workable, but not very good, impeller from my parts motor, soon after acquiring the J 20. Wasn't too careful sealing the pump housing as the plan is to do a full service and restore, after sorting things out. (Not such a good idea). The water volumed seemed okay.
* Checked the motor with a thermal imager, at all RPM ranges on the river.
* Was getting temps on and around the head averaging in the high around 140s F. One spot was reading 159 F. (According to my manual 160 represents overheating).
* Ran the motor in a tank, and clamped the telltale hose off; and this caused the motor to run a bit cooler.
* Replaced all water pump parts and followed installation requirements to a tee.
* Re-checked the motor with a thermal imager, at all RPM ranges on the river.
* Now areas on and around the head average around 132 F, with the hottest spot not exceeding 152F.
* The water coming out the exhaust relief port is only warm at full throttle, but still hot at idle.
* The tell tale water never goes above luke-warm temperature.
* Running within specification, and is a very nice motor. Now onto cosmetic restoration.
 

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