overheating

[GLEN DEUTSCH]

I started this earlier, but it got lost somehow. I have three Johnson’s that are running hot. 1956 15hp, 1962 18 hp, 1963 18 hp. All pump fantastic water,but after a few minutes, it comes out boiling hot. Impeller replaced, thrmostats test good. Heads removed. All pasages 100% clean. Exhast plates removed. 100% clean. No dirt anywhere. How can a motor run hot yet pump gallons of water. Not a nubie. Been doing this for ten years, Frustrating

[fleetwin]

Hmmm, three different engines doing the exact same thing…. You mention that the water coming out is very hot, but have you measured powerhead temperature?

[Matthew Pieklik]

OEM impellers? Had a sierra impeller in a merc 110. Wouldn’t pump water. Put an oem impeller in and worked fine. on the other hand , I have used sierra impellers in older Johnson Evinrudes with no problem. Maybe a bad run of impellers.

[GLEN DEUTSCH]

Thanks,fleetwin and Matthew. Powerhead temp is 190 +. Another thing. I don;t have a parts diagram for 1956 but the 1957 model has that “cover plate” under the powerhead gasket. There wasn’t any on my 1956. I had one laying around and I tried to put it on but it would not fit. Shift lever hit it.Couldn’t be impeller because it thows a “fire hose” of water

[fleetwin]

OK, seems like too much of a coincidence…. Seems like these engines have something in common causing the powerhead to run at 190 degrees F…. Plenty of water discharge, so sure doesn’t seem like a pumping issue. Almost seems like the water is short circuiting somewhere bypassing the powerheads.
How about pulling off one of the thermostat covers and running the engine to see how much water is coming out….
Can you think of any service procedures you did on all three of these engines? Perhaps an incorrect base gasket or something like that….
Did these engines ever cool correctly? If so, when did that change?
I know you mention that plenty of water is coming out of the discharge, but did you use OEM impellers?
Again, the symptoms you post makes it seem as though water is somehow bypassing the powerheads and dumping out of the exhaust reliefs? Water tubes, bent/split open?
Are you running the engines in a barrel or on the boat in the water? Sometimes, test tanks are not our friends….

[GLEN DEUTSCH]

Thanks fleetwine. Gives me somethingnto work on

[GLEN DEUTSCH]

Eureka! I’m not running through the streets naked, but I feel like it. Something fleetwin said. All three motors have the same problem. Common denominater is me. Got to thinking about where the heat is comming from. Fuel burning and friction. In my desire to get the last bit of RPM out of these larger engines, I leaned them out to the point that the friction heat was more than the water pump could handle.The water pump is basically the same as a 10hp. Allowed more fuel and the overheat is gone.

[fleetwin]

Boy, you must have had em dialed in just perfectly! Glad all is OK now….

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