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Author Topic:   1991 Johnson 90-HP Overheating

Montauk77 posted 09-05-2014 04:29 PM ET (US)
My 1991 Johnson 90-HP outboard engine was over heating. Assuming it was the cheap Sierra thermostats I installed earlier in the season, I decided to remove them. I re-installed the housing without the thermostats, valves, and springs and gave the motor a flush with a vinegar-water solution. While flushing in the bin without thermostats, the engine ran fine, without overheating, and hot water exited above the water intake on the lower unit. This led me to believe my theory on the thermostats was correct. I re-installed new Johnson thermostats, springs, and poppet valves last week, and fired it up. No hot water ever exited the lower unit, and the overheating buzzer came on.
Thinking the overheating was being caused by faulty thermostats I removed them again, and tested them on the stove. All was good. Re-installed them again, and the engine is still overheating. What else could it be? The confidence steam outlet flow is strong, leading me to believe the water pump is okay. Thanks for any suggestions.
contender posted 09-05-2014 07:57 PM ET (US)
You say the water pump is okay, but when was the last time it was changed? If you change the water pump, buy the entire kit from OMC and DO NOT USE AN AFTERMARKET WATER PUMP. Just becaause you are getting a spray [from the nozzle of the confidence stream outlet] does not mean the pump is 100-percent. Make sure you have good water pressure from your hose and a good connection to the lower unit to get the water. Good Luck
Montauk77 posted 09-05-2014 10:39 PM ET (US)
Thanks Contender! All things I've considered.
-Ran the engine on the muffs and in a large bin of water since installing the thermostats, and had same result. The water pump was changed last year with OMC pieces, and probably only has 15 to 20-hour on it. Anything is possible here, but I'm betting this is probably not the cause of the overheating.
I'd remove the thermostats again and fire it back up, but I'm out of gaskets. Need to pick more up.
Is there anything north of the thermostats it could be?
seahorse posted 09-06-2014 12:25 AM ET (US)
Use a small triangular file and make two very small notches, about 1/32-inch-wide in the 12:00 and 6:00 position of each white pressure relief valve edges. That will allow the air to bleed out of the thermostat body and allow the cooling water to reach the thermostat elements. It will also help drain the thermostat cavity after motor shut-down and minimize corrosion buildup.
jimh posted 09-07-2014 06:17 AM ET (US)
Moved to REPAIRS/MODS.
Montauk77 posted 09-08-2014 09:26 PM ET (US)
Seahorse, if air were trapped, would the water still flow out of the stream off the engine?
bw1969 posted 09-14-2014 09:15 AM ET (US)
Try cutting off two coils of the poppet valve springs. I will bet if you run the motor at speed it will not overheat. The water just isn't circulating at idle
Montauk77 posted 09-14-2014 07:29 PM ET (US)
I removed the thermostats and poppet valves again today and fired it up. No [overheating]. I'm going to give [the cooling system] a good flush with SALT AWAY or vinegar-mix, and put it back together with the suggestions made here. Will let you know.
deepwater posted 09-15-2014 02:23 AM ET (US)
If you have any rubber hoses like I did in the rear of my 88 SP you may have an inner lining collapsing on one side of the engine power head.
Montauk77 posted 10-10-2014 11:23 AM ET (US)
I [aggressively] flushed [the 1991 Johnson 90-HP outboard that had been overheading] with SALT AWAY and vinegar without the thermostats in place, back flushed it with the garden hose, and re-assembled [the cooling system] with notched out pressure relief valves. All seems to be well. Took [the boat] out yesterday and ran [the engine] for about an hour; the boat hit 34-MPH. I am hopeful [the problem of overheating] is gone. Thanks for the help.

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Author	Topic: 1991 Johnson 90-HP Overheating
Montauk77	posted 09-05-2014 04:29 PM ET (US) My 1991 Johnson 90-HP outboard engine was over heating. Assuming it was the cheap Sierra thermostats I installed earlier in the season, I decided to remove them. I re-installed the housing without the thermostats, valves, and springs and gave the motor a flush with a vinegar-water solution. While flushing in the bin without thermostats, the engine ran fine, without overheating, and hot water exited above the water intake on the lower unit. This led me to believe my theory on the thermostats was correct. I re-installed new Johnson thermostats, springs, and poppet valves last week, and fired it up. No hot water ever exited the lower unit, and the overheating buzzer came on. Thinking the overheating was being caused by faulty thermostats I removed them again, and tested them on the stove. All was good. Re-installed them again, and the engine is still overheating. What else could it be? The confidence steam outlet flow is strong, leading me to believe the water pump is okay. Thanks for any suggestions.
contender	posted 09-05-2014 07:57 PM ET (US) You say the water pump is okay, but when was the last time it was changed? If you change the water pump, buy the entire kit from OMC and DO NOT USE AN AFTERMARKET WATER PUMP. Just becaause you are getting a spray [from the nozzle of the confidence stream outlet] does not mean the pump is 100-percent. Make sure you have good water pressure from your hose and a good connection to the lower unit to get the water. Good Luck
Montauk77	posted 09-05-2014 10:39 PM ET (US) Thanks Contender! All things I've considered. -Ran the engine on the muffs and in a large bin of water since installing the thermostats, and had same result. The water pump was changed last year with OMC pieces, and probably only has 15 to 20-hour on it. Anything is possible here, but I'm betting this is probably not the cause of the overheating. I'd remove the thermostats again and fire it back up, but I'm out of gaskets. Need to pick more up. Is there anything north of the thermostats it could be?
seahorse	posted 09-06-2014 12:25 AM ET (US) Use a small triangular file and make two very small notches, about 1/32-inch-wide in the 12:00 and 6:00 position of each white pressure relief valve edges. That will allow the air to bleed out of the thermostat body and allow the cooling water to reach the thermostat elements. It will also help drain the thermostat cavity after motor shut-down and minimize corrosion buildup.
jimh	posted 09-07-2014 06:17 AM ET (US) Moved to REPAIRS/MODS.
Montauk77	posted 09-08-2014 09:26 PM ET (US) Seahorse, if air were trapped, would the water still flow out of the stream off the engine?
bw1969	posted 09-14-2014 09:15 AM ET (US) Try cutting off two coils of the poppet valve springs. I will bet if you run the motor at speed it will not overheat. The water just isn't circulating at idle
Montauk77	posted 09-14-2014 07:29 PM ET (US) I removed the thermostats and poppet valves again today and fired it up. No [overheating]. I'm going to give [the cooling system] a good flush with SALT AWAY or vinegar-mix, and put it back together with the suggestions made here. Will let you know.
deepwater	posted 09-15-2014 02:23 AM ET (US) If you have any rubber hoses like I did in the rear of my 88 SP you may have an inner lining collapsing on one side of the engine power head.
Montauk77	posted 10-10-2014 11:23 AM ET (US) I [aggressively] flushed [the 1991 Johnson 90-HP outboard that had been overheading] with SALT AWAY and vinegar without the thermostats in place, back flushed it with the garden hose, and re-assembled [the cooling system] with notched out pressure relief valves. All seems to be well. Took [the boat] out yesterday and ran [the engine] for about an hour; the boat hit 34-MPH. I am hopeful [the problem of overheating] is gone. Thanks for the help.