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1981 Johnson 90Hp overheat problem

Keithm18

Regular Contributor
Dear Sirs.
I have a 1981 Johnson 90 Hp fitted to an 18ft Bowrider. I have only had the boat on the water once since I bought it and after only about 5 minutes on the water at around 80-90 throttle I received an overheat buzzer. I shut the motor down let it cool, checked the water intake. All clear and tried again, no surprise I got the same result.

I have since tried to run on muffs (mostly at idle) without being able to replicate overheat. I have heard that running on muffs can compensate for a weak water pump and there is definitely a difference in the tell tale stream when running in a barrel vs on muffs. I also noticed a small water drip from each head during running.

I have ordered the new complete water pump as I don't know what condition it is in and am awaiting it's arrival. I also did a compression check I get 108 psi on the two starboard cylinders and 112 psi on the port side. From what I understand that difference is acceptable

I also torqued the head properly today (Some of the bolts were way too loose, which seems to have sorted most of the drip from the bottom of the heads.

I somehow don't believe that the new water pump will cure my woes and am reading in this and many other forums that the cause of the overheat could be the Thermostats and in one I read that it could be due to one of the motor running too lean due to a weak fuel pump. I am boating on a very limited budget and would like to know if there is someway that I can find out what the actual problem is without systematically replacing part by part that may not be faulty.

I am also based in South Africa and Johnson parts only come through one channel which is very pricey to say the least.

Any help would be greatly appreciated.
 
Overheat usually results in loose head bolts.-------At this time it would be wise to remove both cylinderheads and replace the head gaskets.----Replace the 4 rubber water deflectors ( guides ) while the heads are off.-----------Make sure heads are flat and re-install , torque to 240 inch lbs.---Install a fuel pump repair kit too, very easy to do as well.
 
Thank you for the quick reply Racerone.
Head gaskets are on the way. I will get the Deflectors and fuel pump kit as well. Can you tell me what the fuel pump pressure should be?
 
Agreed I see two on the exploded diagrams. Got the part numbers but seems I have to by the parts separately. There doesn't seem to be a kit (Thermo's, gaskets, springs etc) with all of the parts I need for this motor.
 
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