70HP Johnson Overheating????Please Help.

[PanamaRanger]

I have a 70 HP Johnson about 1992 - 3 cylinder. It is a VRO but that is removed and I mix the oil. At mid throttle and idle it runs great, At wide open after about 15 or 20 seconds it starts missing and choking and the warning horn goes off. I think it getting hot and is going into the safety mode from the thermostat and that is what causes the choking and sputtering. It has a very good stream of water coming out from the tell tale pee hole. I have cleaned the water inlet screens. I think it is getting enough water to be cooling fine at medium speed but it has a problem and gets hot at high speed. Great water coming from the pee hole... Any suggestions please??

 

[Daviet]

Re: 70HP Johnson Overheating????Please Help.

Water out of the peehole only means that the water pump is pumping up to the block, not through the entire cooling system. Could be a thermostat or blocked cooling system. Might want to do a compression test, maybe compression is leaking past head gasket and pressuring cooling system at high speed. Just a thought.

 

[petryshyn]

Re: 70HP Johnson Overheating????Please Help.

if its the same as a 93, check the plastic housing behind the thermostat. It can get plugged up by scale....

 

[HybridMX6]

Re: 70HP Johnson Overheating????Please Help.

Agree, do a basic check of compression to rule out bad head gasket, and pull t-stat and check it out. It could be bad, sticking, or blocked.
Also, if you have a digital temp sensor, check the head and block at different areas to verify it is in fact overheating. If you get even #'s and it still sounds the horn and kicks you into SLOW mode, could be just a faulty temp sensor.
As for the "if it's the same as a '93", it's not. They changed the design from '92 to '93. I have very first hand experience with that, as I was told a motor I had was a 1993, when in fact it was a 1992. Made it very frustrating to figure out why the t-stat for the '93 would not fit, until I figured it out myself. The ID tag was missing from the motor.
If t-stat is all good, and compression test it also good, I would suggest dropping the lower unit, pulling the t-stat, and back flushing the system through the t-stat opening. Put a trash can or something under the midsection to catch the water and whatever else may come out of it, and see if anything comes out.
If it continues to overheat after these checks, your next option would be to pull the head and check the cooling passages around the cylinders for any sort of blockage or build up.

 

[PanamaRanger]

Re: 70HP Johnson Overheating????Please Help.

I pulled the thermostat out and put in a new one. It did not help. I pulled the thermostat out and started the motor for a few seconds with it out and the cover off. A LOT of water comes out of the thermostat housing. I put the cover back on without the thermostat. I ran the motor all over at different speeds without the thermostat installed and with the little black cover on and it runs just fine.....without the thermostat and spring installed. Now it is running great but I have not fixed the problem. I think I saw a little smoke come out when my wife started the engine with the thermostat out and spring out and the cover off....I THINK (not positive) ....it was hard to tell because the water started coming out so fast after a couple of seconds........ I may have a blown exhaust gasket - do you think??? Letting hot gas get into the water when running at higher speeds.....Even with thermostat in, it will run fine at about 3000 RPM...when you give it more and go to WOT it overheats in a few seconds.....It looks like there are two gaskets behind there.....maybe exhaust gaskets....Can the exhaust plate be removed and the gaskets replaced without pulling the powerhead....I think it is a 91...it has a white thermostat and a large spring that fit together into the housing. I wonder if I can do it while on the engine.