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1988 johnson gt 150 running hot

R

rd1

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need some help with this pleas

need some help with this please motor was running hot changed thermastats waterpump motor ran fine two trips wide open to idle maybe thirty min. run time each trip yesterday went fishing twenty min. run time ran hot horn would sound off anything above idle had to idle all the way back today checked thermastats for trash or sand all clean water pump tell tail looks good also fine mist from exaust circulation problem ? need to pull heads? thanks for any help. ROY
 
"Roy, is it actually running h

"Roy, is it actually running hot or possibly a bad sensor?

When the alarm went off did you even just stick your hand under the tell-tale stream?

Before you start craking the head open make sure it is actually overheating and not just a horn yelling at you for no reason..."
 
yes I did check both sensors t

yes I did check both sensors today. yesterday tell tail water warm not hot both heads to hot to hold my hand on them sensors check open to ground cold. horn should sound when wire is grounded?
 
Not to butt in Mr.Lamb but I h

Not to butt in Mr.Lamb but I have same motor and just recently had trouble. What i wanted to say is are you sure it is overheat alarm and not fuel restriction (dirty fuel filter or inlet screens) is there any discoloration of plugs or coating in heads (white/gray haze)that indicate overheat. also to check alarm slip piece of thin stiff wire under connector of tan wire that comes out of head.With key switch to run ground wire if alarm sounds horn is good also those sensors should be open to ground when cold.
 
"Roy, I'm not familiar wit

"Roy, I'm not familiar with these big OMC engines but have been giving it a good read over. There appears to be a valve co-located with your thermostat (pressure release valve).

It appears that it may have the same function as the poppet valves on Mercs. If so, it's purpose is to somewhat restrict the waterflow through the head at start-up etc and then when the heat/pressure increases past a certain point (operating temp or whatever) it will open and allow 2nd stage cooling of the head.

Have you checked these out? may have a seized spring/something corroded etc."
 
"The cooling system and tem re

"The cooling system and tem regulaton operates both by thermostat at low rpm, then by water pressure at high rpm.
To get is as smooth as possible and with as low consumption as possible the temp is about 65C at idle and up to around 2000RPM.
Then IF impeller and cooling system works as suposed to, the waterpressure will open the valve and by-pass the thermostat. This should REDUCE engine temp down to around 45C. Everyone running temp-gauges starts wondering - but it is as should be. If not - change impeller and check cooling system! Note: Running on muffs at 2000 will probably NOT drop the temp - not enough water. Has to be done at sea!
What I would reccomend in this cas is to drop the lower unit and check/replace the big rubber seal for the exhaust. If that is missing/dammaged exhaust will go into the water-pum when pressure /rpm get up."
 
ok i replaced the bypass valve

ok i replaced the bypass valves and gromets when i replaced the thermastats also as i said ihave replaced waterpump checked pump today all looks ok exhaust seal looks ok. i have been reading manual on line looks like a thing called a water deverter in water passage of head what i can understand it is arubber hose that gives the water a way to flow in the head they can swell and block flow any info.? thanks for help so far ROY
 
"There is nothing in the head,

"There is nothing in the head, but the deflectors in the cylinder water-jackets might block or disturb the waterflow. Heads must off, and You do not actually know until You have changed them. They can not be removed and re-inserted, but have to be replaced if removed. Pos7 on below pic.

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"
 
thanks for the info. and pic.

thanks for the info. and pic. I think i will pull the heads tomorrow and check it out its either the deflectors or corrosion trash etc. everything else checks out.thanks again ROY
 
"I'm having a very similar

"I'm having a very similar problem with my 1979 Johnson 150. I just had a new water pump installed and nothing changed. I've flushed the system, no change. I have on order two new Thermostat kits. Roy, I'd like to know how your problwm is finally resolved. Thanks, Sam"
 
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