130hp Johnson Overheating...

[scottdNH]

Hi everyone,
I have a 1996-98? 130hp Johnson outboard. Last season I purchased it to repower my 21ft Trophy after the Force it came with blew up. It was used and with that I expected some issues, had a fuel pump issue which I got resolved, but I have had an over heating problem which 2 mechanics havent been able to resolve yet. Last season the fix was to just pull the thermostats out and let the water run freely to cool the motor, and that worked fine. Talking to another mechanic we planned on rebuilding the water pump this spring and replace the thermostats. I had that done about 2 weeks ago and got the boat on the water for the first time this past weekend. Water pump seemed to be working fine, water was spitting out freely....but I noticed it was very warm. I started motoring down the lake and opened the motor up to give it a good run, within about 1 minute the overheating alarm went off and I quickly shut it down. I again pulled the thermostats out, while on the water, started it up and the water flowed out very cold and the boat worked great the rest of the day.

Any idea why these Thermostats arent openeing and doing their job? Its basically a rubber seal that should open up and let water cool the motor but it isnt working. This is the 2nd set of Thermostats....and the water pump is rebuilt. So I have no idea what it could be.

I know ive been told by multiple people just leaving the thermostats out and letting the water run freely is fine, will cause no damage and do the job...but I would rather the thing work correctly.

Thanks in advance for any advice.
-Scott

 

[Auxlarry]

Re: 130hp Johnson Overheating...

Actually Scott, the thermostats are required to allow the engine to perform properly. Running too cold does not allow the cylinder walls to expand out and the rings won't seat properly. Quick wear and high fuel usage will follow.
I had the same belief until some on here convinced me to reinstall my t-stats, I have noticed a great difference in the way my engine performs and have seen much better looking plugs after that.

If you can get an Infrared thermometer to check the head temps it would be great but if not you can use the thumb method of checking. Start the engine (water cool) and allow it to heat up, if you can hold your thumb on the outside of the head next to the plug for a few seconds before it gets uncomfortable you should be around 145 degrees. If it burns you immediatly your overheating. Each cylinder should be the same or close.
It could be you have a bad heat sensor in one of the heads. I fought this problem once and finally pulled both the heat sensors out and placed them in cool water while attached to an ohm meter. Slowly brought the heat up and had one close at about 110 degrees the other was somewhere around 160. Replaced both of them and problem solved.

Larry