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Dominick, Marine Mechanic
Category: Boat
Satisfied Customers: 1009
Experience:  25 Years Experience as Mechanic, 20 Yrs Boat Yard Owner/Mechanic, Factory Certified
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I have a 93 Johnson 120 looper ( J120 TLATF) The overheat

Customer Question

I have a 93 Johnson 120 looper ( J120 TLATF) The overheat warning goes off after i get it to around 4800-5000rpm I have replaced the following, the temp switches on both heads, both thermostats i also pulled the heads off and looked around and cleaned
and installed new head gaskets I also replace the complete water pump assy. So yesterday when the alarm went off and went in to slow I took temp readings on the heads and felt the stream, it was not hot at all, the heads were in the 155* +/- ranges. so i disconnected
the tan wires and took off and ran at WOT for a but and rechecked the stream and the heads, temps were fine..and stream was not too hot to keep my hand under So my question is, What am I looking for? could this be fuel or timing related?
Submitted: 1 year ago.
Category: Boat
Expert:  Tan replied 1 year ago.
Hello. My name is***** sounding alarm is overheat. Did you disconnect the sensor on both cylinder heads? The thermostats act as a pressure relief valve also. The thermostats have to lift in and out of the seat under pressure if they are sticking and don't move freely this will cause the engine to run hot or warmer. 155 is on the warm side for full throttle. It should be closer top 140. # ***** is the seat it can corrode and cause the thermostat to not move freely. The springs # ***** & 31 must be in place also.
Customer: replied 1 year ago.
so are you telling me the seat#38 needs to move freely in the head? I did replace the seat when I did the thermostat,
Expert:  Tan replied 1 year ago.
No the thermostat has to move freely in the seat. You eliminated that possibility by replacing the seat so this isn't the problem. Are the springs in place? The one under the t-stat and the one outside the t-stat?
Expert:  Tan replied 1 year ago.
Do this disconnect the temp switch on one head at a time. Lets see if we can narrow down to one side or the other. Now they used 2 different pressure relief and t-stats on these motors. Do you have the same as the diagram I posted?
Expert:  Tan replied 1 year ago.
The idle timing is 8*ATDC and wide open 17-19*
Customer: replied 1 year ago.
I used the exact parts that are in the diagram, When i get back to the house i will check the springs again but they should be in the correct locations
Expert:  Tan replied 1 year ago.
When you had the heads off how did the cooling passages look? When you replaced the water pump did you happen to look at the water tube for any holes? Make sure the t-stat moves freely in the seat. The flow is restricted or you aren't getting enough water. If the water tube has any holes this will introduce exhaust gases into the cooling system causing the temp to run hotter. Post back with findings.
Customer: replied 1 year ago.
good call on the water tube, I did not check it, I will check these items later on and report back
does the water tube just pull down from the power head? or should i be able to just feel around for holes
Expert:  Tan replied 1 year ago.
The tube is permanent. Feel the tube look as best you can. If you wait till dark you can stick a small flash light inside the tube. If you have holes you will see the light coming out through the holes. Keep me posted no rush there are no time limits.

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