Johnson 70 3cyl overheating

[Lord Tywin]

My 89 J70TLCEM started overheating when run above 4,000 RPM. I replaced the impeller and it pumps very well thru the tell tale. I can run it all day at a slow cruise but when I go a little faster it overheats and goes into SLOW mode. I then replaced the temp sensor and it still does it. I'm thinking the next step is pulling the head cover. Is there anything else I should look into before going to that step? Any suggestions or recommendations for performing that step? I have a factory Service Manual to perform the operation.

 

[racerone]

The water coming out of the telltale ---DOES NOT ---circulate through the block !----It tells you that water pump is working.-----And in 1994 there was a service bulletin to relocate the telltale hose from side of exhaust to the top of the block.----Was yours done ??----Repeated overheating will damage the engine.------Step #1 is now a compression test.-----Post your numbers.

 

[oldboat1]

FYI: https://forums.iboats.com/threads/1...g-system-modification-additional-work.527266/

Good thread.

 

[Lord Tywin]

FYI: https://forums.iboats.com/threads/1...g-system-modification-additional-work.527266/

Good thread.

Fantastic reply @oldboat1! Thank you for this!

 

[racerone]

Was the tell tale moved on your motor ?

 

[Lord Tywin]

Tell tale still in factory location but the pump kit had the parts, just not the instructions on moving the port. I will give that a try. Thanks again @oldboat1

 

[racerone]

I think the factory kit has clear instruction to move the tell tale and change thermostat spring and thermostat cover.

 

[ct1762@gmail.com]

also possible it's a faulty pack or harness diode causing it to go into SLOW, but i'd verify it's actually overheating at speed.

 

[Lord Tywin]

I moved the tell tale per the bulletin to the top of the manifold and checked the temperature. It went into SLOW mode while the discharge temp was 123 F. I dont think the thermostat even opens until 140 F. I think I'm going to try it with the thermostat removed and if it does it again I'm going to disconnect the temp sensor and keep the thermometer in the tell tale discharge.
@oldboat1 @ct1762@gmail.com

 

[oldboat1]

Use an infrafred tester to check the head temp (top of head) when the motor goes into slow mode.

 

[Lord Tywin]

will be picking one of these up today and posting the results, thanks!

 

[Lord Tywin]

I got a laser infrared thermometer and ran the boat this evening. When SLOW mode engaged the top of the motor where the temp sensor is, read 142. I moved over to the side opposite the exhaust manifold, where I would assume the cylinders are closer to the exterior of the motor and it read 195 at the top cylinder.

 

[ct1762@gmail.com]

SLOW won't kick in at 195 degrees, unless the emissivity value is off on your temp gun. its supposed to be 202 degrees or so... some were slightly higher. the temp reading that matters is at the top of the head at a fast idle, NOT the thermostat housing. just forward of it, the actual head casting. if its really 195 degrees, you have an issue with the thermostat grommet leaking or faulty t stat.

 

[flyingscott]

I got a laser infrared thermometer and ran the boat this evening. When SLOW mode engaged the top of the motor where the temp sensor is, read 142. I moved over to the side opposite the exhaust manifold, where I would assume the cylinders are closer to the exterior of the motor and it read 195 at the top cylinder.

So is the motor being limited to around 2800 rpms?

 

[flyingscott]

edit

 

[Lord Tywin]

At the top of the motor just in from the t-stat it read around 142 when SLOW kicked in. Yes it drops to high 2,000's (tach not 100%, but close). I found in my service manual how to test the diode so I'm going to do that as well as remove the t-stat completely and see what it does. I really hope I can resolve this as it's just getting progressively worse.

Thanks for all the help, I really appreciate it.

 

[Lord Tywin]

this is where it read 142...

 

[Sea Rider]

That's a 32 year oldie, was run through the years in fresh or saltwater ? If installing new stats, new impeller or complete water pump, temp sensor and all the electric components are doing their homework right that motor has excessive crap already built inside the entire cooling system that does not allow the hot temp inside the motor to dissipate fast enough and triggrs the temp sensor's slow protection mode...

Happy Boating​

 

[Lord Tywin]

Boat's lived its entire life on the same lake. That was my fear that it had gunk built up inside. I really didnt want to start disassembling it because of the age...breaking bolts etc seems like it will be quite a chore. Since it's slowly getting worse, I'm crossing my fingers it's the diode. @Sea Rider New stats meaning stator? Everything else has been replaced from the pump thru the sensor.

 

[oldboat1]

Did you replace the impeller on your motor?