​1989 Johnson 70 Possible Overheating Issue under specific RPM ranges

[sumocomputers]

EDIT: Changed Temp Sensor to Thermostat.

I have a 1989 Johnson 70, and it has a very specific overheating issue at certain RPM ranges. I don't have a very good understanding of how outboards work, so forgive me if I am missing something obvious. I am really looking to see if what I am observing is normal, and if not, what type of troubleshooting list I might go through.

Here is the summary:

When running at around 5 MPH (No Wake Speed) for a few minutes in the San Diego Bay, which is around 1500-1800 RPM, the newly installed Sierra temp gauge/sender will stay steady at about 210F. This seems very high.

The sender unit is the type that just threads into the engine block, and was bought with the temp gauge, with a new sensor wire installed.

When just idling, which is around 800-1000 RPM, the temp will stay steady at about 180F. This seems kind of high, but not too bad.

When anywhere above 2000 RPM, the temp drops to somewhere between 120-150F, even WOT. This seems normal.

I have a new warning buzzer installed, which does work when grounding the factory temp sensor tan wire to ground, though I have not tested the sensor itself, as it seems a little bit involved. The buzzer never goes off when the temp gauge shows 210F at the engine block. I suppose this could be because the engine block temp is probably not the same as the factory one that is internal (water jacket?).

I did recently disable the VRO2 system and now pre-mix at 50:1, but had this same problem before I did this, so I don't think it is related to lubrication.

Based on the little research I have done, it seems like might be related to the impeller, water pump, or thermostat, though I am not sure what does what. I do know the tell tale produces a strong stream at any RPM, and I am pretty sure the previous owner had a new impeller installed recently (I have an email out to him confirming).

Is this temp behavior normal? If not, could someone help me begin troubleshooting with a short list of likely candidates? I do have a Factory Service Manual, and consider myself mechanically inclined, and have worked on cars in years past.

Thanks,

Chris

 

[racerone]

Does the hose for the telltale come off the side of the block ---Does it come off the top ?

 

[sumocomputers]

Does the hose for the telltale come off the side of the block ---Does it come off the top ?

Comes off the side, looks like it has never been converted to come off the top (I just became aware of that conversion). Is it a hard conversion to do myself if needed?

 

[emdsapmgr]

Some facts: your overheats (hot horn) when the powerhead temps reach 212 degrees. The hot horn should come on. The hot horn will silence when the temps get back down to 175 degrees. That's still too hot. Your engine's cooling water goes thru the stat (only) when idling and at low rpm's. At normal idle and at slow speeds, the powerhead temps should be in the 143-155 range. Don't continue running it at the temps you are experiencing... If the stat is stuck shut, you will have no cooling water at low rpm's-(even with a good impeller.) That's the 200 figure. When you get the boat on plane, the extra water flow coming up from the increased water pressure at the lower unit will cause the thermostat to move away from it's seat (it's spring loaded) and a lot of extra cooling water will flow through the powerhead, cooling it significantly. That's the more "normal" temps you are seeing when on plane-above 2000 rpm's. In fact, once the stat moves, you could see even cooler temps than 143-that's normal when on plane at speed. I'd put a new stat in it and visibly check the stat housing to be sure all the components are there, clean and functioning normally. Get an exploded parts diagram (epc.brp.com) or check your factory service manual. I'd be looking for any debris, salt deposits, etc.

 

[sumocomputers]

Some facts: your overheats (hot horn) when the powerhead temps reach 212 degrees. The hot horn should come on. The hot horn will silence when the temps get back down to 175 degrees. That's still too hot. Your engine's cooling water goes thru the stat (only) when idling and at low rpm's. At normal idle and at slow speeds, the powerhead temps should be in the 143-155 range. Don't continue running it at the temps you are experiencing... If the stat is stuck shut, you will have no cooling water at low rpm's-(even with a good impeller.) That's the 200 figure. When you get the boat on plane, the extra water flow coming up from the increased water pressure at the lower unit will cause the thermostat to move away from it's seat (it's spring loaded) and a lot of extra cooling water will flow through the powerhead, cooling it significantly. That's the more "normal" temps you are seeing when on plane-above 2000 rpm's. In fact, once the stat moves, you could see even cooler temps than 143-that's normal when on plane at speed. I'd put a new stat in it and visibly check the stat housing to be sure all the components are there, clean and functioning normally. Get an exploded parts diagram (epc.brp.com) or check your factory service manual. I'd be looking for any debris, salt deposits, etc.

OK, I think the thermostat is something I will give a try. I found someone who went through a lot of stuff on this thread: http://continuouswave.com/ubb/Forum3/HTML/019367.html

Looks like this is what I will need to complete the replacement. Can anyone help me make sure I am getting all the right parts?

0434137 Thermostat Assembly (replaces old part 0394409)

0336416 Thermostat Seal

0329830 Gasket Cover

0320880 Seat Valve

0508235 Gasket Sealant

 

[sumocomputers]

Went ahead and ordered the list below for a reasonable total of about $60, and will report back if this fixes the issue.

Not sure if I need Gasket Sealant, but will see how the current one is installed and check the Service Manual.

​Thanks for the help so far.

0434137 Thermostat Assembly (replaces old part 0394409)
0336416 Thermostat Seal
0329830 Gasket Cover
0320880 Seat Valve
0330273 Thermostat Cup
0508235 Gasket Sealant