9.8 Tohatsu four stroke overheated

[Mary Ella Rose]

After getting mired in weeds I managed to overheat my 9.8 four stroke Tohatsu. It went into "limp mode" that is, rpm dropped dramatically but kept on running long enough to get me back to the dock. Interestingly enough, plenty of water was coming out of the vent hole on the side. I stuck my finger in it and much to my surprise, it was cool, not hot, even though there was steam coming out of the exhaust.

​My questions are:


If the cooling intake was clogged, why was water still coming out of the side vent hole?

Why was the water from the vent cool? If it was overheated, wouldn't it have been hot?

I boat in zebra mussel infested waters, could the cooling passages be clogged with them? How do I check?

What's the normal operating temp for this motor?

I've got a infrared temp sensor, where should I measure at? The cylinder head?

Was the steam in the exhaust an indication of overheating?

Thanx....

 

[pvanv]

What makes you think you overheated? The CD on that motor does not have a temperature sensor, so it did NOT go into a limp-in mode due to temperature.If you aren't familiar with that motor, review the owners manual at http://tohatsu.com/tech_info/own_man_pdfs/Toh_MFS8A3_98A3_003-11082-9.pdf
Low oil pressure will limit RPM to 3000. If your oil is over full, it can foam up and cause low oil pressure. Normal temp for any 4-stroke OB is about 150F. Zebras would typically get into the inlet strainer area if the motor is left down in the water all season. They hate warm passages, so will not get in there while the motor is running.

 

[Mary Ella Rose]

Thanx Paul,



I've had several two stroke outboards but this is the first four stroke so I admit I'm still some where down on the learning curve. I was badly mired in weeds, they had accumulated on the keel cable and on the lower part of the OB so badly we could hardly move. Unfortunately we were in a channel with other boats approaching and I didn't have much choice except to bull my through. I had read about the low speed governor and when the motor slowed to a crawl I assumed that's what was going on. Hence my question about the cooling water vent flow.

 

[Mary Ella Rose]

Should the water coming out of the vent hole be warm or cold? Was the steam in the exhaust an indication of over temp or just because I was working the motor too hard? ​Oil level is at the full mark, but not overfilled. If the sudden drop in rpm wasn't caused by an overheat situation, it sounds like I've got another problem?

Thanx Again

 

[Mary Ella Rose]

I check the motor temp today after idling at the dock for about 20 minutes. It was 157degrees F between the plugs, 147 degrees F below the bottom plug and 167 degrees F above the top plug. Does that sound about right?

 

[Sea Rider]

If lower leg water intakes gets clogged due to algae, plastic bag, even a banana peel, there should be under antiventilation plate another water strainer doing his homework cooling engine down, check if having one. If wanting to know if working temp is correct when at idle, put your palm at rear leg's middle idle relief port, check if you can feel a hot/warm cycle going on repetitively, if so, thermo is working ok. If heat cycle can't be felt while cooling water is outing on palm, change thermo.

Happy Boating

 

[pvanv]

If lower leg water intakes gets clogged due to algae, plastic bag, even a banana peel, there should be under antiventilation plate another water strainer doing his homework cooling engine down, check if having one. If wanting to know if working temp is correct when at idle, put your palm at rear leg's middle idle relief port, check if you can feel a hot/warm cycle going on repetitively, if so, thermo is working ok. If heat cycle can't be felt while cooling water is outing on palm, change thermo.

Happy Boating

No. No sub strainer on the MFS/NSF9.8A3.
Those temps sound quite normal.
The pee stream should get warm, but not scalding.