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Overheating issues, it can't be this hard!

Feb 26, 2004
21,902
Catalina 34 224 Maple Bay, BC, Canada
Really small ones probably work themselves out also I am rough guessing....
No, they don't. There is nowhere for them to go. It is what in engineering is called a CLOSED system. That is why the air needs to be VENTED OUT, as completely as possible. Without getting into the scientific details, tiny air bubbles can and will be pushed through by a pump if these tiny bubbles are a small percentage of the liquid volume. Water pumps cannot pump air. Period. That's why your engine overheats if there is an air bubble in the FRESHWATER coolant side. The freshwater carries the engine heat to the HX where the seawater cools it and returns it to the freshwater loop via the freshwater (coolant) pump. If that pump hits a block of air, movement of even the liquid stops.
Take a look at the freshwater coolant loop on your engine to understand, if you don't already. Here's a good example:
 
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RoyS

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Jun 3, 2012
1,162
Hunter 33 Steamboat Wharf, Hull, MA
Additionally to what Stu has stated, your thermostat may end up in the air bubble. If that happens your engine will overheat due to the thermostat never opening.
 
Jul 5, 2011
598
Oday 28 Madison, CT
OK, interesting. So flipping the discussion, if one is running at normal temps (within spec of Mfg), it is unlikely there is any air in the system.
 
Jan 7, 2011
2,829
Oday 322 East Chicago, IN
I was motoring for an hour or more this weekend, and took the opportunity to take temperature readings on my Yanmar 2GM20F motor.

I was running at about 3000 rpm’s, flat seas, pushing my O’Day 322 at 6 knots.

I used an infrared thermometer and took readings in various areas of the engine:

Exhaust manifold 375 F
Heat Exchanger. 156 F. This was by the neck, but on the actual H.
Mixing elbow. 135 F. Near the top, after the water inlet
Valve cover. 165 F
Thermostat housing. 165 F at least I think that it was the thermostat housing area.

So, when people talk about the engine temperature, where are they measuring the temp specifically?

On the positive side, my traditional stuffing box appears to be pretty dialed in (with GFO packing) with no noticeable water spray at 3000 engine RPMs and the shaft and stuffing box were cool enough that I could grab it (100 F or so).

The little engine ran well…


Greg
 
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Jul 5, 2011
598
Oday 28 Madison, CT
So, when people talk about the engine temperature, where are they measuring the temp specifically?
For me, at the temperature gauge....
 

RoyS

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Jun 3, 2012
1,162
Hunter 33 Steamboat Wharf, Hull, MA
Your thermostat is supposed to keep your engine temperature at the thermostat's listed temperature value. When the coolant passing by the thermostat reaches the thermostat setting (say 160F) it opens and redirects the coolant to the heat exchanger to be cooled before returning to the engine. If the coolant passing by the engine thermostat is below the thermostat's setting then the device remains closed and sends the coolant back to the engine block. This is called by-pass. Eventually the thermostat modulates the coolant flow to result in your engine running at the thermostat setting. Somewhere on your engine is an over-temperature sender. Aim your probe at the casting that the sender is screwed into and that should give you your true engine temperature. If you ever decide to add a temperature gauge, put the gauge sender there.
 
Oct 26, 2008
4,991
Catalina 320 Barnegat, NJ
If you ever decide to add a temperature gauge, put the gauge sender there.
So, my understanding is that the standard Yanmar panel doesn't include a temp gauge and the temp switch is placed at the casting that houses the thermostat. P.O. installed the temp sender on a tee fitting that branches to the water heater. He mentioned to me that the temp gauge always reads a little cooler than the true engine temperature - I typically see 160 when warmed up and running. I'm assuming that the switch engages the buzzer alarm for over-heating. I'm more inclined to hear an alarm before I actually look at the gauge. The gauge is at ankle level and I'm not inclined to crawl down to the floor to look at it frequently. Would you say that I should leave the switch and sender in the current positions or swap them so that the gauge is reading true engine temp. The alarm might go off a little late ....
Front.jpg

Engine is 3GM30F
 

RoyS

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Jun 3, 2012
1,162
Hunter 33 Steamboat Wharf, Hull, MA
Scott, hard to tell how exactly your system is plumbed from the photograph. It appears that the two senders would see the same temperature unless there is some other device modulating the water line to the water heater. If it were me I would leave the switch where it is. You are fortunate that you can hear the overtemp alarm. I can barely hear mine with the engine running. I added a boat horn in parallel to the alarm and mounted it under the helm seat so I would be sure to hear it. Last time it went off I nearly jumped overboard, clutching my chest.
 
Oct 26, 2008
4,991
Catalina 320 Barnegat, NJ
Scott, hard to tell how exactly your system is plumbed from the photograph. It appears that the two senders would see the same temperature unless there is some other device modulating the water line to the water heater. If it were me I would leave the switch where it is. You are fortunate that you can hear the overtemp alarm.
Yes, it seems to me that that both senders are reading pretty close to the same temp - the plumbing at the Tee is coming directly off the water pump housing. I don't know if I can hear the overtemp alarm! I don't think it has been activated in the past 2 seasons.