Thermostat rating / actual operating temp

[Skipper]

Since my high school hot rod days, I've "thoroughly understood" thermostats. ;-)
However, I've been caught in assumptions before. And with better temp data now, provided by today's laser-guided infrared thermometers, thought I'd revisit this.
A thermostat rated at 165 degrees begins to open at approx 165 degrees.
It is not fully open for another 15 or so degrees higher.
Therefore, the motor when fully warmed up should run at 180 degrees(?)
On my marine diesel, it can take nearly an hour to get there, with the coolant running through 2 heat exchangers. The hot water heater takes that long to heat up, so the engine runs at about 160-165 for awhile.
It never really gets to 180. It stops at about 172. -Hitting the thermostat housing with the thermometer, which is probably slightly less temp than the coolant. The panel temp gauge reads consistently 10 degrees lower. It's consistent, so it's still usable. This is in 70 degree Pacific Ocean water.

Would like to confirm that the motor at long-evolved operating temperature should be running at 170-180.

Side note, the exhaust elbow runs at 117 when the motor is at full temp.

 

[Ralph Johnstone]

And the engine is ............................................... ? ? ?

 

[jviss]

A thermostat rated at 165 degrees begins to open at approx 165 degrees.
It is not fully open for another 15 or so degrees higher.

That has not been my experience. I've tested thermostats in hot water, and found that, without exception, they 'popped open' at almost exactly the set point. I had expected them to open gradually, as if they would linearly regulate engine temperature, but they didn't.

 

[Skipper]

And the engine is ............................................... ? ? ?

......................................Irrelevant.

But to satisfy any curiosity
It's a Universal M18.

 

[Skipper]

That has not been my experience. I've tested thermostats in hot water, and found that, without exception, they 'popped open' at almost exactly the set point. I had expected them to open gradually, as if they would linearly regulate engine temperature, but they didn't.

Interesting observation.
I would have expected gradual opening too, especially since Stant Thermostats says there is an opening range of 15-20 degrees, beginning at the rating point.

 

[kloudie1]

The thermostats should begin to open and go toward fully open so that they smoothly control the temperature of the coolant.. Here in the bottom right of the attachment is the characteristic of the Yanmar GM thermostat.. most will look like that .. some will have a faster or slower opening "curve"
Consider this: The water heater is a "bypass" section of the coolant circulation.. if there is low pressure drop there compared to the heat exchanger side, coolant will go thru the heater preferentially, bypassing the heat exchanger.. This can be a source of overheating or the thermostat not being able to control the engine temp.. Not saying that is what ya have, but if the heater was put in by a PO and the fittings are bigger than normal, or the coolant hoses are collapsing or the heat exchanger (coolant side) has deposits .. might be something to look at..

 

[Ralph Johnstone]

On my marine diesel, it can take nearly an hour to get there, with the coolant running through 2 heat exchangers.

I would imagine the two heat exchangers are a DHW heat ex. and your engine heat ex. and not 2 external heat ex.

It takes about 10 min. for our engine (Yanmar 2GM 20F) to come up to 168 deg. F at idle. Once under way, the engine water temp. sits at 172 deg. F (2700 RPM) and 180 deg. F at full throttle (3400 RPM). The mixing elbow sits at 90 deg. F.

Except for the long warm up period, it looks as if you're well within range.

I think Kloudie1 may be looking at the reason for your slow heat up ...............

if there is low pressure drop there compared to the heat exchanger side, coolant will go thru the heater preferentially,

.................... on a cold startup.

 

[Bill Roosa]

kinda depends on the temp of the water the boat is in. I noticed that during early spring the motor runs 10-15 degrees F cooler then at the end of summer

 

[Skipper]

The thermostats should begin to open and go toward fully open so that they smoothly control the temperature of the coolant.. Here in the bottom right of the attachment is the characteristic of the Yanmar GM thermostat.. most will look like that .. some will have a faster or slower opening "curve"
Consider this: The water heater is a "bypass" section of the coolant circulation.. if there is low pressure drop there compared to the heat exchanger side, coolant will go thru the heater preferentially, bypassing the heat exchanger.. This can be a source of overheating or the thermostat not being able to control the engine temp.. Not saying that is what ya have, but if the heater was put in by a PO and the fittings are bigger than normal, or the coolant hoses are collapsing or the heat exchanger (coolant side) has deposits .. might be something to look at..

Thank you.
Mine actually is a loop that flows from the engine hot out at the manifold to the water heater, then to the heat exchanger, then back to the coolant pump. Not through the bypass.
I'm curious about deposits in the HX on the coolant side. Did you mean the raw side?

 

[Skipper]

I would imagine the two heat exchangers are a DHW heat ex. and your engine heat ex. and not 2 external heat ex.

It takes about 10 min. for our engine (Yanmar 2GM 20F) to come up to 168 deg. F at idle. Once under way, the engine water temp. sits at 172 deg. F (2700 RPM) and 180 deg. F at full throttle (3400 RPM). The mixing elbow sits at 90 deg. F.

Except for the long warm up period, it looks as if you're well within range.

I think Kloudie1 may be looking at the reason for your slow heat up ...............



.................... on a cold startup.

Yes, one of the "heat exchangers" to which I referred is the water heater.

The motor has recently been gradually heating beyond what is has historically.
I was motoring 2 hours today and it gradually got up to 185+
This has never been this high before so I'm troubleshooting. I don't want it to gradually get up to overheating.
I've previously motored 5+ hours straight several times (crap, no wind coming back from Catalina) and it never exceeded 170.

 

[Benny17441]

I think Bill has it. The temperature of the raw water entering the exchanger will affect the temperature of the coolant and the engine. The way to test a thermostat is to remove it and place in a heat resistant glass container with a thermometer and then start heating the water while watching the thermometer. Compare to the manufacturer specs and if it opens on point you are done. I believe the engine designer took into account that when the boat is in the water the opening temperature is a little of a moving target. Obviously it is not practical to have one thermostat for summer and another for cooler temperatures. There will be times when the engine may not be running at maximum efficiency but it will hardly be noticeable.

 

[Ralph Johnstone]

I was motoring 2 hours today and it gradually got up to 185+

I know you've checked this already but how is the raw water flow from the exhaust ? You've said that the mixing elbow is 117 deg. F but doesn't hurt to look. Pray it's a plugged water passage in the elbow and if not, then work towards the harder stuff i.e. fouled engine heat exchanger.

 

[jibes138]

Check your HX zincs. As they corrode you can end up with chunks of them in the HX too large to pass through the fittings and restricting flow. My old 2 inch HX only had a cap on one end and of course the zincs were on the end with no cap. The 3 inch HX has a cap on both ends.

 

[kloudie1]

Skipper.. interesting.. the series flow scheme you describe is the reason your water heater takes so long to come to temperature.. Like Ralph, mine and most set up on bypass flow will come up to temperature in just a few minutes.. Set up like yours, the coolant does not flow through the water heater until the thermostat is open.. on the bypass scheme, the coolant is flowing at high flow rates through the heater until the thermostat opens (engine and heater hot) and the engine heat exchanger begins cooling the coolant loop..and the water heater flow decreases a bit. You may have some excess pressure drop in the hoses going to/from the heater or the heater or the engine heat-ex. causing the engine to overheat by not allowing enough coolant flow at higher loads..

 

[jibes138]

My Universal was set up as a series flow as well. Yes it took forever to heat up the hot water. For the bypass to work you must have two fittings to return coolant to the reservoir? The only return on the Universal was from the thermostat housing output fitting to the manifold.

 

[Skipper]

Skipper.. interesting.. the series flow scheme you describe is the reason your water heater takes so long to come to temperature.. Like Ralph, mine and most set up on bypass flow will come up to temperature in just a few minutes.. Set up like yours, the coolant does not flow through the water heater until the thermostat is open.. on the bypass scheme, the coolant is flowing at high flow rates through the heater until the thermostat opens (engine and heater hot) and the engine heat exchanger begins cooling the coolant loop..and the water heater flow decreases a bit. You may have some excess pressure drop in the hoses going to/from the heater or the heater or the engine heat-ex. causing the engine to overheat by not allowing enough coolant flow at higher loads..

Side note:
There is coolant plumbing in the sail locker next to the water heater to allow a partial bypass of it. It's between the in and out hoses prior to the heater, with a gate valve. (Pic)
It's not directional, so I can only guess it's there in case the heater coil clogs. It's OE from Bristol.
When it's closed, the water in the heater heats up faster.
I leave it open, only to diminish the domestic water temp during normal outings. On long cruises the water eventually gets up to engine temp even with the valve open.

 

[Skipper]

I know you've checked this already but how is the raw water flow from the exhaust ? You've said that the mixing elbow is 117 deg. F but doesn't hurt to look. Pray it's a plugged water passage in the elbow and if not, then work towards the harder stuff i.e. fouled engine heat exchanger.

Flow is good. It's always been on the light side. Maybe 3 gpm. I think that's par for small Universals.
Oberdorfer pump. Impeller changed every 2 years. HX is clear. Elbow was new in 2011.

 

[Skipper]

Check your HX zincs. As they corrode you can end up with chunks of them in the HX too large to pass through the fittings and restricting flow. My old 2 inch HX only had a cap on one end and of course the zincs were on the end with no cap. The 3 inch HX has a cap on both ends.

I too have the idioc HX with only one cap. 2". And the end with no cap is where the zincs go. Eyes rolling.
But the HX is clear raw side tubes, and I've poked around for wayward zinc rods through the zinc pencil hole and found nothing. They have broken off the base more than once, but they must have dissolved. I don't know for certain, because there's no feckin' cap there.
In any case, I don't think they would clog the outlet, as the outlet is on top. Seems they would rest on the bottom, like a penny in a toilet bowl.

Correction: the outlet is on the other side of the HX , on top.
Therefore any blockage could only be of the tubes. They appear clear.

 

[Skipper]

Ok I just bled 7 seconds of air from the thermostat housing, then added a pint of coolant to the expansion tank above the water heater.
I didn't think to check this because I'd bled all the air 2 years ago when I replaced the coolant pump and coolant. I guess air found its way in or there was a pocket hiding from the aforementioned system breach.
Will test temps on next dead wind passage.

 

[Bill Roosa]

You should not need to "bleed air" from the engine. the coolant fill cap (radiator cap) does that for you. You should just have to monitor the over flow tank and add coolant as needed after a maintenance event. the air left after a maintenance event will escape when you run the engine and get it to full operating temp. as it cools the over flow tank will only allow coolant in. Air out, coolant in. repeat 3 or 4 times and you have no air in the engine.
If you had air in the engine and no recent maintenance event that would have introduced it then have the radiator cap checked as it is probably bad.