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Old 01-08-2020, 08:45 AM   #1
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Main engine coolant temp drop

For several months, I've had temperature sensors on various parts of the main engine (each alternator case, the thermostat housing, and the junction where the raw water is injected into the exhaust), which I monitor while underway. Over the last few trips, I've noticed that the temp at the thermostat housing has dropped about 5 - 7 degrees Fahrenheit from it's normal cruising temp. I don't mean that it gets to temp then drops - I mean it just doesn't get to the same temp as before. Instead of the normal 185, now it's reaching only about 178 - 180.

The coolant is keel-cooled, and we've been in the Eastern Caribbean for a year, so no change in water temps. We run at the same engine RPM all the time, so nothing there.

My only thought is that possibly the thermostat is not closing completely, or is opening at a lower-than-normal temperature, but could it be something else? I have a spare thermostat, but I don't want to change it without needing to.

How low should I let that temp get before I need to try to fix the issue? I think most diesels run about 180 - 190, but what happens if they run cooler? How cool is too cool?

The temperature at the junction where the raw water is injected into the exhaust has also dropped by about the same amount, during this same period. It's probably related, but I don't know how, as the raw water that dumps into the exhaust isn't used to cool the engine - only the transmission oil, and the hydraulic fluid.

Any brilliant ideas, TF?
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Old 01-08-2020, 08:48 AM   #2
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What's the rated opening temperature for your engine thermostats? If it's 180, then anything from about 180 up to 195 - 200 would be normal.

Have you had the hull cleaned or anything that could impact water flow through the system or over the keel coolers?
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Old 01-08-2020, 09:12 AM   #3
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Suggest you also check whatever sensor you have there. For example, is it a surface applied sensor that is now slightly loose? Even if all is well with the sensor it is possible some other error has creeped in. Perhaps verify with an IR temp gun.


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Old 01-08-2020, 01:27 PM   #4
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+1 for sensors since both dropped and the only connection is they both have a sensor. Maybe even the same part number ?
Check the wiring too since it would be unusual for both sensors to go at the same time.
What does you original temp gauge read ? Any change ? A 5-7 degree drop is nothing to worry about just keep an eye on it. Could be just the outside water temp is a little cooler.
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Old 01-08-2020, 02:19 PM   #5
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If sea water injected into the wet exhaust has dropped in temp too, and all it cools upstream is the gear cooler.... I strongly suspect you are running in cooler sea water temps. What else could explain that?

Diesel t-stats generally have about a 10F operating band from cracking open to full open. So if you get cooler sea temps, coolant temp will be lower too as tstat will close further.

A good check for tstat function is to get up to full temp under load, then come back to idle load and see how far it drops. If it drops way below like 160F, it is not closing fully.

Nothing wrong with any engine running 175F.

Would help to know what engine it is.

Maybe some electrical bias common in your monitoring system? Crappy connection somewhere?
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Old 01-08-2020, 03:02 PM   #6
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Thermostats can do this. My last defective automotive thermo would open too much and left me with no real heat for occupants.
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Old 01-08-2020, 05:55 PM   #7
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Have you adjusted the v-belt tension lately maybe.
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Old 01-08-2020, 08:06 PM   #8
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Is the coolant level staying constant ????????????
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Old 01-08-2020, 08:32 PM   #9
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Change the thermostat and boil the old one to test it, then you have a spare.
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Old 01-08-2020, 09:51 PM   #10
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Have you an I.R. thermometer gun? What does it say vs the sensors?

When you installed the sensors did you use a thermal paste? The paste will help heat transfer and also protect the metal the sensor is mounted to from oxidiation/corrosion which can create a heat barrier.
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Old 01-09-2020, 07:13 AM   #11
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Replies to all of the above comments and suggestions:
- I can't find the rated temp of the thermostat in any of the documentation I have for the engine, which is an Isuzu 6RB1T, an industrial engine. (838 cu in, 6 cylinder, 283 HP, small turbo that I suspect never does anything as I run the engine at only 1300 rpm.)

- The hull has been clean as a whistle since our last haul-out and new bottom paint in August.

- Sensors are attached to the surfaces being measured, but they're both tight and in the same place as always. I have used an IR temp "gun" to verify that the temps are actually dropping.

- The original temp gauge (a Faria analog) has always read somewhere above 200, but it's calibrated only in 25 degree increments. Maybe it's reading a needle's width cooler, but that's hard to say.

- We're in the Southeast Caribbean, and the water temp has been in the 82 - 84 degree range ever since we hit Turks and Caicos over a year ago.

- If it continues to drop, I'll try the full load test - thanks for suggesting it.

- No change in the belt tension - it's something I check before every engine start.

- Coolant level has not changed a bit - it's also something I check before every engine start.

- I didn't know there was such a thing as "thermal paste" - I'll try to find some, because I know the sensors read a little low, and I'd like for them to read closer to what the IR gun says. (Note that I have taken this known difference into account in everything that I've posted.)

- If it continues dropping, I'll pull the thermostat and test it on the stove, but that's a messy job, and I don't want to do it if it's not necessary.

No one seems concerned about it operating at about 175, so I'll keep an eye on it, and take action only if it continues to drop (which would almost certainly indicate a failing thermostat, of which I have two brand new ones).

Thanks for all the input, as always!
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Old 01-09-2020, 08:46 AM   #12
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Does the engine warm as fast as usual? 175 is less of a problem that warmup time. Also, if the stat is indeed stuck, high loading could cause an overheat. WOT running ahead of pirates or storms.
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Old 01-09-2020, 11:07 PM   #13
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I got my thermal paste from an electronics supplier when I used to work. GO to someone like Newark electronics. Maybe even a radio repair shop where you are.
I have also used heavy bodied silicone grease. Prefer the actual paste as there it will conduct heat better.

OR

https://www.amazon.com/s?k=electroni...b_sb_ss_i_1_19
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Old 01-13-2020, 09:07 AM   #14
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Quote:
Originally Posted by diver dave View Post
Does the engine warm as fast as usual? 175 is less of a problem that warmup time. Also, if the stat is indeed stuck, high loading could cause an overheat. WOT running ahead of pirates or storms.
I honestly can't answer the warmup time question, as I've never paid any attention to it. I guess I better start. We never run WOT - in a full displacement hull, we're not going to "run ahead" of anything, regardless of the throttle setting. :-)
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