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Question about engine overheating

Frank Langer

1984 Ericson 30+, Nanaimo, BC
Hi,

The Universal 5416 16 hp diesel engine on our boat always runs at about 160 degrees--warms gradually to that temperature and stays constant with no apparent fluctuation in the gauge. I do regular maintenance--oil changes, fan belt tension, turn grease cup, change coolant every two years, check/replace hoses, etc. The engine has 850 hours on it, and I put on about 100 hours per year.

Today I started the engine before lowering sails (it had operated normally on my way out, and then was off for about two hours during my sail). As I was on my own, it took me about 15 minutes to put away the sails, add fenders, dock lines, etc. As I was ready to put the engine in gear to motor in, I noticed that the heat gauge was almost at the high end. The alarm had not yet gone off, the engine was idling slowly and smoothly, no smoke, and there was exhaust water at the transom. I debated shutting the engine off immediately, but decided instead to slightly increase the idle. The heat gauge gradually returned to it's normal 160 degree position. I motored slowly at first and then at normal speed, and the gauge stayed constant with no further sign of overheating. When I got to my slip, I checked the oil and it was fine--normal level, no change in colour or consistency or smell. The raw water strainer was clean.

So my question is: What would have caused the engine to show overheating? Could it be that I had the idle too low to circulate enough coolant, even though there was exhaust water at the transom?

I don't think I did any damage, but would like to understand what may have caused this, and how to avoid a bit of a scary moment in the future.

Thanks for any thoughts on this.

Frank
 

treilley

Sustaining Partner
Check the easiest and most likely culprit first. Raw water flow. Blockage, bad impeller or worn impeller cover. Mine acted this way and it was the impeller cover. I replaced with a speed seal and now it works great.
 

Frank Langer

1984 Ericson 30+, Nanaimo, BC
Thanks for your reply, Tim. That's what makes this so puzzling--I had checked the exhaust water and it was flowing at it's normal amount, and I could hear it while I was putting sails away, so I don't think there was any interruption. The impellor was checked and the cap was re-greased recently, so I know it's good. The raw water strainer was clean, and there wasn't any evidence of weeds, etc. in the water around the boat.

In thinking further about it, I'm wondering if maybe the thermostat got stuck, and when I increased idle/flow, it opened. Otherwise, my initial thought on perhaps the idle speed being too slow while I was getting everything ready, and when it increased, the temp went down. I'm thinking maybe like a car overheating when it's idling in traffic, and then running at normal temp when underway again. But any ideas would be really helpful.

Frank
 

Glyn Judson

Moderator
Moderator
Overheating at idle.

Frank, I pondered your problem a bit yesterday and then again this morning. And assuming that you don't have a blocked strainer or a constricted exhaust elbow, let me throw out this thought as a possible answer to your mystery overheating problem. Picture the cooling water moving too slowly through the water jacket at idle, the result being that it causes the water to get too hot and the gauge to rightly indicate a potential overheating problem. But increase the RPM's and the cooling water will then move through and out of the jacket more quickly before it can get too hot, the result being that the temp on the gauge goes down and the overheating goes away. So assuming that the first two above are not the culprit, try a little test. Fire up the engine in your slip or mooring and occupy yourself making coffee and something to eat in the galley. Keep your eye on your watch and the temp gauge at the same time and jot down the temp in, let's say two minute increments. How interesting it would be to see the temp begin to get alarmingly high after X-minutes at idle? Then raise the RPM's from the idle speed of 800(?) to, say 1,000 and watch the temp. If no change, then 1,200 and so on and so on. My guess is that the temp would begin to drop at 1,200 even possibly 1,000. It would be interesting to see what speed would be necessary to get it all the way back to 160 or what ever is normal for your engine. What do you think of my hair-brained idea? Keep us posted as to whatever you do to cure the problem. Glyn Judson, E31 hull #55, Marina del Rey, CA
 

Frank Langer

1984 Ericson 30+, Nanaimo, BC
Hi Glyn,

Thanks for your reply, and for taking the time to ponder this problem! The possibility of the coolant running too slowly through the engine at a slow idle is one of the two causes I had contemplated (the other being a stuck thermostat). I am sure it is not related to an obstruction/blockage or a problem with the impellor.

I think your idea of a test at the dock while I sit with a coffee or drink and watch the gauge sounds quite boring, but would probably be effective. I don't have a tach on my boat, so won't be able to report the RPMs accurately, but I can report on approximate throttle position as a proxy. I will try this when I'm at the boat next, and report back. I'm hoping to go sailing on Tuesday, and will try to replicate the events of last day--ie. running the engine to leave the dock, then shutting it down for about two hours while I sail, then starting it and running it at slow idle to see if the gauge shows overheating like last time. I think this would be a more accurate test than just running it at the dock, though that would be possible if I can't get out on Tuesday.

Thanks again for taking the time to reply.

Frank
 

Frank Langer

1984 Ericson 30+, Nanaimo, BC
Follow up on engine overheating

I sailing and tried to replicate the same situation and sequence as last time, when the temp gauge showed the engine beginning to overheat. The temp gauge stayed at it's normal temperature, with no sign of heating up, even though I took about the same amount of time at a slow idle. So it doesn't appear that the engine will normally begin to overheat if the idle speed is too slow, ie. if the coolant is not circulating fast enough.

But that still leaves me at a loss to understand why the temp gauge spiked last time. I am still sure there was no obstruction, as the exhaust water was discharging as usual; the pump and impellor are good, the engine sounded normal and did not smoke or smell. Although it could have been something in the gauge itself, I sort of doubt that, because it returned to normal operating temperature once I started to get underway, and has worked fine since.

A real puzzle.... :confused:

Frank
 

bayhoss

Member III
Overheating Puzzle

Hi Frank, It sounds to me like you have a thermostat that is not reacting to temperature as it should. If you are seeing water in the exhaust then you are moving water thru the heat exchanger. I will from time to time catch some of the exhaust water in a cup and put a thermomenter in it. That will tell you how much heat you are removing. If all is well there, check the heat exchanger. One thing that will also do this is an obstruction inside the heat exchanger itself. I had a similar problem, at low rpms the obstruction (sea weed) would hold in place and block the tubes. At higher rpms the force of the pump would pull water thru.

Hope this helps,
Frank
 

Frank Langer

1984 Ericson 30+, Nanaimo, BC
Thanks, Bayhoss,

I'll do the temp check on the exhaust water as you suggest, and keep an eye on it. As I couldn't replicate the overheating when I went out this last time, I''ll monitor it carefully. I did replace the heat exchanger three years ago (about 250 engine hours ago), but it could be that something small got caught in there, though the raw water strainer should catch that, and it was clean...

No matter how much you learn about boats, there's always another puzzle...:)

Thanks again.

Frank
 

Gary Peterson

Marine Guy
If you have a hot water tank plumbed into your fresh water by-pass connections you might have a bubble of air in the top of your thermostst housing where the temperature sending unit is. Do you have a functioning overflow bottle connected to your heat exchanger fill neck? These are supposed to remove coolant bubbles that can cause your fresh water pump to air lock and stop circulating coolant. Increasing engine speed could have started the coolant flow again. Reminder-I had to replace my water temperature sending unit because sometimes it would cause my electric(stock) temp gage to start reading hot for no apparent reason. A VOM meter confirmed that event.
 

Frank Langer

1984 Ericson 30+, Nanaimo, BC
Thanks for your reply, Gary. I do have an overflow bottle, and coolant levels are at normal levels. I haven't done any work on the cooling system recently, so there should be no new air bubbles anywhere.

I wonder if the temperature sensor might be beginning to fail, like you said yours did. I'll keep a careful eye on the temps, but I've been out twice since the initial overheating situation, and everything has been normal both times.

Thanks again.

Frank
 
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