engine overheat guidance

[qazvick]

Hey all,

I'm pretty familiar with gasoline engines but this sailboat is my first jump into diesels. I imagine there's not a whole lot that's different but obviously you'll know more than I.

My '83 H33 is randomly overheating. The high temp alarm will beep at me anywhere from 5 mins to 1 Hour of run time. I've also run it for an hour, shut it off, then 3 mins later turned on the engine again to a high temp alarm, but only for a couple mins as it turned off on its own. Sometimes I have smoke out of the exhaust when the alarm is going off, and when that happens, the engine gets hot enough to burn residual oil off the exterior of the engine, causing smoke to rise off of it and out of the Companionway. Othere times, the smoke won't happen. The only consistent thing I've noticed is that every time the exhaust does smoke, the engine is hot enough to burn off that residual oil causing smoke to rise up out of the companionway. And vice versa.

I've replaced the impeller Even though I had great flow out of the water pump. The sea strainer is clean. The belt has tension. I don't believe it's the wiring to the high temp alarm corroding because it is actually overheating sometimes.

Any help much appreciated. Thanks!

 

[Benny17441]

Don't know what engine you have but will assume a closed system with coolant with an exchanger cooled by raw water. There could be a few causes for the seemingly random overheating. When a raw water impeller fails it usually breaks up the vanes into small rubber pieces which could get lodge and move around inside the hoses and cooling passages causing intermittent water flow blockages. Check the water intake clam and any strainers for debri accumulation. Another reason could be a sticking thermostat in the closed coolant system. The exchanger also can get internal build up scales thus obstructing coolant flow. I would start with the raw water system and remove all hoses and clean them out or replace them. Dig into the exchanger passages with a bottle cleaner brush. Once you have insured the raw water system is good then move to the closed system. Remove and test the thermostat and make sure the hoses, engine passages and exchanger passages have adequate coolant flow. You have reported actual overheating so I will skip the phantom overheating alarms due to sensor and wiring faults. Lastly repeated overheating may cause a head gasket to fail but these fault is seldom random in nature. Check the coolant level and take notice if there is any loss of coolant. Obviously no loss would be a good thing but if you find a loss then you would need to track it down to an external or internal leak. The alarm buzzer will usually go off well before the engine reaches a damaging temperature but without a temperature gauge it would be wise to promptly shut down the engine . Hope this helps and good luck.

 

[qazvick]

I'm sorry I didn't specify and thank you for the detailed response. It isn't a closed system. The engine is a Yanmar 2 Cyl. Water goes straight from the sea cock, through a strainer and into the water pump. This also would leave me to assume there's not thermostat. If there is one, where would it be located? I've also been considering installing a temp gauge But am not quite sure off hand where I'll plumb that into.

 

[Johnb]

If you read the recent posting about an overheating incident causing a burnt piston it is obvious that prevention is way cheaper than cure.

Most (but not all) of the problems will be in the raw water because it is (maybe) salt water, comes in complete with marine growth, trash etc and gets injected into the exhaust at the mixing elbow.

The typical path for raw water is
through hull intake with strainer
seacock
strainer
raw water pump
heat exchanger
anti siphon valve
exhaust manifold passages
mixing elbow
muffler
all connected by hoses of various sizes.

As a general statement, if the raw water can flow freely through all of these parts and if the pump is working right, all will be well with the raw water side. Most of this system is not that difficult for an owner to take apart and examine, the biggest problem most likely to be frozen/corroded nuts and bolts. In any event, if you don't do it you will have to face these issues sooner or later and the problems won't be any less, just more urgent.

The fresh water side is typically less problematical, especially if it has been well maintained. The thermostat and hoses being the first things to pay attention to.

The purpose of the long winded posting is to make the point that it is better to make the whole system right, than to look for "the problem" that is causing occasional overheating, especially if it is some years since the last general overhaul.

 

[qazvick]

Couldn't agree more with you on the prevention as I'm well aware of the consequences over a nasty overheat. Looks like I'll keep on troubleshooting.

 

[rardiH36]

I'm sorry I didn't specify and thank you for the detailed response. It isn't a closed system. The engine is a Yanmar 2 Cyl. Water goes straight from the sea cock, through a strainer and into the water pump. This also would leave me to assume there's not thermostat. If there is one, where would it be located? I've also been considering installing a temp gauge But am not quite sure off hand where I'll plumb that into.

Sorry to respond like this, but even with the above "added" information, not enough. Find out exactly what engine model you have (Yanmar has quite a few "2 cylinder" versions). And whether raw water cooled directly into the engine block or the raw water goes through a heat exhanger which cools anti-freeze that circulates through the engine block.

Considering buying this at Amazon:

http://www.amazon.com/Yanmar-Inboards-1975-98-Tune-Up-Manuals/dp/0893300497

You likely have a 2QM20 or a 2GM20. This book covers both and gives in-depth instruction for such things as location and testing of the thermostat. And trouble-shooting or repairing everything about small marine diesel engines. It's a much better reference manual than random responses from the internet ... no matter how helpful our forum members try to be. With the more knowledge you learn from the manual, then if still stumped you will be better able to construct your questions.

 

[Bill Roosa]

That is easy. Random overheating is the engine thermostat. Those things are temperamental when they get old. Work sometimes and don't sometimes, sometimes they open half way....... temperamental.

 

[Bill Roosa]

BTW the only difference between a diesel and gas engine is the ignition system is replaced by the fuel injection system. Think of the injector pump as a coil.
AND
the cooling is with water to water not water to air so it is much smaller.

 

[qazvick]

Sorry to respond like this, but even with the above "added" information, not enough. Find out exactly what engine model you have (Yanmar has quite a few "2 cylinder" versions). And whether raw water cooled directly into the engine block or the raw water goes through a heat exhanger which cools anti-freeze that circulates through the engine block.

Considering buying this at Amazon:

http://www.amazon.com/Yanmar-Inboards-1975-98-Tune-Up-Manuals/dp/0893300497

You likely have a 2QM20 or a 2GM20. This book covers both and gives in-depth instruction for such things as location and testing of the thermostat. And trouble-shooting or repairing everything about small marine diesel engines. It's a much better reference manual than random responses from the internet ... no matter how helpful our forum members try to be. With the more knowledge you learn from the manual, then if still stumped you will be better able to construct your questions.

No that's a very fair and deserved response. I didn't even realize there were multiple types of 2 Cyl yanmars as options on my boat. I bought and read a book on general marine diesels but never came across the one you showed me. I'll pick it up, troubleshoot more, and return if I can't figure it out.

Thanks again.

 

[rardiH36]

Dear Qazvick:

I appreciate your very considerate response. I had paused several times before hitting the "post reply" button because I thought I might have been a bit too forward. Actually, I even waited a day after observing your post before replying.

As Bill Roosa already observed, something in the thermostat circuit was also my first thought. But from what I remember from a teenager, thermostats are designed to fail in an open position... that is coolant will circulate unimpeded and the engine will run under the design temp. But the intermittent hot/normal is baffling. Almost seems that something is rattling around in your coolant circuit. Sometimes restricts and sometimes doesn't. I have other observations from my experience about the alarm going off. But that has more to do with the temperature sensor rather than the actual engine temp.

If you have clear access to your engine, you should be able to visually determine what is likely the thermostat housing. Then a simple matter to remove it. Should be closed. Put it in a pot of warm water on a stove. If you have an anti-freeze system, it should start to open at about 70C. If you have an seawater cooling, the thermostat is different (as is the housing to prevent an owner from installing the wrong thermostat) and it will start to open at about 45C. I got these numbers from the official Yanmar manuals for the QM series and the GM series which I have. When you determine your engine model and whether its a sea water or anti-freeze system, I could take a photo of the relevant pages and post them for you.

You also asked about adding a temperature gauge onto your engine. By chance, another discussion is now going on in another SBO forum:

http://forums.hunter.sailboatowners.com/showthread.php?t=169799

I did something quite similar several years ago. I have a trigger-happy temp alarm. When/if it does go off, I can peer at my installed temperature gauge and know that the engine really isn't over heating. Reading the above link however, I am intrigued about the device that just bolts on to the engine.

 

[rardiH36]

Just thought of something else you should be aware of. On my and also other Cherubini Hunter models, Hunter took a sea water cooled Yanmar 2QM20 version and modified it to anti-freeze by the addition of an externally mounted heat exchanger. The modification however retained the seawater thermostat and housing. So even though these engines have anti-freeze coursing through their blocks, the engine runs at the lower 42c-55c (~130F) temp range rather than the ~160-170F range of the engines actually designed for anti-freeze cooling. Let me know if you do have one of these modified engines. There have been several threads with the details.

 

[Johnb]

For grins and giggles here are some before and after pictures of my heat exchanger refurb project.

I choked a bit when the radiator shop quoted $176 for cleaning the heat exchanger, but when I got it back, immaculately cleaned, all gasket mating surfaces made perfect and the whole unit spray painted I was very pleased.

My curiosity was piqued about the failure mode of wax pellet thermostats and from what I could discern it is possible for them to stick in any position but most likely closed. I do figure though that if they are in well maintained coolant they will last a long time.

 

[qazvick]

Thanks for this. 99% positive this doesn't exist on my boat. Unfortunately I'm away for business and won't return until July 4. So I shall return after meddling with it. Thanks again guys

 

[Alctel]

I'd prepared to bet 10 bucks you have a partial blockage somewhere on the raw water system side, maybe an impeller blade.

 

[Sailm8]

Most of the Yanmar GM series manuals are online in this site: http://hunter.sailboatowners.com/in...ds&Itemid=287&brand=2&id=0&model=gen&page=gen

 

[kito]

I assume you are seeing water still exiting the exhaust when in these overheating phases?. My bet is a sticky thermostat.

 

[Nelsonhow]

I had the same problem with fy Raw Water Cooled Yanmar 2 GM

1) Make sure that there is no blockage between the external strainer (thru hull) and the ball or gate valve on the raw water inlet. I hope that someone replaced the original Hunter gate valve with a ball valve. If not, now is good time to do so. I had a bunch of small muscle shells suck in mine which was causing an intermittent overheat problem..

2) While you are at it remove each hose and make sure that that each one is in good condition with no blockages or internal deterioration . There may be a flap or something inside one of the hoses that intermittently blocks the flow. It's not likely but possible and easily checked. Also check to make sure that none of the hoses are collapsing when the engine is running or overheating. Only the hose between the thru hull and the raw water pump has suction so check this one first. It's an easy and cheap check.

3) Check that the mixing elbow is not clogged. If you pull the hose off the top you can do a quick visual check. If you can easily blow through the hose you're probably OK. But if in doubt you'll have to pull the mixing elbow of to really check. I wouldn't do this until you run through the easy checks that follow.

4) Make sure that the thermostat cover (that goes over the thermostat) is on with the arrow pointing up. The arrow is sometime covered by the end of the hose and it's easy to put the cover on upside down because it fits both ways. There are two hose barbs on the cover. The top barb (with the arrow pointing up) goes to the exhaust mixing elbow. The bottom barb has a hose that routes the cooling water back into the engine block. Good Old Boat magazine had a good article about a boat that had an intermittent overheat problem due to the thermostat cover being put in upside down. And while your at it replace the thermostat. It's easy to check by tossing the old thermostat into a pot and heating it up checking the temperature with a thermometer, it should open at between 165 and 185 degrees but why bother, it's a cheap replacement at about $10 and easy to do.

5) Replace the raw water pump impeller with a new Yanmar impeller. This is standard maintenance. Note any cracked or missing impeller vanes. Good Old Boat Magazine also had an article where an intermittent overheat problem was due to the rubber impeller vanes slipping on the metal bushing in the center of the impeller. The vanes started to slip only under load. So replace the impeller.

6) While you have the pump cover off to replace the impeller see if there is any wear on the inside of the pump cover. The pump cover is a flat brass (I think) plate and is very soft. the impeller rubs on the inside of the cover and does wear away the cover. If it's worn it will not pump well. A replacement for the GM 2 costs about $15.00. Get a new paper gasket (about $4.00) too.

If these don't solve the problem then by all means check and if necessary replace the mixing elbow. You might also try descaling the motor, but my guess is that the problem is either a blocked hose or a raw water pump in need of servicing.

Good luck and let s know what you find......

 

[Stephen Trapani]

He should be able to rule a flow problem in or out (assuming it's entirely raw water cooled) simply by sticking his head off the stern and looking for the water coming out the exhaust, right?

I am so attuned to flow problems that the sound of the motor running better have the sweet sound of exhaust water hitting the surface water or I'm on it in a heartbeat.

 

[kito]

He should be able to rule a flow problem in or out (assuming it's entirely raw water cooled) simply by sticking his head off the stern and looking for the water coming out the exhaust, right?

I am so attuned to flow problems that the sound of the motor running better have the sweet sound of exhaust water hitting the surface water or I'm on it in a heartbeat.

Exactly. Start with the simple solutions first. If you are seeing flow from the exhaust when overheating then take the 5 minutes to just remove the thermostat and see if it overheating stops. If not then look into water flow restrictions in the mixing elbow, strainer, impeller etc. But I bet it's the thermostat.

 

[py26129]

These diagrams may help a bit.

Cheers

Matt