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Old 04-25-2012
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Yanmar 3GMF Overheating

Folks,

Last season my 3GMF started overheating after 10 minutes at full throttle (4000rpm). Previous seasons she would run at full for hours with not a complaint. Raw water system is clear. New impeller. Regular oil and filter changes. The one thing thing I did differently before last season was change the coolant. It had been green and I replaced it with Canadian Tire's Diesel Coolant which was red, which I put it in full strength. Could this have affected the heat exchangers efficiency? There are no other indications that I can see.
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Old 04-25-2012
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Re: Yanmar 3GMF Overheating

Some cooolant is mixed 50/50 with water. Some is pre-mixed.
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Old 04-25-2012
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Re: Yanmar 3GMF Overheating

You already mentioned the usual suspects. Now is the time to be more thorough.

1) What are the directions on Canadian Tire's diesel antifreeze? Full strength or diluted?

2) What does your engine specify for coolant? Does Canadian Tire's product fill the bill?

3) Is the impeller you installed an OEM part for that engine? Heat is an engine's worst enemy. Overheating shortens the life of a diesel engine and we all know how expensive they are to replace. This is not a good place to save a few bucks.

4) When you say the raw water system is clear, do you mean you checked the entire path from the raw water intake through-hull through the mixing elbow? Let's go through the steps:
-- Did you run a screwdriver or pipe up the intake through-hull to ensure there's no obstruction, such as barnacle growth?
-- Did you ensure the strainer is clear?
-- Did you clean the heat exchanger? You'll need to do that every few years. I did it when I changed the fresh water coolant.
-- Did you examine the mixing elbow and clean out the crud?

If you don't want to go under the boat to check the through-hull, you can short-cut by closing the seacock, taking the engine's raw water hose off the strainer, then opening the seacock for a short time to assess the flow. Assuming your strainer output is well below the water line, the flow you see there should be greater than the flow you see out of the water muffler. If it isn't you found your choke point. If you don't want the water running into the bilge, I'm sure you can put a temporary hose on it and run it to a bucket.

If the strainer is vertically mounted as mine is, the exit is near the water line and the lack of pressure can be deceiving. You can take the strainer off and do the same test with just the seacock, but then you need go to the trouble of getting a good seal on the threads when you reseat the strainer.

Good luck. Hope this helps!
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Old 04-25-2012
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Re: Yanmar 3GMF Overheating

Also, any chance your old impeller broke off and there are pieces in the hoses or heat exchanger? Is the radiator cap on the fresh water tank holding pressure? Are you getting the right amount of water out the exhaust? The exhaust elbows block up on on some of the Yanmar 3 series. Sometimes its the just nipple, sometimes its the elbow.
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Old 04-25-2012
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Re: Yanmar 3GMF Overheating

Quote:
Originally Posted by gordofloats View Post
Folks,

Last season my 3GMF started overheating after 10 minutes at full throttle (4000rpm). Previous seasons she would run at full for hours with not a complaint. Raw water system is clear. New impeller. Regular oil and filter changes. The one thing thing I did differently before last season was change the coolant. It had been green and I replaced it with Canadian Tire's Diesel Coolant which was red, which I put it in full strength. Could this have affected the heat exchangers efficiency? There are no other indications that I can see.
Pull and clean your mixing elbow. Obstructed mixing elbows seem to be a common problem on these engines. In theory you should get about 500 hours on an elbow but I have helped several fellows in our marina with the same engine that have found otherwise. A cleaned out elbow will not last as long as a new one but will work for some while.

FWIW...
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Old 04-25-2012
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Re: Yanmar 3GMF Overheating

Thank you all, really useful responses

I know my raw water sea-**** is clear - I always check it ever since overheating after leaving a weedy berth a few seasons back. And my old impeller came out looking new. I haven't checked the mixing elbow so I'm heading down to the boatyard now to do just that. How do you clean the heat exchanger? Just flush it with water? I've just gathered from reading other threads that green and red coolant shouldn't be mixed. I should've done a little more searching before doing that change last year. The problem is that none of the recommended coolants in the manual are available here so I did a little guesswork.
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Old 04-25-2012
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Re: Yanmar 3GMF Overheating

I just got nautically censored. Of course, I meant seacock.
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Re: Yanmar 3GMF Overheating

Quote:
Originally Posted by gordofloats View Post
Thank you all, really useful responses

I know my raw water sea-**** is clear - I always check it ever since overheating after leaving a weedy berth a few seasons back. And my old impeller came out looking new. I haven't checked the mixing elbow so I'm heading down to the boatyard now to do just that. How do you clean the heat exchanger? ....
Depending upon how many hours you have on the engine, a simple counter-flow flush might work, but not well. Some will advise you to pull the tube bundle and have it "boiled out" but that may not be necessary (particularly if the mixing elbow is obstructed). You can get a pretty good cleaning of the tube bundle by disconnecting the raw water input line and the nipple to the injection spout on the mixing elbow and temporarily connecting some hose that you can lead back to a bucket to each. Connect one of the hoses to a small submersible bilge pump ($20-$30 at West Marine) that you immerse in the bucket (powered with jumper-leads to your battery) and fill the bucket with a mix of "Barnacle Buster" or "RydLyme" and water (following mixing instructions on either). With the pump activated, this mix will circulate through the heat exchanger and back to the bucket. Let that run for a few hours and then discard the mix and replace it with fresh water and circulate that for awhile. Counter flow, i.e. in at the mixing elbow end of the heat exchanger and out at the raw-water end seems to do a somewhat better job of "cleaning" the tubes. Finish by connecting your garden hose to one end of the loop and flush with that for a few minutes (you'll be surprised at the amount of sand and gunk you wash out) and then reconnect your raw water lines and injection nozzle.

Before going to all of the trouble of the foregoing, however, check the injection nipple on the mixing elbow.

FWIW...
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Old 04-25-2012
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Re: Yanmar 3GMF Overheating

You should always replace the thermostat(s) if an engine has overheated. I would also check the belt and tension that drives the raw water pump. You didn't say if it only overheats when run for long periods at full throttle, but if it runs fine at a normal speed of 70-80% of full then it may just be the belt slipping a bit. Is the 3GM rated to run at full rpm continuously, I thought it was only supposed to be run at full intermittently?
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Old 04-25-2012
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Re: Yanmar 3GMF Overheating

4000 rpm is very high for this engine. I have a 3GMF and run it at less than 3000, usually around 2700 which seems to be its sweet spot at hull speed which for me is around 6.5K. Below 2500 for extended running, these tend to glaze the piston walls. If I remember right, the 3GMs reach peak torque at around 3000 (I don't have my service manual torque curves here to give you the exact #). If you need to do 4000 to get to hull speed it may indicate that your wheel is too small. At "full throttle" I don't think you should be able to run the rpms up that high. That high, you may be vulnerable to overheating with any kind of small abnormality. Check the thermostat as mentioned by jrd. Also check your sending unit to be sure you are actually overheating.
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Last edited by smurphny; 04-25-2012 at 11:49 AM. Reason: grammar
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