overheating 3QM30

[bmjpv]

I''m new to this forum but old with my problem, I've had my hunter with the 3QM30 for about 12 years but this intermitant overheating problem has plaggled me for years. I'm a retired auto shop owner so I know a little something grease and dirt, but my product knowlege is not enough on this yanmar. What it's doing is at idle running about 110F but when it goes into gear (today tied to the dock) it'll run up to the 240F range before I turn it off. All impellors were good yesterday ,salt water strainer and flow looks normal and clean. thermoatat has been replaced in the past and all hoses are below the filler neck with no air pockets! I've always felt that evertime It runs too hot that a little life comes off the headgasket. Is this maybe what's going on while the engine is under load? How tough are these headgaskets? I need ideas to get this dependable again. thanks, joe

[never monday]

Joe,
QM's are verry strong. I'd put them on par with 454 in ability to withstand an overheat.
There are a few things that I wonder about in your scenerio. Is the QM raw water cooled or closed cooled? What transmission is atached? WHen was the last time the cutlass bearing was replaced?
What is the color, & quantity of smoke being produced in gear and out?

[bmjpv]

Good to hear a responce,I've got a saltwater impellor and strainer and a closed engine (antifreeze) system with other pump/impellor system. Good to know that the headgaskts are tough,that job I didn't want to do! After sitting for awhile It started rignt up ,smooth and would run to 3200 ,unloaded ,It had a little bit of smog and a small fuel slick that both cleared up. I had the pump and injectors rebuilt about 3-4years ago, what's next? The cutless b,earing was done 6mo ago and the trans I'll have to look up,doing that now thanks joe

[SkiprJohn]

Aloha Joe,
When you put it in gear how much rpm can you turn before the temp starts to climb? Is the prop or running gear fouled? Is your prop too large? Lots of questions I know but if the engine runs to 3200 and doesn't overheat then I think you can eliminate an overheating problem and start looking at a running gear problem. Just a thought.
Kind Regards,
JohnL

[bmjpv]

Capt, Ya, my engine will do 3200 out gear and about 2300 tied to dock in gear,very little smoke at loaded/max trottle. I have the boat cleaned every month because the water is soo warm right now,cleaner/guys i've used for years. Question, how long can I hold this engine at max RPM out gear, safe to do this?? I have been thinking along the lines of gearbox to prop problem . Can gearboxs cause engine to overheat? I've had the same prop since day one for me, now that I think of it maybe a time to down size the prop and retest, God I might be getting smarter?? I'd like to hear all ideas! thanks joe

[Alan Wheeler]

2300 from 3200 is far too much. 2850 is about as low as it should go underload/max throttle, 2900 preferable. Soudns like you are seriously over pitched. This will cause major over heat issues.

[talmeg333]

lots of answers but none correct. your heat exchanger must be pulled and dropped in muratic acid. it is quite simple...remove alternator so you have access to remove exchanger. there are 2 bolts or 3 on front cover and 2 or 3 on rear cover depending on type of exchanger. once alternator and cover plates are reomoved take a 2x2 of pine and a mallett and drive it towards the dir. that alternator was b4 you removed it..may take a little force, but do not over do it as the exchanger is made of brass and copper. once removed, simply drop it in the acid for about 2 min..
You must drain the heat exchanger of coolant before removing end caps. there is a drain plug at the base of it usually midway. you will also need new o-ring gaskets for the end caps. around $8-12 for the pair, there might also be a rubber seperator strip about 2 " long on the one end that seperates the 2 sets of tubes which would come with the o-ring kit if that model heat exchanger requires it....
Another problem could be your oil cooler on the back of the engine..it looks like a "V" shape..it could have build up in the cooilng jacket....try the heat exchanger first as your only expense with that is the o-rings and coolant....if that does not cure your issues..then remove to the oil cooler..the reason I say this is because a new one must be purchased plus new oil galley bolts and 4 copper washers..the oil cooler is around $60 to $200 depending on style.. I am a marine mechanic here in Key West part time...Any ?'s feel free to E-me at talmeg333@yahoo.com..
J.D. Reef Point boat repair and hull cleaning....Key West Fl.
I am a sailor myself and also live aboard my Morgan 382..please feel free to verify this info with a Yanmar dealer

[talmeg333]

Would like to add one more thing for all who are reading....In my past and present exp.. These two items in the fore mentioned and of course impellars are the #1 killers of motors.....many people look for the technical side of things when in reallity it is as simple as the sun rising...everyone do yourselves a favor and treat your lady good.....clean your heat exchangers and replace your oil coolers when needed you will get much more life out of the old girl....I recommend cleaning the heat exchanger atleast every 2 years....also if you see a little milk in your coolant...don't panic right away..9 times out of 10 it is just the oil cooler..and it is a 30 minute to one hour repair to put a new one on depending on how seized the bolts are....

[DeepFrz]

I would also suggest you check the exhaust cooling elbow. If this gets gunked up you will have a reduction in raw water flow which may not be noticable but can cause overheating.

[bmjpv]

On my overheating problem I pulled the heat exchanger last year( lucky to have and extra) and both looked good. Now I'm thinking ,why when I bought the boat 12 years ago did he have an extra heat exchanger? I will get that looked at and cleaned ,good coverage! As for the prop and engine speed, Please make this clearer, my loaded in gear RPM is aprox 2-2300 RPM and my unloaded RPM out of gear is 3200, What is safe unloaded RPM ? and what is the engines best RPM when proped correctly? I've felt over the years that I might have too much prop, I also forgot what size I do have, any sizes that work with my engine/boat size . Again it's a Hunter 37'. thanks joe

[SkiprJohn]

Aloha Joe,
We've had many overheating problems on this forum. The majority were cured with a prop repitch. Search in the archives for proper prop size and related issues with overheating. My suspicion is that the prior owner also had overheating issues and the wrong size/pitch prop. Its nice to have a clean exchanger too and hope that it is. Alan stated it correctly. If you can get 3200 out of gear without overheating, you should be able to turn at least 2850 in gear without overheating. If you've never been able to do that then your prop is the wrong size and/or pitch.
Kind Regards,
JohnL

[Alan Wheeler]

lots of answers but none correct.

Telmeg333 that's a rather strong opinion of your ability to diagnose isn't it.

As for the prop and engine speed, Please make this clearer, my loaded in gear RPM is aprox 2-2300 RPM and my unloaded RPM out of gear is 3200, What is safe unloaded RPM ? and what is the engines best RPM when proped correctly?

Ok, firstly, a Diesel engine is governed. This means you can safely rev it to maximum RPM when in neutral.
Next issue is loaded RPM. The engine needs to be able to reach close to max RPM. A rule of 10% is the usual. So as I said above, if you can get 3200 from it, then 2900 would be what I would expect to see when in gear. This is full throttle and is not your cruising RPM. If you can only get 2300 max throttle, then you are something closer to 25-30% of max RPM which is way too much. This results in excessive load placed on the engine and results in excessive heat produced. Plus at 2300RPM, your Hp figure will be way down. So the boat will not be as responsive as it could be. Plus very uneconomical and in the end, very bad for the engine.
Do you know what the current propellor pitch is?
If you can give the the gearbox ratio, I can give you a very close approximation of the right pitch.

[never monday]

I agree with Alan

[bmjpv]

Good morning, Thanks alan/john and all for that info on props/RPM ,This problem that I have had to deal with was never given my full attention until now. Without acting like I have this overheating fixed, I do feel that I have been given the correct knowlege to resolve my problem .As a retired auto shop owner I've always had a general idea about what I had to deal with but didn't have the product knowlege or the focus to resolve it. I'll recheck/repitch prop and clean the heat exchanger and rerun the engine. I'll repost with results at a later date. Again, thanks to all for the input and direction. joe

[Ex-Calif]

What is meant by intermittent?

Does it happen some days and not happen others?

There may be more to this than meets the eye. Although the low RPM under load may indicate overpropping it may indicate an inability to produce full power.