Yanmar 1GM Overheating

[FairhopeBill]

Team,

I've got a 1982 Yanmar 1GM, on a Cape Dory 25D, that is overheating after about 15-20 minutes running at idle. Seeking advice.

Here is what I've done so far:
- Replaced the thermostat
- Replaced the mixing elbow
- Checked/replaced the raw water pump/impellar
- Checked / cleared all hoses
- Flushed the system with acid based solvent

During the flushing process I observed great water flow cycling from the bucket out of the thermostat and back in to the bucket. Once the engine heated up, I think the thermostat engaged and the water flow reduced by 3/4 flow and within 2 mins the overheating alarm went off. I also noticed the liquid moving through the engine was getting hot just prior to the alarm engaging.

My next option is to take the head off and just overhaul the whole thing by removing the head gasket to make sure all the cooling ports are clean and make sure there are no stray broken impeller pieces blocking the flow of water through the head. I've not raised the yet but after that I'm not sure what to do, therefore seeking sage advice from this forum. Thoughts?

Fairhope Bill.

[LittleWing77]

Why is water flow reduced by 3/4s when the thermostat engages? Seems to be related, as the overheating alarm is triggered just after the cooling flow is reduced. Something in there is expanding with the heat and blocking your cooling system somehow, sounds like.

Side questions: When you took the old impeller out, were there any vanes missing or pieces broken off? Was the impeller installed correctly and the replacement identical to the one you removed/or is the correct replacement, as specified in the Service Manual?

You are seeing cooling water coming out of the stern exhaust as the engine is idling, yes?

[sv_pelagia]

Does it overheat without the thermostat?

[lockie]

Might not be relevant, but I once had a cooling issue that persisted despite replacement of impeller, thermostat, hoses etc. The cause turned out to be some sort of hard marine growth inside the CW thru-hull, which I only detected by poking a long screwdriver through with the valve open.

Cheers, Graeme

[Compass790]

Welcome to the forum Fairhopebill, good advice upthread, I dont know how the 1GM is setup but a good tip for the future is to put a strainer after the water pump if possible to stop broken off impeller blades from blocking cooling water galleries. Otherwise +1 for try running without the thermostat for a start. You certainly have a serious issue if it overheats at idle

Following with interest

[FairhopeBill]

Thank everyone for your replies.

I ended up taking the head off today and there were significant clogs in the cooling ports. I’ve been cleaning them with a pipe cleaner and solvent cleaner. My guess is that’s the problem. Once I get all of the new gaskets I need to reinstall I’ll complete the job and let you know how it goes. If you see something I don’t in the photos, please let me know.

To answer some of the questions, there was water coming out of the exhaust but local captains stated that it wasn’t enough. Clearing the ports I think will be good.

Bill

[FairhopeBill]

Here is where I am with the cleaning so far…

Bill

[FairhopeBill]

With attachments…

[LittleWing77]

Originally Posted by FairhopeBill

Here's where I am in the cleaning so far....

BEauty!!!

Veddy nice. Your engine will thank you for this!

LittleWing77

[FairhopeBill]

Originally Posted by LittleWing77

BEauty!!!

Veddy nice. Your engine will thank you for this!

LittleWing77

Thank you LittleWing77. I hope it works!

[Compass790]

What does the bore look like?
I'd expect a bit of water damage as your head looks rusty.
I'd chuck a new set of rings it it as well if you can pull the piston easy enough. Well you could measure the ring gap anyway.

[LittleWing77]

Originally Posted by Compass790

I'd chuck in a new set of rings it it as well if you can pull the piston easy enough.

+1

Might as well, eh? Head's off anyway...

LittleWing77

[jimbunyard]

Have seen worse blockages, it's surprising that what is shown would make the engine overheat at idle.

That the water flow reduces after the engine heats up would seem to indicate that something is amiss.

Assuming the engine is raw water cooled, do you have the right temperature t'stat, and is it installed in the right direction?

Won't hurt that you have it apart, and if the cylinder looks sketchy, a honing and rings might be appropriate, providing it isn't too difficult to remove the engine, and perhaps a valve lapping as well -- depending on how well the engine ran (besides the overheating) before.

Of course, if you go that far, you may as well throw in rod and main bearings, and you'll have a 'shade tree overhaul', probably good for at least several hundred hours...

[LittleWing77]

Originally Posted by lockie

Once had a cooling issue that persisted despite replacement of impeller, thermostat, hoses etc.

The cause turned out to be some sort of hard marine growth inside the cooling water thru-hull, which I only detected by poking a long screwdriver through with the valve when open.

This may be a consideration as well - Ghoulies creeping into the raw-water inlet.

Here in the Great Lakes, it's zebra mussels that love these spots...

What's the local bugbear in Fairhope, Alaska?
LittleWing77

[Chayal]

Originally Posted by sv_pelagia

Does it overheat without the thermostat?

Might be good to confirm your new thermostat is working and not stuck. I’ve been burned by a dud before.