2GM20 thermostat

[prv]

Just had an email from Kindred Spirit's new owner. He's taken her to the Scilly Isles (dead jealous!) but is having some engine trouble - the overheat alarm went off despite plenty of water coming out of the exhaust. He's not the most mechanically-minded of chaps and has emailed me asking for advice.

My gut feeling is that it's a duff thermostat (raw water engine, so the only way you can pump water but not cool it is if all the water is going through the bypass). Though of course it could also be a dodgy sensor and the thing isn't really overheating at all. I've pointed him at the test procedures in the workshop manual (boiling the parts in a saucepan) but I suspect he's a bit reluctant.

The specific question I wanted to ask is what happens if you remove the thermostat entirely - does this result in water going round the block at all times, hence overcooling at startup but running ok once warm, as a get-you-home? Or does it still send all the water round the bypass, or perhaps you can't even reassemble the housing without a thermostat in place? I don't know as I never dismantled the thing during my ownership, and I don't have the workshop manual here at work.

There is a "spare" thermostat and temperature sensor on board, but I inherited them with the boat and I don't know if they're actual new spares or old ones which the previous owner kept for indeterminate purposes. To the best of my memory, the sensor looked clean and new and the thermostat didn't.

Any other advice on overheating raw water 2GM20s welcome, bearing in mind that I'm working via intermittent emails and he's anchored in some bay in the Scillies. Also, I know that it's not my responsibility to sort his engine out, but he's nice guy and I want to help.

Cheers,

Pete

 

[VicS]

The specific question I wanted to ask is what happens if you remove the thermostat entirely - does this result in water going round the block at all times, hence overcooling at startup but running ok once warm, as a get-you-home? Or does it still send all the water round the bypass, or perhaps you can't even reassemble the housing without a thermostat in place? I don't know as I never dismantled the thing during my ownership, and I don't have the workshop manual here at work.

There is a workshop manual that you can download from http://www.motoren.ath.cx/

BUT although it clearly states that the freshwater cooled engine should not be run without the thermostat it is not clear if the same applies to the raw water cooled engine.

 

[mtb]

to start with it could be a blocked manifold, if he takes the thermostat out check for blockages at that point which isn't going to show much, then pop it back minus the thermostat. Try it again to see if it clears, also blocking the bypass hose may help
He may well have the strip the manifold out to clear it out. With out the thermostat it will run cold which wont do the engine wear rate or fuel consumption any good
Be sure to tell him not to allow cold water in to a very hot engine the cast iron may crack

cheers
mick

 

[vyv_cox]

In general it is not good practice to run a raw water cooled engine without the thermostat. I have not studied the Yanmar but for Volvos and especially Bukhs it can result in overheating as water will go through the bypass preferentially. With reduced flow through the block local overheating can occur which will result in insoluble salt deposition.

 

[prv]

I have not studied the Yanmar but for Volvos and especially Bukhs it can result in overheating as water will go through the bypass preferentially.

Well, that's what I wondered - it would depend which way round the thermostat operated and how the ports are arranged. But looking at the parts diagram in the manual, it does appear that with no 'stat in the housing the water will just go straight through the bypass. So operating with no thermostat and the bypass in place would not be a good idea. However, you could have a degree of manual control by clamping and releasing the bypass pipe up the front of the engine. Leave it open at first, then progressively clamp shut as the engine warms up. This would be useful to know, if I still had a 2GM20 rather than a new Volvo

Pete

 

[vyv_cox]

I looked at the diagrams in my manual. The Yanmar arrangement looks very similar to the Bukh, where the thermostat acts as a diverter valve, closing the bypass and opening the hot water flow out of the engine. I suspect that removing the thermostat would result in most of the coolant bypassing.

 

[prv]

I looked at the diagrams in my manual. The Yanmar arrangement looks very similar to the Bukh, where the thermostat acts as a diverter valve, closing the bypass and opening the hot water flow out of the engine. I suspect that removing the thermostat would result in most of the coolant bypassing.

Exactly, that's what I meant.

Pete

 

[Appledore]

Not sure that my similar problem with Yanmar 1 GM 10 might help, but see here:

http://www.ybw.com/forums/showthread.php?368040-Yanmar-1-GM-10-Again!!

After flushing the engine (overnight), we went to France and back (230 miles) with no further water temperature alarm problems. So, really, I never did find the 'actual' cause of the alarm problem.

 

[vyv_cox]

A cause of overheating on the Bukh engines is fouling of the injector point into the engine, conveniently placed behind the flywheel where it is very difficult to access. For some reason the injector is a perforated tube with a closed end, rather like an old-fashined gas poker. The holes become blocked with salts, resulting in poor flow to the engine. It appears from the fuzzy diagrams in my Yanmar manual that the injector is a simple fitting, so it may not suffer the same problems as the Bukh, but I would check just in case.

Here is a page from the engine coolant section of my yacht systems course, showing the cooling arrangement of the Bukh. The top fitting on the Yanmar is very similar, i.e. the stepped arrangement of the inlet/outlet hoses, so I strongly believe that it works in the same way.

 

[charles_reed]

From experience with my 2GM20 the problem is down to the thermostat having failed open - that means the raw-water goes straight through the bypass and the engine gets no coolant.

 

[prv]

From experience with my 2GM20 the problem is down to the thermostat having failed open - that means the raw-water goes straight through the bypass and the engine gets no coolant.

My gut feeling is that it's a duff thermostat (raw water engine, so the only way you can pump water but not cool it is if all the water is going through the bypass).

Pete

 

[Spyro]

I looked at the diagrams in my manual. The Yanmar arrangement looks very similar to the Bukh, where the thermostat acts as a diverter valve, closing the bypass and opening the hot water flow out of the engine. I suspect that removing the thermostat would result in most of the coolant bypassing.

That is exactly how it works, I have a 2gm20 raw water cooled.