Coolant problem when starting from warm

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Sidedrum

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My Yanmar GM20 (raw water cooled) developed a problem on my last trip. If I started the engine from warm, the coolant water wasn't coming through causing the engine to overheat. I checked the obvious (the stuff I know how to check!) - the strainer was clear, the impeller OK, the water pump turning - but it happened twice when I started the engine from warm. The water was coming through fine again when I started the engine after it had cooled down - and remained flowing as the engine warmed up.

Please can anyone advise on possible causes?
 
Air getting into the intake pipe, probably from either a loose clamp from the cap of the strainer. Could also be a worn water pump face plate or squish plate allowing water to drain down after you switch off.
 
This is a strange one! My strainer is level with the top of the engine so when I take the lid off to clean it the intake pipe will empty under gravity. This means that it takes longer for water to exit the exhaust, but the pump still self primes OK so I'm not inclined to think it's air in the intake pipe.

I don't see how it could be the thermostat either. Surely if the engine's warm, that should already be open? If it opens OK when the engine is started from cold, why would it not do so when started from warm?

It's possible that the intake has become obstructed - perhaps by a plastic bag or the like sucked up underneath it. After a period of time - which would also enable the engine to cool down - it's floated away. Another possibility (and I don't know why the situation should be different when warm to when cold) is that the impeller shaft is spinning but the impeller isn't. I understand that in some cases the impeller rubber is only bonded to the metal inner that is driven by the shaft and that all can appear well to a visual inspection when the two have parted.

I'm baffled really..... But do let us know when you find the cause!
 
Thanks for the replies - don't think it can be a thermostat problem since I found out there was a problem when the alarm went off which I assume is triggered by the thermostat.

The intake was definitely clear - the second time it happened we were becalmed for over an hour, so I took the cover off the impeller. (Better that than sit out on deck with my crew, trying to look nonchalant as we drifted with the tide, all sails set, all sails limp ....) With the seacock open there was definitely water coming through. I know that with a raw water cooled system you get a build up of crystals over time, so I wonder if that's the problem - but I don't see why it would only happen when starting from warm and not when the engine warms up when starting from cold. I'm at the limits of my (very limited) engine knowledge here ....
 
Don't confuse the thermostat with the temperature sender. The thermostat mechanically regulates the flow of cooling water through the engine and so is an integral and important part of the engine cooling system. The temperature sender is part of an electrical circuit that monitors the temperature of the coolant in the engine and alerts you to a high temperature. They are two completely separate components.

A raw water cooling system is essentially very simple. It consists of an inlet seacock which is (usually) connected via a rubber hose to an inlet strainer from which another hose takes the water to the raw water pump. The rubber impeller within the raw water pump is turned either by a direct mechanical drive from the engine or by a belt. I don't know which it is on your engine as I'm not familiar with it. The raw water pump pushes the water to the thermostat housing via another pipe which could be metal or rubber. My understanding is that, if the engine is cold, the thermostat directs the raw water such that the majority of it bypasses the engine water jacket and is injected straight into the exhaust. If the engine is warm, the thermostat will direct the raw water through the water jacket of the engine (to absorb heat from the engine) after which it will be injected into the exhaust.

Possible problem areas starting from the inlet are therefore:

  • Obstruction in the inlet / seacock
  • Obstruction in the hose from the seacock to the strainer
  • Obstruction in the strainer
  • Obstruction in the hose from the strainer to the raw water pump
  • The impeller being damaged, not turning or not turning fast enough
  • Blockage in the hose / pipe between the impeller and the thermostat
  • Obstruction or partial obstruction within the engine water jacket
  • Blockage between the thermostat outlet and exhaust injection point
  • Blockage in the exhaust manifold

The fact that you get good coolant flow when the engine is started appears to rule out all of these with the possible exception of:

  • The impeller being damaged, not turning or not turning fast enough
  • Obstruction or partial obstruction within the engine water jacket

The following is a long shot based solely on the fact that you only report a problem when the engine is started from warm...

When you start the engine from cold, the thermostat will allow water to bypass the engine's water jacket and be injected into the exhaust. There may therefore be little resistance to water flow in the system and the raw water pump is therefore able to supply water ok. As the engine warms up, the thermostat starts to divert water through the engine jacket. This may increase the work that the pump has to do as it is now pushing the water through smaller bore water passages which may be partially blocked. However, because the load comes onto the pump gradually, it can keep up with it and continues to supply raw water.

When you start the engine from hot, the thermostat is set from the start such that the water has to flow through the engine's water jacket. This load all at once may be too much for the pump and expose a weakness somewhere. An imperfect analogy would be pulling away in a manual car. Let the clutch up gently and the car will move away. Quickly take your foot off the clutch and the car will stall as the shock load is higher.

If the raw water pump is belt driven, is the belt too slack such that it slips and doesn't turn the pump or turns it too slowly?

Is the impeller damaged? To be absolutely sure that it isn't, I would remove it to make sure that the rubber part of it can't move independently of the metal centre. If you have a spare, swap them to rule this out.

I can't think of anything else it could be other than a blockage of the inlet by something like a plastic bag that just happened to occur when the engine was already warm and float away before it was started from cold.

As I've admitted, this is a long shot(!) but in the absence of any other advice I'd certainly look closely at the potential issues I've described with the raw water pump before looking any further.
 
Houleaux,

Many thanks for such a clear and detailed response. I really appreciate you taking the time to alleviate my ignorance, and now have a much better idea of how a raw water cooled system works, as well as the difference between thermostat and temperature sender.

You've given me a couple of possibilities: as the pump is belt driven, and since accessing the pump requires removal of one of the belt's pulleys, it's certainly possible that I've left the belt with too much slack; and although I've checked the impeller (new this season) I was checking for wear in the rubber and not slipping around the metal centre. So I'll start with those two.

I'm confident that it's not a blockage of the inlet - it's unlikely as it happened twice, and the second time it happened I opened the seacock with the top of the strainer off and again when I'd taken off the impeller cover, and there was a good inwards flow both times.

Heading up to the boat tomorrow, so I'll look at the two things above and report back here in due course.
 
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