2nd Shake Down Cruise Failure.

[Capn Jack]

I think unfortunately that there can be a lot of conjecture about this sort of thing, because there is so much that can cause this problem, from the circulation pump to the impeller to blockages.

Looking back, your impeller has been changed at the annual service, it is working but the techy says it could require priming....whats all that about?? Since it wasn't overheating last year therefore the pump would have worked then!!

Next put your hand on the manifolds and see if they are excessively hot, this is a good givaway to let you know if there is a problem with them.They are either very hot or cool (assuming the engine is up to temp).

If the manifolds were new in 2003 then they are overdue for change but it doesn't mean this could be the cause, which is why I asked about the change.

My next suggestion would be to check to see if you have a good flow of water through the engine.....you should do as dpb says and connect muffs but to let you see the water flow through the engine. It won't neccesarily let you know if the pump is working perfectly because of the water pressure from the hose, but it will let you know if there is a good flow.

[salamis]

Try running it up without the thermostat in and see if the engine still gets hot.
My guess it has failed and will not open to heat.

[Gazjen]

Or Thermostat mounted up side down (if possible)

[Capn Jack]

Try running it up without the thermostat in and see if the engine still gets hot.
My guess it has failed and will not open to heat.

Yep and as Gaz and Salamis have said, if it is the thermostat playing up you'll see it if you run it on muffs. It should be the same as all other Mercruisers when started the water won't circulate fully but when the thermostat opens the coolant flow changes.

[dpb101]

Try running it up without the thermostat in and see if the engine still gets hot.
My guess it has failed and will not open to heat.

Yep and as Gaz and Salamis have said, if it is the thermostat playing up you'll see it if you run it on muffs. It should be the same as all other Mercruisers when started the water won't circulate fully but when the thermostat opens the coolant flow changes.

I am struggling with that concept Cap'n! I dont think you visually see any change to the flows....

The clue I think is that it has gone from fine to not fine suddenly, therefore unlikely to be manifold related as that is a more gradusl effect.
Certainly points to checking what has been done in between first.

[Gazjen]

I don't understand how ageing manifolds can make your engine over heat? unless your saying they are blocking the return of water back through them

[Capn Jack]

I am struggling with that concept Cap'n! I dont think you visually see any change to the flows....

Unless I'm wrong here, when the thermostat is shut, there is minimal water flow to the manifolds and only to the elbows and block, the coolant basically circulates with a small amount exiting via the elbow sand through the exhaust outlets. When the thermostat opens the manifolds, block and elbows all receive coolant. This is usually plain from the flow from the exhaust outlets in the sterndrive so a greater flow appears.

Ageing manifolds will slow the flow of water around the block, although to be fair, overheating because of this usually manifests itself when at speed...JP said that it overheated when at idle the other day.

The symptoms are all pointing towards a stuck thermostat, or the impeller or impeller housing, however I wonder if it could the water circulation pump failing. This would certainly cause the problem at idle.

I still go with dpb's idea of muffs to check flow, plus you can see if there is an increase of flow when the thermostat opens. Only put the tap on a little though, just enough to wet the impeller and not enough to pump water around the engine untill the engine fires then increase if you need to a little.

[JORIDAPILOT]

All this conjecture is certainly interesting and for the benefit of all, an update following another visit by my engineer.
He took the water pump apart which was fine, no evidence that the impellor had been running dry. He took the thermostat out checked all round and put it back.
Fired up the engine and gave it 2000rpm for 5 seconds or so once it started and then reduced to idling. He then monitored it for 20 minutes. And by careful observation of the temperature guage could tell that the thermostat was opening and closing by the needling swinging up and down slightly. I have a piece of tape on the guage for normal so he was able to judge by that. In short, nothing wrong.........
This of course he told me by phone, I did not see for myself, however he has always been reliable and reasonable so I have no reason not to believe him.
I hope to get down to Saxon one evening or at the weekend to move the boat round to be lifted out. If I have time and the engine is OK I will take a trip down to Southampton Water to thrash the nuts off her, as they say I need to have her lifted as we are off to Cornwall next week and another weeks growth on the hull will negate my two days of solid polishing. I have already marked the surface with a slight coming together with the timber edge of the pontoon. I must sort out the fender spacing a bit better
These events do serve to knock the confidence that you normally have in your engine, I will now be forever expecting it to go wrong, perhaps I had better buy a rowing boat.

[markanddawn]

All this conjecture is certainly interesting and for the benefit of all, an update following another visit by my engineer.
He took the water pump apart which was fine, no evidence that the impellor had been running dry. He took the thermostat out checked all round and put it back.
Fired up the engine and gave it 2000rpm for 5 seconds or so once it started and then reduced to idling. He then monitored it for 20 minutes. And by careful observation of the temperature guage could tell that the thermostat was opening and closing by the needling swinging up and down slightly. I have a piece of tape on the guage for normal so he was able to judge by that. In short, nothing wrong.........
This of course he told me by phone, I did not see for myself, however he has always been reliable and reasonable so I have no reason not to believe him.
I hope to get down to Saxon one evening or at the weekend to move the boat round to be lifted out. If I have time and the engine is OK I will take a trip down to Southampton Water to thrash the nuts off her, as they say I need to have her lifted as we are off to Cornwall next week and another weeks growth on the hull will negate my two days of solid polishing. I have already marked the surface with a slight coming together with the timber edge of the pontoon. I must sort out the fender spacing a bit better
These events do serve to knock the confidence that you normally have in your engine, I will now be forever expecting it to go wrong, perhaps I had better buy a rowing boat.

Or even a rag and a stick or is that going too far sailing, now that is something I can help or advise on

[JORIDAPILOT]

Or even a rag and a stick or is that going too far sailing, now that is something I can help or advise on

definately going too far, I don't mind crewing on one now and again but buy one...not whilst there is petrol on the forecourt

[markanddawn]

[Gazjen]

I am struggling with that concept Cap'n! I dont think you visually see any change to the flows....

Unless I'm wrong here, when the thermostat is shut, there is minimal water flow to the manifolds and only to the elbows and block, the coolant basically circulates with a small amount exiting via the elbow sand through the exhaust outlets. When the thermostat opens the manifolds, block and elbows all receive coolant. This is usually plain from the flow from the exhaust outlets in the sterndrive so a greater flow appears.

Ageing manifolds will slow the flow of water around the block, although to be fair, overheating because of this usually manifests itself when at speed...JP said that it overheated when at idle the other day.

The symptoms are all pointing towards a stuck thermostat, or the impeller or impeller housing, however I wonder if it could the water circulation pump failing. This would certainly cause the problem at idle.

I still go with dpb's idea of muffs to check flow, plus you can see if there is an increase of flow when the thermostat opens. Only put the tap on a little though, just enough to wet the impeller and not enough to pump water around the engine untill the engine fires then increase if you need to a little.

Water flow wouldn't really change because when the stat is shut water is diverted to the manifolds and out when it's open it's allowed around the engine, well technically the stat allows the hot water out.

I had a drawing somewhere for mercruiser coolant flow for when I was going to fit a warm air heater