Overheating Mermaid

[iain]

Any suggestions as to what is causing my overheating problem would be welcomed before I give up boats and take up golf.
I have a Mermaid Majestic 6 cylinder Turbo intercooled engine which started overheating about two months ago. It has a bypass thermostat and I and mermaid felt that this was probably the cause so it was replaced. However this didn't cure the problem, although it was a bit more intermittant. (Temp guage going up to 100c then suddenly dropping to 50 before slowly creeping up to 83ish which is about normal) I checked out the fresh water pump and it was leaking slightly from the tell tale and will need replaced in the near future but according to Mermaid this should not affect the temprature. I have stripped all the fresh and raw water circuits down and cleaned them and there is plently of raw water flow at all times. When I refill the circuit with coolant and it heats up the header tank starts to boil off and vent its brown contents all over my once nice clean engine. I have tried venting the system at the highest point on the turbo until I get water flowing but after five minutes or so if I vent it again there is more steam. I have spent literally hours venting it in this way and thought she might be drawing in air. I have triple checked all hoses and connections and the head gasket seems OK so if it is drawing air then I have no idea from where.
Mermaid have been very helpful as usual but I cant help thinking they will be getting fed up with me by now. SO any suggestions?

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[longjohnsilver]

Have you rodded the stack in the header tank? And the coolers? Is the impellor OK?

Chris, Cyril and others at Mermaid are the experts, always helpful. Are you sure the new thermostat is working as it should? Maybe worth checking.

I'vr hsd exactly the same problems as you in the past, unfortunately the engine gave up the ghost in the end and I replaced it with a factory rebuild 4 years ago. Not expensive and so far so good, had problems with the head gasket and Mermaid replaced it 3 times before they realised they had a faulty batch of gaskets. All done without any charge to me, excellent service.

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[Nauti Fox]

Sorry to say it sounds like a problem I had with one of my GMC's.I had to bleed the coolant before starting or it would overheat.I tried everything else but in the end had to have the heads off(not fun or easy as they are a hernia inducing 10.5 litre cast iron V8 lump).Turned out it was a very badly made head gasket which partly covered the coolant holes and had finally let go.
Regards Al.

<hr width=100% size=1>No dear,the water goes in the other one.

 

[brianhumber]

Assuming heat exchanger has also been checked out for bits of plastic etc that could move and cover/uncover tubes. I am concerned you seem to be getting a lot of air/gas in your primary circuit but any water in the hottest part of the circuit will 'flash off' into steam when the system is unpressurised. If it is air, unfortunately this would indicate trouble with the head/ or head gasket. However before looking closer in this area, recheck all the simpler items first again, pipes and heat exchanger including the injection nozzle into the exhuast. Is the temp guage actually correct etc
There is quite a bit of expansion to the primary circuit between 20C and working temp so if the circuit is filled right up you will expand water out of the filler cap.

Do you overheat on full load or tickover? do you get 'air' on tick over or loaded?

These problems can very frustrating as in my exprience sometimes the more you look the less you see. Somebody thens comes along and points out the obvious! so don't get disheartened.
Brian

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[omega2]

We get a flash overheat alarm on our 180's if we give them the gun too early in the warming up period, we let them warm gently at about 1500 rpm before we go for it. Also we had a head gasket that was blown between 3/4, caught the water in a bottle and put it back in the header tank when it was cooled off. If you do this you will see what you are losing, as taking the cap off whilst warm will vent anyway even with a perfect engine, some engines will find their own water level, and as far as I am aware the system is self venting. Whatever you do, do not try to drive through a high temperature, or you could seize the engine, just ease off and let it drop. Cannot understand why you are rising and falling though unless the sender/electrics are at fault. Good luck with it.

<hr width=100% size=1>J HAMER

 

[omega2]

Sorry I mean let them warm up under load, do NOT try to warm up off load this will do nothing for the engine.

<hr width=100% size=1>J HAMER

 

[gus]

It might be also worth looking at your pressure cap on the header tank. I had a similar problem due to a cap that wasn't holding the pressure.

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[oldharry]

The varying temprature may be caused by the coolant dropping away from the sender so that it is only measuring the heat of the casting it is mounted in, and not the actual coolant temprature. It may also be nothing more than a faulty sender! I had an Astra blow its radiator on the MWay, and the temp guage did exactly the same as yours - rose a little then dropped to a low reading - until the block had warmed up to near meltdown! The guage only registers the temprature in the immediate vicinity of the sender - not the coolant temperature elsewhere in the engine!


Otherwise the symptoms you ndescribe suggest a compression leak into the coolant - usually via the head gasket, or a crack in the castings. If the cylinders have wet liners, a liner seal may have failed - particularly if the head has been removed recently and has disturbed the liners.

You need to know whether there is excess pressure building up in the coolant system. From a cold start, if there is pressure on releasing the radiator cap before it has properly warmed up - and particularly if there is a rush of coolant when the cap is relieved - then this indicates a compression leak into the cooling system, which needs to be tracked down.

You say the header starts to 'boil off'. If this starts with what appears to be a stream of froth or tiny bubbles in the coolant, getting worse as it warms up, looking almost as if someone has put a few drops of washing up liquid in the coolant, then you have a compression leak. If so let us just hope it is the gasket and not a cracked casting resulting from an overheat caused by the coolant loss from the water pump.

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