Coolant overflow - its a mystery

[Chris_B4]

My starboard engine (1997 Mercruiser 250hp 4.2 litre D-Tronic diesel) has been overflowing it's coolant header tank and everything I’ve read here suggested head gaskets.

Engine chap came out with me to do some tests and not as straight forward as it might seem. Pressure testing the coolant system showed a leak but could not be traced. Turns out there is no carbon monoxide coming off coolant filler so can't be exhaust or combustion pressure, but the coolant is frothing so definitely gas getting in. There is also no exhaust type smell from header tank, no water in the oil and the coolant is not discolouring. Concluded it must be charge air from the intake side of the engine so may be inlet manifold at fault.

Got the intercooler off at the weekend (eventually). Engineer came on Monday night to pressurise the coolant circuit for me so we could see if there was any leak in/around the inlet manifold. Absolutely b****y nothing, coolant circuit held 20psi for ages. Stuck intercooler back on and ran engine at idle to get it warm, still no leak.

Seems I’m going to have to put everything back together again so more tests can be done, but not sure what.

Please, has anyone any ideas how air might be getting into coolant.


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[philwebb]

I am not familiar with your system, but here are a few thoughts. Dip you oil and look for
a creamy froth - this indicates cylinder head leaks on most engines.
Otherwise if you can`t find the leak it must be pretty small - you could try a
proprietary leak sealer eg Radweld from Halfords. If air comes in one way , surely
water must go the other way. Might have to apply a bit of pressure to the water to make it find the leak, then the leak sealer will do its work.
If you do strongly suspect the head gasket though this is not a good solution, as you dont want water in the cylinders.
Is there any way that the compression in the cylinders could be tested by your
engineer? A low reading on one cylinder may give you a clue to where the problem lies.
Regards, Phil

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[philmarks]

Sorry not familiar with engine. Are you sure it's air? I have problem with leak in heat exchanger, raw water gets in to fresh water circuit (end cap fault on heat /exch) and blows cooling water out.

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[Chris_B4]

Once the coolant overflow bottle is full I can return all of that coolant to the heat exchanger. If sea water was leaking in the heat exchanger would remain full and I have been told I would probably see salt crystals around the filler.

Thanks for the suggestion anyway

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[Guest]

Engine Gremlins. Crashing around at sea upsets their digestion and leads to flatulence. Try going slower, or add Gaviscon to the coolant.

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[burgundyben]

how much does it over flow?

you could just leave it till its really bad and then it will be more obvious to find?

<hr width=100% size=1>Sod the Healey - I think I'll buy an E-Type.

 

[spannerman]

What temp are you getting on the engine, is it running hotter then normal?. If so then it could be a blocked cooler or thermostat sticking or insufficient flow from seawater pump, this will cause overheating, hence boiling over which can appear to be gas coming coming from the header tank. Older Volvos are renown for this.
You say the leak test showed a leak, if its internal i.e.headgasket you will almost certainly see white smoke in the exhaust. It would be worth pulling the injectors to see if one has rust on it, and inspect the cylinders while pressure testing to see if coolant appears. One last thought are you filling the header tank to the brim, if so when the engine heats up coolant is forced out into the recovery tank, and will be sucked back in as the engine cools, I assume you know this as we have had this many times with customers not leaving an airspace an inch or so below the filler neck, to allow for expansion so the problem is induced as they keep filling the header, and it becomes a self perpetuating situation.
Hope you get to the bottom of it, Steve

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[VicS]

This could be due to an air pocket in the system due to a blockage in a small water way somewhere. As the engine warms the air expands and coolant is forced out. When it cools down the coolant can be returned to the system.

I had the problem with a car once. I overcame it by fitting a reservoir (plastic bottle) putting the overflow pipe down to the bottom of it, & slightly modifying the pressure cap so that when the engine cooled down it sucked the ejected coolant back into the engine. Over a few cycles the problem resolved itself until the next time the system was drained. The annoying thing with the car was that it didnt originally have an expansion tank so all the expensive new antifreeze simply went on the ground.

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[Chris_B4]

Thanks for your thoughts Steve

Bizarrely, even when the recovery tank is full and some has overflowed into the bilges the engine temperature remains normal. No white smoke visible and no other signs of a head gasket fault which is why we don't think it's that at the moment.

Pulling the injectors seems a sensible thing to do, any other tests you can suggest for when I get it all back together ?

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