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[Mikes1670]

For the second month in a row I have just had to top up my coolant bottle with 50/50 mix - on each occasion it took approx. half a litre. I am doing mainly short runs at the moment. Is this normal? There are no apparent water leaks that I can see  :-?

[MutantCav]

There is a good chance that your Heater Bypass Valve is leaking...it leaks onto the exhaust and evaporates so you dont generally see the leak...have you have the aircon on a fair bit recently??

[Mikes1670]

No I haven't used the aircon that much recently as I have been doing mainly short runs - how would that make a difference?

[Jimbob]

the aircon, can, but not always affect how the HBV leaks.

It is a very common, probably the most common cause of omega water leaks.  Unfortunately not the most accessible component, but not that bad either.

[tango]

If your losing that much water then thers definitly a leak
And most likely the hbv
Have had a look around the coolant rad to see if there's any water dripping anywhere

[Gaffers]

It may well be the Hbv but it isn't always.  Mine was a little more complicated and the Hbv change did nothing.  Whip the scuttle off and sit a small shallow tupaware container underneath the Hbv.  Run the engine for an hour or so and see if it is leaking.

[feeutfo]

It should use no water at all. Hbv diverts the water according to heater or ac settings via the vacuum pipe on the top of it. So heater settings affect water loss depending on where the leak is on the hbv itself. Sometimes they leak dramatically dumping a good few litres on the floor, other times they only leak slightly depending on heater settings so the leak can almost appear seasonal.

However if the leak is slight and constant with no sign of your coolant colour or residue in and around the hbv area at the back of the plenum under the scuttle, then the next place to look is usually in the v of the engine, oil cooler cover plate, stat housing, coolant bridge, or water pump in order of likely hood. Pressurizing the coolant system can help here via a homemade tyre valve sealed into a spare water bottle cap. Or simply seal your mouth round the neck of the bottle and blow into it. Little pressure is needed.

Hth

[feeutfo]

Also as said, if the leak is slow as yours is, it's by no means unusual for the coolant to evaporate off due to heat before any sign of it hits the floor. However there will be residue apparent, usually down the back and around the bell housing.

If you take it to an independent do NOT allow them to tell you it's head gasket  related as that's complete 'dangle berries' and equally common on the omega.

[Mikes1670]

Cheers for that I will investigate further at the weekend