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[Andywill]

The car (2.0lt petrol) has on its last 2 trips (10 mile) lost a couple litres of water & was overheating.
I tried to recreate this today & it all seems to be working fine.
The fan is running when it gets hot. Should I be looking at replacing the thermostat? or could it be something else.
Any advise would be most welcome.

[Markjay]

It is difficult to say if the car losses coolant first then overheats, or it overheats first then the coolant is lost through the pressure relief valve in the expansion bottle filler cap.

Check for external leaks first. Not sure if 4-cylinder cars have HBV, if so this is a suspect. Then perished hoses or leaking expansion bottle cap.


If no external leaks visible then check for cold spots on the rad when at normal operating temperature. A good flush and coolant replacement with Vx Red Coolant might be a good idea (but again only iof no leaks are visible)


Other possibility failed water pump. What year/mileage is the car, and do you have its service history?

[SP_3.2]

It sounds like the HBV may have gone same thing happed to me a few years back and that had to be replaced. Not sure where they are on a 2.0 but on the v6 there at the back of the enging.

Hope that is of some help.    

[Andywill]

Thanks for your help.
Its a 1998 2.0lt, 110,000 mls with a full service history.
There are no visible leaks.
What is HBV & what does it do?

[TheBoy]

Thanks for your help.
Its a 1998 2.0lt, 110,000 mls with a full service history.
There are no visible leaks.
What is HBV & what does it do?

HBV is a valve that stops coolant entering heater matrix if not required. Only exists on cars with A/C

[Craig_R]

Yep 2.0L have a HBV.

To find it lie under your car on the drivers side and look up where the bottom of the steering box is and its up on the left.

Have a look for any stains around it or on it from the anifreeze.

HBV is heater bypass valve.

Black thing with three hoses and a vac hose

Craig

[Andywill]

Is the HBV only a problem if it is leaking?

[ians]

Its a common fail point.  eg I have had 3 Omega's and changed .....  3 HBV's.

You may find it only leaks when AC is switched on (or only when off),   but coolant loss can be dramatic.
As said previously you will only have an HBV if you have AC though.

[Andywill]

I do have ac so I must have a HBV.
I'll have a look this evening to see if its leaking.
Thanks to all for the advise.

[ians]

Andy,

On my 2L access was best from underneath,  but with that amount of coolant loss you should see it dripping.

The bugger itself is a circular black plastic thing with three water pipes and one vacuum pipe on the top.
Replacement is a bit fiddly but easy enough, but make sure you get an original Vauxhall part (£30 quid if you get trade)

Cheers

Ian

[Andywill]

There is no sign of any coolant leak on the ground when it is parked.
Which did make me think originaly that the cause of the water loss was overheating not the other way round.
But as I said earlier, today it is all working well.
I may well change the BHV anyway.
Is the thermostat easy to change? or is this an unlikely culprit.

[magnul]

Thermostat on 2l is easy to change. It's fitted with two inverse torx bolts where the top hose from the radiator meets the engine block. You could test it by immersing it in boiling water to make sure it opens. It should start opening at 92C.

[TheBoy]

When my hbv leaked, it only did it under certain circumstances, hence couldn't locate for ages.  Heater on full showed mine, but others have had different results...

[cem_devecioglu]

Small leaks are very hard to find..Sometimes dont let traces on the ground..Only solution is opening the air inlet and check around the HBV and oil cooler area for coolant traces...

[Andywill]

Thanks for all tha advise.
I think I'll change the HBV & check the thermostat for starters.