head gasket td5

Hi,
What are the symptoms of a cylinder head gasket about to fail on a 1999 td5 90?
The car has 101k miles and the temp guage drops down from quarter scale to nearly zero when accelerating. Should I be looking to get it replaced?
I can also hear a tapping sound when accelerating between 50 - 60 mph. I was told this may be the exhaust manifold. Any ideas would be welcome. and also any ideas on cost to put it right?

many thanks

My first post!

Why do you believe you have head gasket failure? I don't own a land rover but I am a fan so I do not know if the normal running temp should be on the half way mark so one please confirm this.

If rises to quater and then drops while moving I would asume your thermostat is stuck open and will need replacing. When at a stand still it is wide open so the temp still rises as air is not being passed over the radiator. When you are moving the water is cooled too much as the thermostat is not restricting the flow of water. I don't own a land rover so I don't know where the themostat is located but the usual position is on the engine where the top radiator hose goes into the engine.

The temp gauge on a td5 runs just above center when normal temp, are you loosing water ? are you loosing power ? temp gauges normally go up not down when loss of coolant or overheating otherwise whats the point,could be dodgy temp sensor or connector located near top hose,the thermostat is located in a y shaped white plastic bowl look at front of engine where hoses join hope this helps.

Mr Tigapilet has confirmed for me that the coolant is not even reaching normal tempreature. This means the cooling system is working too well because it is likely that the thermostat is stuck open.

Thanks guys, but the problem turned out to be water loss and a fuaulty clutch pressure plate. TD5 now for sale looking for a disco 200 or 300 tdi. Have a look on the classifieds section

Regards

Iain