Could be a few causes 1 is that you have an airlock in the cooling system, This you need to leave your engine running with the radiator cap off until the engine is hot and the air trapped inside bubble up until its all gone, But first check your radiator itself to see if its not full of crap!
The other is your cooling system needs flushing out as its full of gunge best way to do this is to buy rad flush and pour it in the radiator and leave it for a day or two then flush your system completly out.
Note if petrol engines are involved the timing could be out thus causing it to run too hot, Make sure you have it checked! And the other is put in a higher temp thermostat,
Ian mechanic, April 2009
