sounds like you got an air lock, see the pipe coming from the bottom of the rad, follow it up till you get to the pipe its joined to, then on that pipe a bit further up you will see an 8mm screw, you need to unscrew that with the header cap off, let it drain for a bit till the air comes out, then put it back and top up the coolant to max lvl.
another good way to see if you have an air lock is let the engine tick over for a while, and squeeze the pipe on top of rad, if you have an air lock this will become hard to squeeze, plus if you run your car with the airlock in the coolant then your HG will blow.

 

L series don't get airlocked unless there is hgf.

Check that the water pump isn't broken (or rather the drive to it)
Check that the thermostat is working properly (if the rad is getting hot it probably is).

 

L series don't get airlocked unless there is hgf.

Check that the water pump isn't broken (or rather the drive to it)
Check that the thermostat is working properly (if the rad is getting hot it probably is).[/QUOTE]

guys i think ive found where its leaking from ! when it gets tht hot it seems to be leaking from the bottom hose on the expansion tank ! it only does this wen its being high revs ! im going to nip it up tomorrow in day light but the pressure shouldnt be tht high ? surely ? its topped up with coolant an like said before they dont get airlocks ! ive looked for a bleed nipple but i cant see it an sure the dervs dont have one ? thermo is working as it flows the coolant round nicely but soon as its high revved it boils up an starts boiling up ? im so confused with this bloody thing ! ive put a tub of Kseal in hopin tht would sort it but no joy ! i lose no coolant if i dont go above 2.5 revs but anymore an it starts to boil an the pressure is so much it leaks from the said hose ! grrrr bloody thing ! please help

K seal won't help.

Check the water pump drive. If it is ok and you are 100% sure the thermostat is working, and the water comes out of the cap rather than any pipe leak then it is HGF. Check the cap works too though!

 

or your have a leak someone where in the coolant system,

my 620 (Basically the petrol version of the diesel engine) had a pin hole in the radiator and did this. A new radiator sorted it for me as the collant could expand and allowed air into the system.

 

guys i think ive found where its leaking from ! when it gets tht hot it seems to be leaking from the bottom hose on the expansion tank ! it only does this wen its being high revs ! im going to nip it up tomorrow in day light but the pressure shouldnt be tht high ? surely ? its topped up with coolant an like said before they dont get airlocks ! ive looked for a bleed nipple but i cant see it an sure the dervs dont have one ? thermo is working as it flows the coolant round nicely but soon as its high revved it boils up an starts boiling up ? im so confused with this bloody thing ! ive put a tub of Kseal in hopin tht would sort it but no joy ! i lose no coolant if i dont go above 2.5 revs but anymore an it starts to boil an the pressure is so much it leaks from the said hose ! grrrr bloody thing ! please help

 

how too ?

how do i check the water pump drive m8 ?

 

Remove the big hose to it
Remove the aux belt
Stick your finger or a screwdriver in there
Turn the PAS pump pulley with your other hand. You should feel the impeller move.
Try to stop the impeller moving by jaming it with something then turn the pas pump pulley by hand. It should remain locked. If not chances are the drive between the PAS and the waterpump has broken.

IF all turns as it should then waterpump can be regarded as working fine (assuming no water leaks!)