Land Rover Repair: too much pressure on cooling system, range rover county lwb, temperature gauge

Question
I have a 93 Range Rover County LWB with a 4.2L. My question is, what causes for my coolant to overflow out of the reservoir only when I drive it for a while. The temperature gauge reads normal all the time. It doesnt overflow when idling. I dont see any signs of overheating. The fan works fine. The thermostat was replaced and it does the same thing.

It seems like its building up too much pressure in the cooling system. Can you help me with this problem please. Thank you.

Answer
Hi Chris,

could be a blockage.  Pressure test the radiator and then the engine itself.  Flush the coolant and replace with fresh coolant.

Obviously, the coolant is boiling over for some reason.  I would suspect the radiator first and test accordingly.  I hope its not a blown headgasket.

Best of luck,

JohnMc