Join Date: Apr 2012
Location: Wexford, Ireland
The easiest way to be sure about your engine temperature is to point a laser thermometer at the thermostat housing where you will get a more accurate reading than what the dashboard gauge displays. I purchased a laser thermometer on eBay for approx. £30 and with it I can monitor what the temperature is when the thermostat opens. That's the easiest way to check your thermostat operation.
If your thermostat is opening the top hose to the radiator should get very warm, almost too hot to hold with your hand for a prolonged period and if the viscous fan and the radiator are doing their job properly the bottom hose from the radiator back to the engine should feel much cooler by comparison.
A couple of years ago I replaced my viscous fan coupling using a Land Rover V8 1970-94 part, it's Britpart number ERC 2849 and I purchased it for £55 (a total of £77.99 including tax & postage) from Island Spares, Kent, UK, that was a lot cheaper than the OE part and it has done the job nicely so far. It's not always easy to tell if a viscous fan is defective, one way of testing it is to spin it by hand when the engine is cold and it should spin roughly a quarter turn, then spin it when the engine is hot and it should feel much stiffer. Also, there should be little or no side play if you try to rock it. Another indication of a bad viscous coupling is when the engine gets abnormally hot when stuck in traffic but cools again quickly when moving faster on the open road. Replacing the viscous fan is relatively easy, you just need to tilt back the header tank and remove the top portion of the fan cowl and you will have access to the 4 nuts & bolts that hold the fan & coupling to the water pump shaft, once removed and on a bench you can separate the fan from the old viscous coupling.
If your radiator fins are clogged with crud, flies or leaves it will hamper the cooling process too, I occasionally blow mine with an air hose, removing the grill aids access as the air-con condenser is in front of the radiator. Grill removal is easy, just 3 bolts at the top and 2 at the bottom from memory. It's important to flush and replace the coolant every couple of years to reduce the possibility of corrosion in the cooling system, there are good instructions in the manual on draining & flushing the coolant.
The cooling system is pressurised which increases the boiling point of the coolant, if pressure is escaping the boiling point will reduce causing the system to boil at a lower temperature, check your header tank seal at the filler cap, the O-ring at the level probe and the steam valve gasket for signs of coolant leakage and check that all coolant hoses are in good condition.
Based on your findings I think the probability is that the viscous fan coupling needs to be replaced but it's difficult to say from this distance. Good luck.