I have a '97 Discovery 1 XD with 122K miles I purchased 4/13. Problem now with gauge showing max temp overheating after driving seven or eight miles at 45mph from cold start. Opened both heater temp controls to max heat with blower set on HIGH to remove excess engine heat and NO HEAT came out center vent positions. Car has oe black radiator expansion tank with no apparent visible hose, radiator, head, block coolant leaks. Fan clutch spins one rev after hot engine stop. Previously replaced top plastic radiator cap with brass one and topped coolant in expansion tank to fill line after purchasing inspection. Replaced thermostat and gasket and serpentine belt. No heat now coming from heater with both controls on max heat yet A/C blows cold and no apparent coolant leaks or anti-freeze smell from vents or heater core under dash. Any suggestions on next remedy and where I can source a reasonable new expansion tank pressure cap if that is the problem?

 

If no heat, then low coolant. Anything over 50% on gauge is overheating. The heat core is driven from a hose port just to the passenger side of the thermostat. If coolant low, nothing but a little hot air in heater core. Of course, the heater core could also be blocked, you can take hoses off and reverse flush.

Check coolant level cold, should be half way up inside chamber. Chamber needs to change out, it will crack and dump all coolant one day.

$10 thermostat, plus a new gasket, would be a good idea. I use a 180F model. Don't buy a "fail safe" model.

 

When I bought vehicle did a routine inspection and found low coolant level adding A GALLON to black plastic expansion tank fill line with premix Zerex coolant also replaced thread worn plastic top radiator plug with aftermarket brass plug. Put in new thermostat but not sure of temp range although it did have a brass flange jiggle vent. Next will replace thermostat again with 180' Stant Superstat #45858 and check hot radiator for lower fin row cool spot obstructions. Don't know what to think about lack of heater core heat possible obstruction, will try backflushing heater core supply lines.

 

Thermostat spring goes inside the block of course. Jiggle device goes at 12:00 position, to pass any steam pocket. Lower fins should not be more than 10F colder than top fins. If clogged, rad can be rodded out by a shop, mine cost $75.

 

I drove that 97 Disco XD today in 85' mid day sun with A/C set on max cool for half an hour and temp gauge never moved past normal range after idling for ten minutes and then a ten minute gas stop and a couple ten minute heat soak stops with engine off while temp guage stayed steady after restart and fifteen minute drive with a couple traffic lights. No coolant or white smoke coming out exhaust pipe or external engine leaks. When I drove yesterday before changing thermostat temp gauge was pegged on red line for couple minutes until shut off with couple minutes to cool down to continue drive. After thermostat replacement and adding a gallon of coolant yesterday heater is now blowing hot. This overheating-normal temp on again-off again engine temp issue has got me frustrated. I will take short drives and monitor coolant level and heat guage closely until confidant it won't let me down.

 

If you can get an OBDII scanner, smart phone app, or Ultra Gauge, you can see temps rising before they hit trouble areas. Gauge really can't be trusted. Note temp in this pix.

When low on coolant, the roof of the intake is a steam pocket, minimal heat because the heater hose is that outlet on top left of pix, thermostat may not have enough hot water on it to operate correctly, and heat sensors may stop having contact with water. Like a pot of pasta on the stove, you can put hand in the steam, but it is not wise to put hand in the boiling water.

If thermostat has a jiggle device, or you drill a 1/8 hole in it, that aligns to 12:00 position to pass the steam pocket.

The engine heats the water. But the thermostat has to be swimming in that hot water to know when to open. If thermostat is taking a steam bath, he's on vacation.

The Ultra Gauge pix is from a D2 with a fever. It can also show battery volts and other live data, plus read and reset codes, plus display what you want, and has alarm set points you select incase driver is distracted.