How to replace a Land Rover Discovery thermostat

Back in one of the first post's I made, I said that the temperature gauge in my Discovery was not working. So my first port of call was to check the connections to the sender unit, but these turned out to be fine.

Now that I have been driving the Discovery for a few weeks now, I have noticed the gauge did move a little, especially when travelling up long hill's where the rev's were between 3000 & 4000rpm. The gauge was still not in the correct range though, and in addition to this, I thought that the air output from the heater wasn't as warm as it could be. My suspicions now turned to the thermostat.

The purpose of a thermostat

The purpose of the thermostat is to regulate the water temperature in the engine. When the engine is cold, the thermostat blocks water from entering the radiator and just allows the water to circulate around the engine - allowing it to warm up quicker.

As soon as the water warms up (88 degrees Celsius on a Disco), the thermostat opens, and allows water to flow to the radiator for cooling.

A faulty thermostat will mean the water runs hot if it's stuck closed, or cold if stuck open (my problem). Either way, it's not allowing the engine to run efficiently.

So the fact my temperature gauge moves a little when the engine is under load going up hill, and the heater is luke warm, made it point towards the thermostat.

Thermostats are cheap, and fairly straight forward to replace. Mine cost £7.40 from Brookwells, with the new gasket costing 61p! So one afternoon, I set about changing the thermostat on my Discovery.

This is how I did it.

Changing the thermostat on a 200Tdi Discovery

So before we get started, here is a list of what you need to complete the job:

New thermostat (pictured above)
New thermostat gasket (pictured above)
8mm spanner/socket
Flat blade screwdriver
Copper grease
Stanley knife blade

In the photo you will see the position of the thermostat in the engine. You can also see two of the three 8mm bolts that need to be undone to change the thermostat. (They form a triangle when viewed from above) In addition, you can see the hose leading from the thermostat to the top of radiator. This will also need to be undone.

1: Undo the hose clamp and pull the hose off the thermostat housing

2: Undo the 3 8mm bolts that hold the thermostat housing in place.
Caution: Apply gentle pressure to these bolts, and if they feel stiff, then use some WD40 to help loosen them a little. One of the bolts sheared whilst I was undoing them, which meant we had to drill out the bolt, and replace it.

3: Once the 3 bolts are undone, then the thermostat housing should just lift off with some gentle pressure to reveal the thermostat.

Side view of thermostat clearly showing the sheared front bolt

4: This part isn't necessary if all your bolts have have come out fine, but as we sheared the front bolt whilst removing the thermostat housing, we had to drill it out.

I had planned to re-tap the aluminium casing, but this wasn't necessary on the front bolt as I could drill right through and just put in a new longer bolt with a nut & washer on the other end.

So starting with a small drill bit to put in a pilot hole, I drilled progressively larger holes until the whole bolt had been removed. We used a magnet to remove all traces of metal swarf from the water, and below the drilling area.

5: Remove all traces of the old gasket from both mating surfaces using the Stanley knife blade. Use some light grade wet & dry paper to remove any stubborn gasket. When you are finished, it should look like this:

Thermostat housing - clean mating surface for gasket

Gasket mating surface now clean and ready for new thermostat & gasket

6: Take the plastic protector off the rim of your new gasket and place it into the water filled hole. Make sure you place the spring side into the water.

7: Now place your new gasket on top, and line up the holes. Place the housing on top of the gasket & align the holes using the bolts.

9:Put a small amount of copper grease onto the bolt threads before doing them up, so that if I ever need to undone them again in the future, then they should come out easily and not shear!

10: Carefully tighten all the bolts so they are tight, but don't overdo it or you will strip the threads from the aluminium casing! Replace the top radiator hose, and tighten the hose clamp.

The completed job - notice the new extended bolt on the front

11: All that remains left to do is to take the Land Rover for a test drive. Make sure the test drive is long enough to get the engine up to temperature. Once up to temperature, pull over, pop the bonnet, and check for any leaks around the gasket.