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Replacing a 300TDi CSW thermostat


Les Henson
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Replacing the thermostat on this engine is very easy and straight forward - very little coolant is lost, there's a minimum of tools needed, there's no gasket as such, and you barely even get your hands grubby.

Make sure the engine is cold before starting work.

The thermostat is right at the top/front of the engine sandwiched between the top radiator hose alloy elbow, and the thermostat housing:-

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Remove the top radiator hose, and disconnect the thin bypass hose and tuck it out of the way.

There are also two plugs underneath the alloy elbow, carefully unplug them - the wire connections are quite fragile and will snap easily, so be careful.

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Two 10mm head bolts hold the elbow to the thermostat housing - they are fairly accessible, although the bottom one is tucked away a bit. These bolts are usually quite tight, so 3/8" drive at least to undo them.

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With the two bolts out, a gentle tap with a soft mallet or a bit of careful leverage, and the elbow will come off. The thermostat will be either stuck in it or still in the housing.

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There's no gasket, but a rubber ring that sits on the lip of the thermostat. This seal is reuseable if it's not damaged.

Thermostat with rubber sealing ring still on it.

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Put the rubber seal (or a new one) on the new thermostat, then fit it in the housing - NOT the elbow. It will only fit in a certain position, so make sure it sits flush in the housing before re-fitting the elbow. Coolant loss is minimal, so a small top up is all that's required. There's a plastic bleed screw of the same type as the radiator on top of the thermostat housing - these shear off very easily, even when being careful. If you feel that you should bleed out the minimal amount of air that might get trapped at the top of the thermostat housing, they buy a replacement plastic one, or X-eng sell a metal replacement item that will last.

If you are unsure if the thermostat is faulty, then there's a simple test that even a solicitor can do! :rtfm:

Place the thermostat in your wifes very best saucepan, add some water and bring to the boil. At boiling point the thermostat should be fully open, and if you then lift it out of the pan, you should be able to observe it slowly close as it cools down.

Les. :)

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I take it you didn't photograph the procedure for checking that the thermostat is operating correctly? It might be an idea to post a note on how that can be done, with the appropriate warnings and disclaimers of course :rolleyes:

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So what are the 2 thermal switches/sensors in the elbow for? My 1995 300Tdi doesn't have them (it doesn't even have the lumps in the casting, its a plain elbow) and this is the first time I've seen them in a pic of a 300Tdi... :unsure:

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