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How to: Remove/Replace Thermostat and Coolant Sensor (Z18 XE, Zafira A and others)

Discussion in 'All Model Vauxhall / Opel / GM / Holden How2' started by Jim Reynolds, May 19, 2015.

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  1. Jim Reynolds

    Jim Reynolds Member

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    Essex, UK
    Zafira A Auto 1.8 Petrol
    Reproduced from the thread here ... http://www.vauxhallownersnetwork.co...s-coolant-jetting-into-expansion-tank.429447/ . Details on removing and replacing the Thermostat and Coolant Sensor on a Z18-XE Engine (1.8L Petrol)

    The thermostat lives inside the thermostat housing which is on the front side of the Z18-XE engine near the top left (standing in front of the car). The coolant sensor is bolted into the thermostat housing - a cable into a brown connector.

    1. Drain the coolant using the drain bolt. I didn't bother with this and the coolant spill was pretty small when removing the thermostat ... not sure where the drain bolt is ! A basin under the thermostat would have caught most of it I reckon.
    2. Remove the nameplate shield on top of the engine using two torx bolts (may not be necessary but affords more working room).
    3. Remove the Coolant Sensor cable by squeezing the clip on the connector and wriggling it off gently.
    4. Remove the coolant inlet hose by using Pipe Wrench Pliers or similar to compress the spring clip that holds the pipe on and then wrestle the pipe from the Thermostat housing inlet.
    5. Now is a good time to remove the coolant sensor. It is awkward to get at and requires a 19mm ring or long socket. Be careful: I broke my old sensor removing it because my long socket was not long enough to accommodate the sensor's connector. I had to replace it using a ring spanner but with very little room to rotate it. The sensor becomes more accessible once the thermostat housing is removed but, if the sensor is screwed in tight, it is difficult to get enough torque to remove it while holding the thermostat housing.
    6. The thermostat housing is connected to the engine block using two 10mm bolts - one visible at the top and another hidden beneath it. The lower one can be accessed using a short extension bar or a long socket. I located the bolt using a socket on the extension bar then added the ratchet to it when found. I found it most accesible by standing to the left of the car then reaching down past the air filter.
    7. With the two bolts removed, the thermostat housing can be removed and carefully moved above the engine block . There is still a hose attached but I saw no need to remove it.
    8. Note how the thermostat itself is secured within the housing by two trapped metal tabs. To remove it, you need to press the tabs down to compress the spring and rotate the thermostat to free the tabs. I removed the old one using my Pipe Wrench Pliers to compress the spring. The new thermostat was provided with a very stiff 'toilet paper tube' to assist with fitting and that was much safer.
    9. Check the housing and its mating faces for damage. If any is seen and severe, consider replacing the housing - mine was fine. It is worth removing deposits with a plastic scourer to assist a clean join when you replace it. Also remove any gunk from within the tubes to get it out of the system. Avoid metal scourers etc - use plastic only.
    10. Check the gaskets on the housing mating faces. For me: spares were supplied with the new thermostat along with jointing compound but my gaskets were in good condition so I chose to leave them on. If you choose to go the same way remember to check for leaks after the engine has been running for a bit and fit the new gasket if you see any.
    To test the removed thermostat for function, lob it in a pan of hot water and see if it opens as the water comes to the boil. There are numerous Youtube videos that demonstrate this. Mine did not seem to open more than a tiny amount when tested so I reckon that was my issue.

    To test the coolant sensor, use a Multimeter set to measure ohms (x1000) and connect the probes to the two connectors inside the sensor socket. At lower temperatures the sensor will read high resistance (thousands of ohms) and at higher temparatures it will read a few hundered ohms. You can heat a pan of water again for this though have care to keep the connector out of the wayer. Again, youtube is your friend. It is actually easier to test while still fitted to the car so maybe test before you start dismantling the thermostat housing!

    I bought a replacement sensor and thermostat for about £30 UK from Europarts. I dare say ebay has them cheaper.

    1. Fit the thermostat into the housing and secure it by compressing it and twisting as per above.
    2. Bolt the coolant sensor back into the thermostat housing. Not too tight - torque it to 20 Nm if you have a torque wrench (I didnt). I put a dab of copper ease on the threads to prevent it seizing into the housing ... hopefully !
    3. Present the thermostat housing back up to the engine block and secure it using the top bolt first then the lower. Again, not too tight - 20Nm. Again, I used a little copper ease.
    4. Reconnect the top coolant hose by compressing the clip and wresting it onto the thermostat housing inlet.
    5. Reconnect the Coolant Sensor connector - it only goes on the one way round.
    6. Top up the coolant in the expansion tank to the fill-line and screw the cap on TIGHT!
    7. Start the engine and do a quick visual check to make sure there is no coolant leaking anywhere from around the thermostat or expansion tank. Obviously ... turn off and sort it if necessary !
    8. You need to clear any airlocks from the cooling system. The Zafira does this automatically - just run the engine at around 2.5 K RPM for a couple of mins then turn the engine off and top up the coolant as needed.
    9. Let the engine warm up and watch the temperature gauge for a bit to make sure it settles at a reasonable level. I saw a little steam as coolant (and rain) cooked off the engine surfaces.
    10. Shut the bonnet and Grab a couple of litres of 'emergency' coolant and take the car for a spin.
    11. Assuming all is well, replace the nameplate shield on top of the engine.
    If anyone sees anything dumb in the above, let me know and I will correct.
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