How-to - Coolant temperature sensor swap out B5.5 1.9 PD

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ct.p
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How-to - Coolant temperature sensor swap out B5.5 1.9 PD

Post by ct.p »

You know it, you love it, you've read about it, you've probably thought about it sometimes...

The infamous VAG group coolant temperature sensor. These go 'bad' a lot and when they do, they;

Cause hard cold starts
Cause under / over fueling by telling lies to the ECU
Erratic gauge readings
And a whole host of other maladies that prevent toefficient running of our cars.

Below is my adventure in changing it for the second time since I've had the car. The first one the original one gave up the ghost at about 100,000 miles, the second one which I'm replacing here was a cheapy off e-bay and has lasted about 25,000 miles and two years. I splashed out and bought a genuine VW/Beru one this time! Also, although I have not done this this time, it is a good idea whilst the coolant is drained down to change the thermostat as they also fail on a regular basis. Better still, do the sensor, thermostat, waterpump and radiator when you next do a cambelt change as they all fail / wear out and it makes sense to do them all in one go when the cooling system is drained down.

My car is a 2004 AWX engined 1.9 Passat TDI.

The job itself is quite straight forward and the only tools you need are;

10mm socket (engine cover bolts)
Philips screwdriver (air filter cover bolts, solenoid bracket)
Slipjoint / water pump pliers (for backing off the coolant hose clips)
Hopepipe attached to a tap! (for flushing the system out)

Anyway off we go...

Below,the engine!
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If you have them, pop out the three engine cover nut covers. I say if you have them as I'm sure it is standard practice for any "mechanic" to throw them in the bin at the first service. I've had three of these cars and all three had these covers missing. I replaced them each time at a grand cost of about 75p each.
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Pull the engine top cover off, take care around the dipstick.

Pull the rubber away from the front of the pleunum cover, and carefully wiggle the plenum cover out.
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Next remove the air filter cover screw caps
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Undo the screws and remove the cover
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Undo the single screw that holds the solenoid bracket to the air filter lid, unclip the lid, undo the air hose jubilee clip and carefully remove the the lid releasing the air inlet hose as you do. You have to wiggle the vacuum lines out the way. Also disconnect the MAF sensor!
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Removing the air filter lid is not strictly necessary but it makes access to the sensor so much easier and only takes another 5 minutes to do. If you have small hands and are double jointed have a go without taking it off.

Anyway, here it is, the old one with the plug already off.
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The VAG connector plugs defy all logic, sometimes they fly off with a simple press of the lug with your finger nail, other times you have to do battle with a screwdriver. Best advice is to practice on one that is easily accessible, they are nearly all the same so you should easily find one, in fact the one on the bottom of the coolant expansion is probably best in this instance.

New one, 4 pins
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Old one, me pulling the 'C' clip down and off. Once the clip is off, pull the sensor out and prepare for a deluge of coolant! You could of course drain the system down properly first but I couldn't be bothered as I was flushing and renewing the antifreeze anyway. Don't forget to recover the old 'O' ring which is likely to be stuck in the engine housing.
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Old and new side be side
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Check the new one is in correctly and reconnect the plug.

It is a good idea to start filling up with new coolant now but make sure all the hoses are on. Fill up with antifreeze first to the correct capacity, on the Passat the system takes 6 litres so I added 3 litres of neat antifreeze and then topped up with water. That way you know there is the correct amount in the system.

Anyway. You need to bleed the system. Once initally full, I run the engine for a few minutes to circulate the coolant a bit and mainly toensure the heater core is full. Switch off. Undo the three screws holding the coolant tank, undo the coolant level sensor plug so you can lift it higher. Back off the left hand heater hose until the white highlighted hole is just beyond the end of the spiggot and lift the coolant tank up. This is the bleeding bit. By raising the tank any air is quickly expelled and you should see coolant coming out of the little hole! Still holding the tank up and with coolant coming out of the little hole, push the hose back onto the spiggot.

Pic below shows the heater hoses with the left hand one completely off its spiggot. Ignore the water coming out as I was flushing the heater when I took the picture.
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You may need to repeat the bleeding process if the heater doesn't get hot.

That is it, obviously everything goes back in the reverse order of taking off. In fact I took the photos as I re-assembled everything which explains there being water everywhere.

Few words of WARNING.

Do this on a COLD engine as you don't want to scald yourself.
Check and check again that all hoses are on and the spring clips are done up.
Check all electrical connectors are plugged in.

And most importantly, make sure you thoroughly wash away any split antifreeze as it is a DEADLY poison.
Bog standard B5.5 Highline Estate TDI

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