Failed coolant sensor. If you have under 100,000 miles, there's a warranty extension in this part for your 1.8T.

 

My 2000 passat got the coolant temp problem last month.
I replaced the coolant sensor which is the green one that people mentioned earlier.
However, after the replacement, it didn't fix the problem.
Then it ended up being a failed thermostat and the problem is that the thermostat is stuck open so the temperature won't go up and it hardly reaches the middle point.
I replaced the thermostat two weeks later and my coolant temp is behaving back to normal.
I suggest that you go and get your coolant temp sender replaced. if you have warrranty, it's free. if not, go buy the green sensor($12) and replace the sensor by urself and there's a lot of writeups about this DIY.
If replacing the sensor doesn't fix the problem, then it's the thermostat for sure.
Replacing the thermostat is easy too and the only thing is that you need to drain all of ur coolant out and reach the back of the water pump and there's a black hose which is the part I circled in red, the hose has two screws on it, unscrew them, and you will see the thermostat sitting right in there. Take out the old thermostat and the O-ring, replace with your new thermostat and O-ring(NOTE: the direction of the thermostat is very important, remember the direction when you are taking off the old thermostat)
once this is done, screw the hose and put back/tighten the clamp.
Fill with the factory G12 coolant(1:1 mixed with water) and start the car and let it run for 15mins to see the coolant level and add more if needed.
Go for a test drive and watch the coolant temp and it should go up to 190 F in about 15 to 20 mins local drive.
If you have any questions, just pm me.
Good luck!

Here's the picture of the water pump:

The black hose in the red circle is the one you need to find, the thermostat is sitting right in there.
You should see this from under the driver's side:

 

Be carefull as on my 2004 Passat the waterpump is in the engine block and the termostat is on the side of the engine right where the oil dip stick is.
Mine is a AWM engine

As for the temp sensor, its hidden right on the backside of the engine. A pain to get to, so there is a writeup. Remove the heat shield over the turbo and reach to the back. Took me like 1 hour to get it out and back in as you need to guess sometimes. Just be take your time it's not hard.
Good luck.

 

Thanks, ctobio and dumochi !!

I am over 100k miles and so out of warranty. I plan to change the sensor first and try it out... Thanks for the deatiled reply, dumochi. Hopefully that fixes it, if not, I'll have to get the thermostat fixed...

One thing I observed also is that when I turn the heat/cold setting to "Hot", I do get hot air. I thought I heard that the cabin air input is heated by passing over a tube containing the coolant reflux and from this I thought that the coolant is certainly getting hotter (even though my indicator did not reflect it then). Is this line of thought correct?

Thanks once again!

 

Any liquid-cooled car engine has a small device called the thermostat that sits between the engine and the radiator. Its job is to block the flow of coolant to the radiator until the engine has warmed up. When the engine is cold, no coolant flows through the engine. Once the engine reaches its operating temperature (generally about 200 degrees F, 95 degrees C), the thermostat opens. By letting the engine warm up as quickly as possible, the thermostat reduces engine wear, deposits and emissions.


My old thermostat was stuck open so that my coolant temp is always low and never hit the middle point. I did get hot/warm air from the heater and I think you should try the sensor 1st since it's a cheap/easy fix. If this doesn't work, then it's 99.9999999% that you have a failed thermostat :lol:

thermostat is about $23 and it took me about an hour to do the job...well, 40 mins of it is about draining out the coolant and filling up the coolant...lols...
Good luck!
let me know if you have any questions :thumbup:

 

Thanks again, Dumochi.

I tried to find DIY posts for the coolant sensor replacement but I see that the procedure can vary depending on the year. Can anyone direct me to the one for 2003 1.8T Passat (if it exists)?

Thanks again!

 

You can get in there by removing the heat shield on the left side of the engine and sticking in one arm down there. Then use a long flat head to reach the CTS.

There's a writeup on here for the AWM engine.

http://www.passatworld.com/forums/sh...d.php?t=183526

 

it may also be a broken plastic impeller on the waterpump, and is more likely than a bad thermostat on these engines.

 

if it would be the water pump it would be doing the opposite... when you give it gas, the temp would go down and when you let go, it would go up....

 

If you have access to a VAG-COM, see if the temperature of the gauge as being recorded by the instrument panel is the same as what's being recorded at the ECU. These sensors have two circuits internally, and usually only one fails. If the temperatures are varying greatly, then it's a sensor. If they're both the same... I'd consider the possibility that the sensor is fine and that there could be a cooling issue of some sort.

 

Thanks, all, for your helpful replies. Yesterday, I noticed that it (the temperature) just stayed at 90 deg. F all the time. Today however it was back to normal :crazy: mmm... Anyhow, shall try to replace the sensor to start with and see how things look...

Thanks again, guys...

 

Good luck

 

I went to a Indy yesterday and they fixed it for me. It was the coolant temperature sensor.

Thanks guys, for all your inputs.

 

Thanks for the great info guys!

 

where IS this coolant temp sensor? I just picked up a 99 V6 and have the same issue where my engine temp gauge sometimes wont go above 90.

 

http://www.passatworld.com/forums/showthread.php?t=143153

 

http://www.tobio.info/passat/howtos/coolantsensorhowto.html
http://www.passatworld.com/forums/showthread.php?t=143153