Thermostat or temp sensor?

Hey Everybody,

After reading alot of posts, i can't figure out what the problem is with my jetta mk4 04 TDI (AXR).
Temperatures are around 0 degrees celcius right now, the car takes 15-20 minutes to ''warm'' up to 90C of coolant temp.
Problem is that once it reaches 90C, and i drive 100kmph for example, when i go off the gass and brake, the temp goes down towards 80C.
Also when i open the heater at max, temp seem to drop a little.

Need to get the engine to work to get to 90C again.
When the tempgauge says 90, upper radiator hose is warm, lower is cold.


Temp sens problem?

Thermostat regulates temp and temp sensor tells the computer what tempature the engine is running at...

So, thermostat it is!

It sounds like your tstat is failing but since the CTS is very easy to replace, you might try that first. If you do go for a new tstat, get a factory version, not an aftermarket. Search is your friend for how-to's

Idea' was that maybe the CTS was giving misreadings to the t-stat. But if you guys say t-stat ill go with that

heard the aftermarket versions are indeed crap, dunno why.
The aftermarket green CTS don't matter that much i believe?
Any special brands that are marked as factory that i can order? Pictures on OEM and aftermarket don't seem that different?


On my way to find some partcodes

My Golf also takes 15 to 20 minutes to warm up when the ambient temperatures are in the 20's F. It's always been this way and like the OP's the temperature will fluctuate when I come to a stop especially with the blower on. I'm on my third t-stat from the dealer as well as third CTS. I monitor my engine temperatures with an Ultra Gauge.

JB05 said:

My Golf also takes 15 to 20 minutes to warm up when the ambient temperatures are in the 20's F. It's always been this way and like the OP's the temperature will fluctuate when I come to a stop especially with the blower on. I'm on my third t-stat from the dealer as well as third CTS. I monitor my engine temperatures with an Ultra Gauge.

Click to expand...

So it's normal?

might be normal, also might be that your thermostat is not closing all the way.

you could close off part of your grille and see if that helps.

How are you reading the coolant temps?

Sounds normal to me. My Golf is equipped with an auto transmission which supposedly helps for faster warm-ups compared to a manual transmission equipped TDI. Urban driving, with more stop-n-go, increases warm-ups also. So there are a lot of variables to consider.

Soullancer said:

Hey Everybody,
After reading alot of posts, i can't figure out what the problem is with my jetta mk4 04 TDI (AXR).
Temperatures are around 0 degrees celcius right now, the car takes 15-20 minutes to ''warm'' up to 90C of coolant temp.
Problem is that once it reaches 90C, and i drive 100kmph for example, when i go off the gass and brake, the temp goes down towards 80C.
Also when i open the heater at max, temp seem to drop a little.
Need to get the engine to work to get to 90C again.
When the tempgauge says 90, upper radiator hose is warm, lower is cold.

Temp sens problem?

Click to expand...

With temperatures around zero degrees, this could be normal. You need to check the actual temperature rather than reading the gauge to see what's going on.

VeeDubTDI said:

With temperatures around zero degrees, this could be normal. You need to check the actual temperature rather than reading the gauge to see what's going on.

Click to expand...

Ill hook up VCDS and see what's going on with the temps.
Only thing i see is the gauge dropping back when braking

Keep in mind that the gauge on the dash will say it is 90 when it is only seeing 75.

logging in VCDS should show you if there is a sharp knee to the coolant temp curve at around 90 c.

meerschm said:

Keep in mind that the gauge on the dash will say it is 90 when it is only seeing 75.

logging in VCDS should show you if there is a sharp knee to the coolant temp curve at around 90 c.

Click to expand...

Ill get back with the log, Thanks!

Lots of good advice...

But, for what it's worth, to sort of clear-up things, the T-stat doesn't have a clue what the Temp Sensor is doing. And, neither does the Temp Sensor know what the T-stat is doing. There is no monitoring connection between the two other than the Temp Sensor monitors coolant temp.

Also, the Coolant Temp Sensor has two independent functions, it sends info to the ECU independently of the info sent to the Gauge in the Dash.

The reason I say the info is independently sent to each is that occasionally the Temp Sensor (wiring issue in my case) in my ALH Vanagon will default to 418f causing the GPs to not activate. However, at the same time, the gauge in the Dash operates normally. And, I have seen the CTS default to -40f (not mine) causing the engine to run extremely rich but at the same time the gauge in the Dash was operating normal.

I'd go for a new T-stat rated at 190f (what ever that translates to centigrade).

87,7 Degrees Celsius

I guess that is the Factory T-Stat specs?

pretty sure the oem thermostat is spec'd for 87C

The newer engine temp sensors only put out one signal. several computer programs in the car use the signal for different purposes, including the one that decides what value to have the dashboard temp indicator show. I am not the expert, but it will be informative to see what the sensor(s) offered to the VCDS says.

the coolant temp sensor can only tell you what it reads, but the temperature of the coolant as it exits the engine, over time, can give insights as to how the thermostat is working, or not working. sometimes this is easier if you read other items to correlate with, such as vehicle speed.

Another thing to consider is that our t-stats are more analog in nature; meaning they open and close because as it opens it allows cold coolant in to immediately close it again. A scan gauge or ultra gauge will show this fluctuation along with VCDS.

Analog because it's a bi-metal material? which responds on the cold coolant that passes.
Maybe it's perfect normal behavior , to warm up, cool down, warm up etc. Because of the opening of the t-stat at 90C and getting ultra cool fluid at these temperatures outside.


Ill get the scandata tomorrow

Yes, that's right with respect to operation of the T-stat.

The T-stat operation is rather subtle. In other words, a properly functioning T-stat just don't pop fully open at a given temp. It gradually begins opening based on it's rating and may began to close as the cool coolant from the Rad flows over it before it ever attains full open. That process would seem to be more likely during cooler/colder ambient temps than on a hot day in mid-summer. And, as some have indicated, speed of the vehicle would affect heat loss due to the increased air flow across the engine and Rad all the while more speed generally means more fuel burned, thus increasing the amount of heat generated!

bimetal thermostats are used in some on/off applications, like some furnace system house thermostats (now they use computers and components to sense the temperature)

Automotive thermostats are operated by a wax which is formulated to expand at specified temperatures. The wax pushes a piston against a spring, which opens or closes the valve to control coolant flow.

They have a "start to open" temperature, in our case, 87 C plus or minus a bit. The service manual has a table to check thermostats which says start to open at 87 or 85 (depending on which year) and a fully open temperature, about 20 degrees higher.

The coolant sensor is on the other end of the engine from the thermostat, so you really have no idea what exactly is going on, but you can see if it is working from the change in slope as it gets to temp.



Here is a graph from my 09 that shows coolant temp(blue) and footwell temp(orange). ( I turned the heat setting all the way up after a while to see how high it would go, and then turned it back down.) this was a drive on surface streets and highways, ten or fifteen minutes.



here is another one with coolant temp (C) and vehicle speed (Km /h)

These are not winter time, which you can see from the starting temps.

I made these from the VCDS logs by opening with microsoft excel and using the graph function provided. VCDS comes with a graphing program as well.

Interestingly, some new cars (like the 2012 - 2014 Passat TDI's CKRA engine) have a very wide thermostat operating range. In the case of the CKRA, the thermostat starts to open at 87C and is fully open by 135C (!). This is done so that the engine runs hotter (and more efficiently) when under load. My 6.0 PowerStroke is the same way.

http://www.partsbase.org/vw/passat-pa-us-2012-12106-coolant-pump-coolant-regulator/

Looks like there is also a temp sensor much closer to the thermostat in the Passat..

wonder which part of the coolant circulation system it measures, since there look to be three or four fluid connections on that housing. also how many other coolant temp sensors are on that car.

(my Jetta has one ( J83 I think) near the Radiator to turn the fan on when it gets too warm, but readings from that one are not available in VCDS)

Locating the ETS on the coolant flange at the end of the head provides a reading that more closely represents the temp of the engine.

Engine temp in the area of the T-stat, which is the location of all coolant "return" would be considerably cooler than at the end of the head.

I'm not sure which engine/Passat you are talking about, but typically, other than the sensor on the coolant flange, there isn't another one. I'd like to see a pic of it.

The temp activated switch on the Rad is quite different than a "sensor" that provides information across a range. The Rad switch is either ON or OFF based on it's activation temp. As the coolant gets hotter, most of them do have a second circuit activating a high speed on the fans. And, you are correct, that switch is not monitored by the ECU or accessible via VCDS.

This is a figure of the CBEA on my 09 TDI. Note the Thermostat, #18, is on the mid-left front, while the temperature sensor G62, #5, is on the right rear side.

The second coolant sensor on my car, on the radiator hose, G83, is listed in alldata as a sensor with readings, not just a switch. (both are two-contact thermal-resistance sensors you can check by disconnecting and checking the resistance against a temperature-resistance table.)




The link in post 20 shows a sensor(26) in the thermostat housing, for the 2012 Passat, but there is more going on than just a simple thermostat housing. I am not sure what this particular sensor is for, if it is in addition to or replacement for, the two in my car. Part number of the temp sensor leads me to think it, too, is a resistive temperature sensor. Looking around the parts site, that Passat also has a secondary electric coolant pump.


sorry this is going a bit off-topic for the original poster.

The T-stat and CTS in the above drawing is typical of the pre-CR engines as for location of each.

As of the drawing in the link of Post #20, it appears that unit (21) is the Coolant Regulator. Thus, I would assume it contains the T-stat. Also, I'd guess the top smaller pipe is the return for coolant that comes from the EGR cooler, oil cooler, heater core, and expansion tank. It is that coolant which provides the heat to activate the T-stat and I suspect VW concluded that would be a good place to obtain Temp info of the engine!