Audi A4 B6 1.9 TDI 130 km: Coolant Temperature Issues - Thermostat, VAG, Sensor & Heating

Hello .
The problem, gentlemen, is that I have a problem with the temperature of the cooling system. Audi, after connecting to the vag, does not show any errors related to the temperature. The indicators in the engine and instruments group in the collant temp field showed results very similar to each other (+ - 2 degrees on a cold and a warm engine). I have a new thermostat fitted.
Yes, it is.
The car normally takes a temperature, only the maximum it reaches is about 70 degrees. Warm air flows from the air vents. However, when I turn on the heating to max, it drops still.
When, for example, stands in a traffic jam, the temperature also drops to about 55-60 degrees. When I start driving, it goes up to 70 and no more.
I talked to the mechanic and he says that some temperature sensor must be changed.
I am asking for help where to look for the cause

It looks like a leaking thermostat or air in the system. One question because my friend did not write whether the readings under the Vag were the same as the friend wrote here or correct?

In the engine group the value shows 69 degrees and instruments 71. I have such values. During a stop, both values drop evenly. The mechanic claims that the temperature sensor on the back of the engine is the reason for this.
As for the thermostat, it was the same on the old one as it is now on the new one. Nothing left this change.
I don't know where to look for the reason; (help

There can be several sensors. There are usually two in diezles. Circular in the socket on the cooling hose on the engine block. Unfortunately, these types of sensors often cause problems. First, locate the sensor and measure the voltage on it against ground.

Tomorrow I will let you know what the values are.
So it is possible that the temperature sensor could be the reason for this anyway?

Yes, they can cause a bad temperature reading. The cause is moisture that gets inside through cracked seals on the sensor housing.

Gentlemen, today I replaced the temperature sensor, the green one on the back of the engine, with a new febi, and it is the same. Warm air flies from the air vents, proportional to the set temperature, and the temperature on the board remains unchanged.
Can vagiem diagnose the problem further? I just don't know what to do with it; /

There is one more option. As a rule, there is a voltage stabilizer in the indicator module, you would have to measure the voltage on it. From what I remember it should be 10V. I do not remember how it was in your Audi if the reference voltage to the indicators was from the stabilizer or the computer. Otherwise, you would have to check the ground wires, especially those on the posts. They like to loosen from moisture and vibrations and also become covered with tar.

Listen, buddy today, when I connected the audi to the vag, I noticed it.
In the engine block, temp 54
In instruments temp 47

Is it not a big difference? I will add that now the indicator in the car shows 50 non-stop and there are no errors related to the temperature.

I would like to ask for help

Added after 1 [hours] 52 [minutes]:

I can't really heat it to 90 degrees in any way. With these 54, as shown by vag, the upper pipe is quite warm and the lower one is definitely cooler and that's quite.
I called the mechanic, told him all this and he said an interesting thing.
He says that you can suspect the visko fan that it may be working all the time at high speed. What do you think about it, how could I diagnose it that it works too fast at such an engine temperature of about 50 degrees?

Hello, your problem is that the engine is underheated and the thermostat is responsible only for that.
Try to clamp the lower hose to the radiator and warm the engine carefully if you are already 90 degrees
then check how the heating works, if it is weak, it is most likely a rinsing heater.
Greetings.

now, when after riding a lot of km on the board I have about 50 degrees, warm air is flowing from the vents. is proportional to the climate control settings. it looks ok at 20 degrees, the air is really warm on the climatronic.

new thermostat, new temperature sensor. I have no ideas. what do you think about this viscose? /

Hello. If the thermostat is closed, and at 50 degrees it must be closed, then in the absence
circulating the fluid through the radiator, there is simply nothing to cool
You have my phone number at the bottom of the post.

So in your opinion, the thermostat only comes into play anyway?

jano720 wrote:

Your problem is that the engine is underheated, and the thermostat is responsible only for that.

Not necessarily, since my colleague claims that the heating works flawlessly. To find out, you need to put the thermometer into the expansion tank and measure whether the temperature rises above these 50. Rubber hoses of the radiator up to 80'C should remain cold. Something will circulate convection there, but not much. Otherwise, even a new thermostat has the right to let it in due to a poorly fitted gasket or faulty factory assembly. I have already had such a defective one. Another problem may be air in the water system. I understood from the description that my colleague has a problem with incorrect temperature indication than engine underheating. I did not take it into account.