Fluctuating water temperature gauge, 1.9 TDI AVG

Hello,
"Over the past few days, I've noticed that the water temperature in my Audi A6, model year 2001, with a 1.9 TDI AVG engine, is no longer reaching 90 degrees Celsius." Today, the needle stayed at 60 degrees the entire time.
I then stopped the engine and felt the hoses under the hood. Everything was extremely hot (after 1 hour of city driving). I closed the hood, started the engine, and the temperature gauge showed 90 degrees. Drove another 2 km, the display showed 60 again, stopped again, waited for 5 minutes with the ignition off, turned the ignition back on, and the display showed 90.

What's going on here?

Thank you in advance for your suggestions.

First, determine whether the problem is related to the indicator itself or if the engine temperature is fluctuating.

- Display malfunctioning --> possibly. Temperature sensor defective.

- The thermostat is no longer closing properly, which causes the engine to stall while driving.

--> After a cold start, the radiator warms up synchronously with the engine temperature.
"...but only when the thermostat opens." You can do that.
Check that first thing in the morning.

The reading will reach 90° approximately when the water temperature reaches around 65°. That's intentional and normal.


EDIT: Engine-KB. AVN does not exist.

Hello, thank you for the reply.
1. Engine identification code AFN
2. Those were my assumptions as well, but now I don't know how to proceed. What should I do? How should I start? What and how should I check first?
Thank you .

stepnogorsk wrote:

Hello, thank you very much for the response. 1. Engine identification code AFN 2. Those were my assumptions as well, but now I don't know how to proceed. What should I do? How should I start? What and how should I check first? Thank you

AFN? That definitely didn't exist anymore in 2001. Please either post a scan of the entire document or take a picture of the first page of the service booklet and black out the Vehicle Identification Number (VIN).

That's an AVG, okay, now we have it. Sorry, I'm completely mixed up.

Okay, so I was just driving. I drove 10 km at 80 km/h, then 14 km at 160 km/h. The temperature gauge went up to about 82 degrees. Then, when I got off the highway and drove about 50 km in about 5 minutes, it went back down to 60. Engine off, wait 2 minutes, turn on the ignition, display shows 90, start the engine again, drive at 50 km/h for 3 minutes, display shows 60 again.

Quote:

2. Those were also my assumptions, but now I don't have a plan for how to proceed. How should I proceed now? What and how should I check first?

Let's start with the simplest thing: check the thermostat.

You probably have to open the hood.

How? --> It's stated above; you'll need to think for yourself.

Okay, as soon as it cools down, I'll give it a try.

To further delay the time-consuming process of opening the hood, another inspection could be performed with the hood closed.

If the temperature gauge drops back down to 60 degrees while you're driving at low load, you could try to find a safe place to pull over and let the engine idle. To speed things up, increase the speed.

If the display turns back on and then remains stuck at 90 degrees within the fuzzy logic range, this again indicates a faulty thermostat.

Quote:

To further delay the time-consuming process of opening the hood, another inspection could be performed with the hood closed.

Or, you can use VCDS and compare the engine temperatures from the engine control unit (ECU) with those displayed on the instrument cluster.

Could the fuel gauge needle also decrease along with the temperature gauge needle?
I used to have a Golf 3 with an AHU engine. It was a widespread problem.
Both pointers occasionally moved to the left by approximately the same value.