Thermostat not opening, coolant hose remains cold

Hello.

I drive a Seat Ibiza TDI Sport, manufactured in 2002, with 130 horsepower and an engine code of ASZ.

Approximately 6 weeks ago, I noticed that the oil temperature was just below 120 degrees Celsius. I then checked and found that there was no coolant left in the expansion tank.
I then had to add about 2 liters of water to reach the minimum level. After that, the oil temperature dropped back down to 90 degrees.

In my garage, I discovered that the head gasket was faulty.

What have I done?

- Head gasket replacement includes all other gaskets.
- Valves cleaned, re-lapped, and valve stem seals replaced.
- Thermostat replaced.
- Temperature sensor G62 replaced.
- Replaced thermostat on the radiator.
- Cooler replaced.

Now, let's get to the actual problem: The thermostat isn't opening, and the radiator hose remains cold.
According to the workshop manual, coolant was added, and the unit was run for approximately 2 minutes at 1500 RPM, but the fans did not start.

I've installed various new thermostats, including original Audi/VW ones that are supposed to open at 87 degrees Celsius.
I also installed an identical thermostat that was supposed to open at 80 degrees, but that didn't solve the problem either.

The temperature gauge on the speedometer stops just below 90 degrees, which wasn't the case before; it used to always reach a steady 90 degrees.

I'm at my wit's end; I've been working on this engine for six weeks now, with no success.

My mechanic suggested that there might be a crack in the cylinder head, which could cause excessive pressure in the system and prevent the thermostat from opening.

I've also noticed that it's now pushing coolant past the expansion tank.

I hope I have explained my problem clearly, and I hope you can help me.

Greetings from Switzerland.

Hello,

Please check if air bubbles are coming out of the hose connected to the coolant return line into the expansion tank, either when the engine is idling or when it's slightly warm. There should be a steady stream of coolant without any air bubbles, like when urinating.

If the cooling system is fully filled and reasonably bled, there should be no air bubbles in the coolant stream.

If you see air bubbles, it could be due to a crack in the cylinder head, a leak in the cylinder head gasket, or a leak in the EGR cooler.

Is the coolant oily, or is everything clean?

If the engine overheats despite a new thermostat, it is essential to check the water pump. Depending on the design, the impeller may come loose or stop rotating properly, which will cause the engine to overheat because there will be no circulation of coolant.

Best regards and good luck, Rainer.

Hello Rainer,

Thank you for your quick response.

Based on your description, I suspect that you might need a new cylinder head.

The coolant stream, as you described, is not constant but rather bubbly.

I replaced the timing belt, including the water pump, about 3 months ago. My Seat doesn't have an AGR cooling system; I just checked.

The coolant is clean, so there are no oil residues.
The cylinder head gasket has been recently replaced.

"I've also noticed that when I'm driving and the engine is warm, if I stop briefly and turn off the engine, it then has difficulty starting after a short period of time, about 5-10 minutes."

If the car is left standing for a longer period, e.g., one hour, it starts up immediately again.

Best regards and thank you.

Stephan.

Hi there,
Have you ever held one of those thermostats in a pot of water and brought the water to a boil? A cooking thermometer can be helpful for this.
You shouldn't expect the thermostat to open within 2 minutes of a cold engine starting. Is there no bleed valve? (Is a repair manual available?).
hg
Herbert.

Hello Herbert,

Thank you for your input. Of course, I don't expect the engine to reach operating temperature after just 2 minutes; however, according to the workshop manual, this is the procedure to help it warm up faster.

I tested all the thermostats in boiling water, and they opened between 85 and 90 degrees Celsius.

This model does not have a bleed screw; the venting occurs through the expansion tank.

Regards,

Stephan.

Hi there,
So, the thermostats are working properly.
I'm surprised that the venting is only done at the expansion tank. It's usually not located at the highest point of the system. What does the repair manual say about bleeding the system? Does it only mention the vacuum method?
Then you should look for the highest-positioned hose connection, which is usually a connection to the heating system in the water tank. Carefully and gently detach the hose from this connection while it's under slight pressure and direct it towards the expansion tank.
hg
Herbert.

Hello Herbert,

As mentioned before, I followed the workshop manual instructions exactly.
Coolant drained, thermostat replaced, and all hoses reattached.

Next, fill the system with a 40% mixture, using 5 liters, then fill the expansion tank to its maximum level and start the engine. Once the operating temperature is reached, the thermostat for the main circuit should open and the fans should turn on.

The original workshop manual does not mention anything about a bleed screw or any other method for bleeding the system.

I've decided to replace the cylinder head.
I hope this solves the problem; if not, then I'm definitely at a loss.

Regards,

Stephan.

Hi there,

Quote:

The original workshop manual does not mention anything about a bleed screw or any other way to vent the system.

What do you mean by "original workshop manual"? I'm reading about bleeding/filling using vacuum pressure.
hg
Herbert.

Here's a small tip: Try drilling a hole, about 3mm in size, into the thermostat and see what happens.

You remove the expansion tank and hang it as high as possible for venting.

Sure.
I've been there too, I've drilled a 3 mm hole.
The result was that the temperature reading fluctuated depending on how the car was driven and didn't remain constant. For example, when driving at high speed on the highway, it would rise to just below 90 degrees Celsius under load. However, if I drove at 50 km/h or 60-80 km/h, it usually dropped below 70 degrees Celsius.

@Herbert,
Yes, I know what you mean about bleeding the system under vacuum, but I've been told by various mechanics that it can also be done in a more conventional way.
Fill the system with coolant, start the engine, turn the heater on full blast, and wait until the thermostat opens and the fans kick in.

Regards,

Stephan.

Quote:

System befüllen, Motor starten und Heizung voll aufdrehen und warten, bis der Thermostat öffnet und die Lüfter anspringen.

This can take a long time . You have to get rid of the air bubble in the water treatment circuit, and that's either done by vacuum filling or by opening it at the highest point in that circuit.
There's a hole drilled in one of the fittings on mine.
hg
Herbert.