Overpressure in Cooling System - Blown Head Gasket?

Hello.

Unfortunately, I've been experiencing significant overpressure issues in the cooling system lately. After a longer drive (approximately 20km, as the car doesn't really warm up properly beforehand), the expansion tank starts to build up extreme pressure. Sometimes, steam even escapes from the (new) cap (which opens at 1.5 bar). When the car is parked and I return later, about 1 liter of water has evaporated. The coolant level in the expansion tank is below the float, so the warning light illuminates. I then have to top it up, and the same problem starts again. Currently, I have to top it up almost every day.

When I open it while it's still warm, the following happens: It makes a bubbling sound when I open it!

There's way too much pressure in the pot!

A CO test came back negative, although I'm not sure how accurate those tests are.

It's questionable what could be the issue. In my opinion, the thermostat seems to be working normally, and the car gets very hot as usual, but it also stays hot. The coolant hose from the cylinder head is also clear.

I have another identical car as a reference.

So, is the ZKD the answer then? Are there any other tips?

"If I were certain, I would exchange it immediately. However, it's questionable whether the head is okay. It might have a crack - I initially thought it worked well for short distances but was problematic for long distances. But the current temperature is low, and the 10km commute to work is only just getting the water to operating temperature."


"Maybe someone else has a tip - it would be a shame to get rid of the car, even with only half the mileage it should have. I'm sure if that's the case, the previous owners didn't treat it well - there's probably a reason for that. Although, it's been running relatively smoothly for me over the past 1.5 years."

Besides that, if I were to remove the head, it would be advisable to have it inspected - pressure test it and possibly repair the valves. The shaft seals should also be replaced, and perhaps the hydraulic lifters as well, to ensure everything is clean. That would involve quite a bit of expense. Simply replacing the gasket is cheaper, but there's no guarantee it will work.

THANK YOU

Sure, here's the translation:

"Hello"

Oh, by the way: This problem started about a month ago, when I drained about two-thirds of the fluid from the container to refill it with concentrate in order to achieve the correct antifreeze level. This happened at the same time, but I don't see a connection... ?

Are the coolant pump and coolant thermostat okay?

I think the thermostat should be okay. I recently replaced the pump along with the timing belt – of course, something could still be wrong with it – but what?

I don't understand how this pressure could occur if the pump or thermostat weren't working properly. How could I check this?

I can't just start checking everything randomly – and that's exactly my problem. If I knew it was the water pump, for example, it would be easy to replace it. If I knew it was the head gasket, that wouldn't be a problem either...

THANK YOU!!!!

Does the cooler get very hot?

That's a very interesting question...

I really relied on being able to accurately gauge when the car would warm up, as I replaced the thermostat a year ago, but I bought a cheap, non-original replacement.

The car, like always (and also the second identical one), takes a very long time to warm up, but once it's warm, it stays warm. I've had the thermostat stuck open twice, which caused the water to cool down on the highway...

I'm testing this with the cooler - thanks in advance.

Technically speaking, if the thermostat is stuck, why would it cause such extreme "pressure"?

Okay, yes - the needle is, as always, pointing at 3 o'clock - that is, directly in the middle - and doesn't move up or down when the device is operating at its normal temperature, so it should fit.

THANK YOU
Sure, here is the translation of the text from German to English:

"Hello"

Okay, I could replace the thermostat – or replace it outright, which wouldn't be a problem. While I'm at it, I'd like to completely clean the system. Does anyone have any tips on how to do that? Are there any specific cleaners that work well?

Then I could at least rule out problems in that area.

roorback wrote:

Technically speaking, if the thermostat is stuck, why would it cause such extreme "pressure"? THANK YOU greeting

The more heat, the more expansion. In a closed system, this leads to an increase in pressure. If you disconnect the small hose on the coolant reservoir in the LL (likely referring to a specific vehicle or system), it should flow slowly.
If the radiator cannot dissipate heat due to being clogged or having insufficient coolant flow, the viscous fan will not operate correctly. This fan contains an oil filling, and the housing should not be visibly oily.
I've never had any problems with Wahler thermostats, although others say the original ones are better.

Hello again.

SO - I don't have a viscous fan; it's an electric fan that's thermally controlled and, either way, never runs.

I've already noticed the oil on the radiator, but I was under the impression that the oil could only be coming from the intercooler!

"I actually have a spare radiator here – I could theoretically replace it."

But first, I'm going to check if it even gets warm.

THANK YOU!

Hello.

Okay, new finding - the cooler isn't getting warm at all. Great! Thanks again for the tip.
Just got a new thermostat - installed it, but the test in the water bath already showed - both open .

Installed in the car - same result - radiator is cold - the inlet at the top is slightly warm - but the outlet at the bottom is COLD. The thermostat housing is warm, but the thermostat doesn't seem to be opening for some reason.

I've now completely removed the thermostat – so, of course, it doesn't get warm anymore. AND – it's still bubbling and boiling.

It doesn't really make sense, somehow.

I also initially used a hose and then a cleaning solution to flush and clean everything. So far, everything seems to be "clear" and okay.

Does anyone have any other ideas?

THANK YOU.

Does the pump provide boost? If necessary, try running the machine at full speed and feel the radiator hoses, or even remove them for inspection.
The AEL is stable, but I wouldn't be surprised by any mileage figures.
You can still perform a pressure test on the system, but the chance of detecting a leaking head gasket that only manifests under heat is low.

I tested the pump yesterday. I opened the top and started it - and water came out.
The heating is working too!

Regardless, it seems like the ZKD (presumably a component) is almost the only thing that remains. But why isn't the radiator getting warm...?

THANK YOU

Sure, here is the translation of the text from German to English:

"Hello"

Hi,
"Could the radiator be clogged? If it stays cold, obviously nothing is flowing through it from the hot water. You've already ruled out the pump and thermostat. Don't drive it too much, you might damage the head gasket."
hg
Herbert.

Yesterday, I thoroughly flushed everything with the garden hose, and then I used a radiator cleaner. I tried various methods, such as unscrewing the hose at the bottom of the radiator that leads to the thermostat, and water flows normally through the expansion tank and the radiator connection...

It's interesting that the temperature sensor responsible for the display (and preheating...) is located at the top of the coolant circuit, right before the radiator. It's questionable how accurate it is if there's no coolant circulating in that circuit. The display always showed a value right in the middle...

Best regards,

Further test - the cooler is not getting warm in the other TDI either - so it seems normal for this time of year .

Last attempt - Use a vacuum pump to extract and refill the fluid in the workshop. If that doesn't work, then remove the head...

Tap the impeller of the water pump; it's still providing some flow!
It's not the first time I've had a negative COVID test.

Test: As soon as the engine warms up (the display shows 90º - be aware of the plateau function), the heating and fan should operate at full power. It *must* produce properly warm air and should not be lukewarm after 5 minutes, nor should it overheat during partial load or idling!

Edit:
Display plateau effect: Above 70 degrees, the display jumps to 90 degrees and remains stuck there until the water reaches 120-130 degrees, only then does it move further into the red zone.
Using VCDS, you can read the ACTUAL display value .

Hello and thank you.

I'm now quite certain that it's the cylinder head. I'll remove it soon and have it tested – I'll see what the engine rebuilder says.

I mean, I think I would be able to tell from the writing itself whether there are any traces of it.

"The water pump was also a possibility, but we checked that, and I tried blocking the supply line – the pump is working. Also, the heating was always nice and warm. My colleague also said that, based on the water flow in the expansion tank, the pump is running."

Currently, the problem is somewhat alleviated - I'm driving without the thermostat, and the temperature gauge always stays relatively low. I've loosened the cap on the expansion tank - that seems to work relatively well. I drove about 200 km this week; it's definitely not a permanent solution. I'm still waiting for an alternative car that I'll be getting - then I won't have to worry about the repair!

I will definitely get in touch and let you know.

Thank you for now.
best regards, uwe