Coolant Leak: Pressure Not Relieving in Cold Engine?

Hi there,

"On my Golf 3 TDI with the AHU engine, the coolant level has been consistently too low for a long time, even when the engine is cold."
When I open the blue lid, it bubbles and hisses – I can't currently tell whether it's releasing pressure or vacuum.

Subsequently, the coolant level will return to its proper position – it rises as a result of opening the expansion tank.

The blue cap was replaced at some point, but nothing changed. I just replaced the water pump and thermostat during the timing belt change – still no difference.

By the way, this has been the case for a long time now without any problems arising in operation – everything is normal.

Nevertheless, I want to know now. What could be the reason for this?

Thank you, and best regards, Bernd.

Quote:

What could be the reason for that?

In 90% of cases, the cylinder head gasket (or head bolt) is leaking, and it's usually a very minor leak in the early stages. If you drive carefully, it might be okay for quite some time. Often, the gas transfer into the coolant only occurs under certain conditions, such as specific temperatures or high load.

"Despite this, it is not precisely known where the air bubble forms, is located, or becomes trapped within the system. Consequently, it is impossible to definitively determine whether a subsequent damage caused by localized overheating will occur or not."

Hi there,

Therefore, I would rule out issues related to the ZK (ignition coil) or ZKD (distributor), as this phenomenon has been occurring for quite some time – several years, and certainly over 150,000 or even 200,000 kilometers.

I've had the car since day one, and I drive about 40,000 to 60,000 kilometers per year – mostly on the highway, and not exactly slowly. Driving 700 kilometers at a time, almost at full throttle, isn't uncommon... Those parts should have failed a long time ago, right?

So, it's not exactly brand new anymore , but it still runs like the first day. Oil consumption, water usage – everything is normal.

What other options are there?

Best regards,
Bernd

Does any coolant leak out when the engine is running in a warm or even hot state? If there's already pressure present when the engine is cool, it must be much higher when it's hot.
If the liquid level rises significantly when opening (more than what would be expected from hose expansion), then there is a larger air bubble in the system, which is why suspicion falls on the head gasket.
The cooling system was properly bled, and neither the radiator nor the heat exchanger are clogged? Has the coolant ever been mixed with distilled water?
hg
Herbert.

"Was the cooling system properly bled? Are neither the radiator nor the heat exchanger clogged? Has the coolant ever been mixed?"

Actually, I didn't bleed anything. I just started the engine, turned on the heating, and refilled the water until the level was correct.
Coolant was never mixed - is it just the red stuff?

The cooling and heating functions are working - I assume everything is okay.
Since the car can easily cover long distances without any issues, I assume that everything should be okay with the engine.

No liquid is escaping. The level remains constant for a long time.
The water pump was leaking again, so I replaced it during the timing belt change. Currently, everything is sealed properly.

The radiator hoses are a bit stiff, but they can still be squeezed.
When I open the blue cap, pressure is released and the coolant level rises; it's currently about two finger widths thick - I just checked.
Question: What is considered normal? Should there be absolutely no pressure when the engine is cold?

Best regards,
Bernd

Quote:

I actually didn't bleed anything... I just let the engine run, turned on the heating, and refilled the water until the level was okay.

Okay, so you replaced the water pump and thermostat, which means there's probably a lot of air trapped in the system. Got it. Make sure to properly bleed the system first.
And – what specific type of coolant was in the system, and what was added to top it up? Red and purple are two different things.
hg
Herbert.

Hi Herbert,

I don't have the exact specification of the coolant memorized right now, but it's either the correct one or a compatible one. It was already in there before, and I just topped it off to replace what was lost.

I thought the cooling system was self-bleeding .
Run the engine with the lid open and keep adding fluid until the level stops dropping – that's it.
How else can I bleed it?
I've done this before as well. The coolant temperature and oil temperature are as usual – everything is normal and unremarkable.

Normally, how does the pressure build up in a cooling system? Or is it simply due to cooling, which causes everything to contract and return to zero overpressure?

Best regards,
Bernd

If the heat exchanger or the pipes are located higher than the connections on the engine side, it won't work. The air will remain trapped inside.
I'm not trying to rule out a potential issue with the cylinder head gasket, but the cooling system should be properly bled first.
On the Audi 80, there was a valve located at the highest point. If that valve is missing, carefully pull the hose at the highest point away from its connection while the cooling system is under pressure, just enough to allow air to escape through the gap.
Is the overflow from the radiator open to the expansion tank?
hg
Herbert.

I suppose that even if there's still some air trapped somewhere after the repairs, which I personally would like to rule out given the high mileage, the system should still be depressurized because... a bubble of air doesn't multiply.

Think about a pressure cooker. It's not airtight either, and yet it can be cold and pressure-free. If we can rule out the possibility of incorrect coolant causing a chemical reaction, then only one option remains: the "little man" and the air pump .

Or what? Does anyone have a more reasonable alternative idea?

Hi there,

I also don't believe it's just an air bubble. Since I replaced the timing belt and water pump, I've driven about 3,000 kilometers – if it were an air bubble, I should have noticed something by now.
Furthermore, I've done this many times before, although this phenomenon has been occurring for quite some time now.

Can a leak in the cooling system cause such a phenomenon?
I found a post online where someone described a similar issue that was ultimately resolved by fixing a leak in the WAPU (presumably a piece of equipment).

I'm going to remove the plastic container at the bottom and take a look underneath. So far, I haven't seen anything.

Best regards,
Bernd

Quote:

Can a leak in the cooling system lead to such a phenomenon?

You have a pressure-based cooling system. Nothing can enter except for compressed or combustion gases. When there is negative pressure (engine cold), the valve in the cap opens.
If the fluid level in the expansion tank rises abnormally when you open it, then something in the system is pushing it up. Normally, any increase should only be due to the hoses contracting as pressure is relieved, perhaps about 1 cm with a warm engine. Otherwise, there's likely an air bubble somewhere, however it was formed.
We only know the background story as you have reported it here.
hg
Herbert.

Hi there,

I'm starting to believe that the problem might be with the ZKD or the ZK, as you suspected. I wasn't able to get a CO test done today.

However, this overpressure has been present since at least the summer of 2016 - I noticed it then and bought a different blue cap at that time. Therefore, I have driven at least 65,000 kilometers in this condition .

I'm just going to leave it like this for now and keep driving it as long as I can. I won't take the cylinder head off again based on mileage. Eventually, it will stop working. It's a shame about this reliable machine.

Best regards,
Bernd

Hello!
I have also been following this topic.
And now, please allow me to also contribute something.

I have a similar engine in my Golf.
As I was driving home from work today, the engine was somewhat up to operating temperature.
I parked the car around 4:00 PM, and at 7:00 PM I checked the coolant level for you.
The engine wasn't completely cool yet, probably around 20°C (68°F). There was no pressure in the water hoses, and when I opened the coolant reservoir, I could feel a very slight amount of pressure, but it was really just negligible.
And I would describe my engine as completely intact and trouble-free after 300,000 kilometers (or miles) and 20 years of use.
I also recently replaced the timing belt, along with the water pump.
and I didn't really bother much about the venting system either.
The water pump generates so much pressure at high speeds that it effectively circulates the entire cooling system, and any air will inevitably end up in the expansion tank. Then you just need to top it off, and everything is fine.

The only exception is if a hose is blocked. This can happen, especially with the thin vent hoses.

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

"Continue:"
I assume that exhaust gases are being forced into the cooling system. Exhaust gases contain carbon dioxide, which is highly soluble in water under pressure. When the pressure is released, for example, when the radiator cap is opened, it causes bubbling in the coolant.
I believe that was also mentioned in the post.

Normally, you can test for exhaust gases in the cooling system with a coolant analysis, specifically by testing for CO2. I haven't done it myself, but I once saw on E*** that if you search for "CO2 test," you can find quite a few options for just a few euros.
It would be necessary to test it with a comparable vehicle in good condition in order to assess the result.

What else?
My Golf car has a financial value of approximately €400 if I were to sell it now. However, it reliably gets me to work every day. In this case, I would simply continue using it and observe how things develop.
"I might also tighten the cylinder head bolts and add some radiator sealant to the engine."

But taking the cylinder head apart? No, I wouldn't do that again.
In the case described here, it's not really serious or acute. It could potentially continue like this for another two years.
If there is an unexplained and significant loss of coolant water, one needs to consider what steps to take next.

It's true that if you disassemble the engine in its current state, it will be difficult to tell whether the head gasket is defective or not just by looking at it.
You'll likely see a cylinder head that has been partially eaten away, and it might still be warped, which could potentially be ground down, but shouldn't be.
And you never really know if it might be something completely different.
In my professional experience, I have seen hairline cracks in the engine block and once a casting defect in the cylinder head. It was like a pore in the material that suddenly opened up after 15 years. Essentially, it was a tiny channel from the combustion chamber to the coolant passage. I was able to clearly demonstrate this using compressed air and soapy water.

Edit:
@ Bernd:
We were writing at the same time. I hadn't seen your last post before I finished my text. But I'll leave it as it is now.

Hi Steffen,

Thank you for checking.

The material value of the car is almost zero – and that's what's great about it. There's no depreciation, etc. It got me from point A to point B, and no other car will do that as cheaply.

Let's see what I'll get as a replacement. That's the problem, actually.
I don't want a new car. There's nothing I can really do to improve it, and with the amount I drive each year, everything will become expensive quickly if I have to rely on the workshop for all the repairs.

Maybe I should just buy another Volkswagen Golf 3 TDI. Unfortunately, there aren't many of them left anymore. They have become increasingly rare in recent years.

Best regards,
Bernd

Here are two things you can test without any extra effort:

Du lässt kalt den Druck ab und fährst das Auto mal eine Zeit lang absolut piano, so dass kaum Ladedruck und so wenig Last wie möglich aufgebaut wird..... Vmax 90-100 km/h.

If the condition changes while it's in a cold state, you'll know.

You can also try releasing the pressure under normal driving conditions while the system is hot, and then continue driving to see if the pressure builds up again. But BE CAREFUL when releasing the pressure. There is a risk of scalding if you open the cap carelessly.

Quote:

Furthermore, you can also try releasing the pressure under normal driving conditions while the system is hot and then continue driving to see if the pressure builds up again.

The pressure will build up again as soon as heat is transferred to the coolant; there's no other way. This is related to vapor pressure. In a closed system, the same temperature-dependent pressure will always be established in and above the liquid.
It will likely be similar with the piano process, but with less pressure and at a lower temperature.
One would have to conclude that more pressure is being generated than expected based on the temperature. And it seems our colleague has already made this observation.
hg
Herbert.