AFN ZKD wieder kaputt? | Posts 16+

Albrecht wrote:

Head bolts and screws can be tightened a second time. I can also confirm that from my own experience.

Sure, I'd be happy to help with that. Please provide the text you would like me to translate from German to English.

My understanding is that expansion bolts are somewhat "plastic" in nature.
That is to say, if screws that have been properly installed are removed, they should be "slightly" longer than new screws that have not yet been installed.

If you very often install them with the specified tightening angle, then, in my opinion, the piece between the thread and the screw head should stretch (and become thinner), causing the pressing force to decrease until it eventually breaks.

Or do expansion bolts only undergo elastic deformation when tightened during normal operation, which reverses when loosened?

Hello Ulf,
Basically, you're right, but those screws should have come off 2-3 times already!!
There is so much security (and an associated surcharge)!
Sincerely,
Günther.

@ Ulf:

Your understanding of expansion bolts is correct.
The screw shafts are plastically deformed, meaning they are stretched. This process is typically used to compensate for tolerances, as the preload of the screw no longer increases when the screw is stretched. The tightening torque or tightening angle is deliberately chosen to induce this state of plastic deformation, which ensures that the necessary preload is achieved.
However, the material only allows for a certain amount of plastic deformation before it fractures (elongation at break). This can definitely happen with repeated use.

In principle, it might be possible to reuse stretch bolts multiple times if, with each tightening, only a fraction of the total elongation is utilized. In reality, the bolt does become slightly longer with each use. This doesn't cause a problem, because the process is initially controlled by torque, and only then does the rotation angle enter the range of plastic deformation.

Regards,
Alex.

Hello Ulf,

I can only agree with my predecessors. Basically, the situation you described occurs. However, you can reuse the screws 2-3 times without any qualms. Especially in the case of meyert.

Best regards,
Albrecht.

Good evening, everyone.

As a self-proclaimed 'scaredy-cat' (that's what they call people who are afraid in the Ruhr area), I'm very worried that any intervention might worsen the situation. After all, so far, all we've had is this gurgling sound coming from the heating system. There has been no sign of excessive coolant pressure or coolant loss.

I already contacted a motor repair specialist on Friday. He offered to perform an exhaust gas analysis in the coolant. That's at least risk-free.

Depending on the result, in the next step I will try to determine whether the screws are still significantly different in tightness. If possible, without loosening them. Let's see if this works, or if it exceeds the torque wrench's measurement range.

If the screws are tightened to different degrees, it would be worth considering either adjusting all of them to the level of the tightest one, or readjusting all the screws from scratch, as Albrecht suggested.

Let's see. Keep me updated.

Regards,
Thomas.

Hello everyone,

The exhaust gas measurement in the coolant yielded no results. The gurgling sound in the heating system disappeared this morning. I can't believe it! Either it's like having toothache while waiting in the dentist's waiting room, or I'm suffering from neurotic perceptual distortions.

I will definitely wait and see for now, and I won't take any action for the time being.

Regards,
Thomas.

Stretch bolts are not supposed to be tightened further, and they should not be reused after replacing the cylinder head gasket. It's possible that by reusing the old screws, they have lost their properties and that, over time, they will gradually lengthen slightly each time they are tightened to the correct torque, which could cause the cylinder head gasket to loosen again.

Hello everyone,

'I also reported yesterday about the gurgling sound in the heating system and suggested that the head gasket might be leaking into the coolant.'
Could it be that it's not the poetry (presumably referring to a poetic description or explanation), but rather the valve that switches from the small cooling circuit to the main one?
With rising external temperatures, this would happen sooner.
The bubbling is intermittent for me, and it completely disappears when the engine is warm.
Or is the cylinder head gasket 'tighter' when the engine is hot compared to when it's cold?

Still hoping to work out a trade.

Michael.

Hello,
Once the valve cover gasket (ZKD) is damaged, it will no longer seal properly.
Best regards,
Günther.

Hello.

Why does the bubbling sound in the heat exchanger disappear when the engine is warm?
Or is there a back pressure building up in the cooling system, preventing exhaust gases from entering?


Michael.

@Michael2

Okay, so the back pressure in the cooling system can't be the cause. The compression pressure and the resulting combustion pressure are many times higher than the pressure in the cooling circuit. Perhaps air is escaping from the heating heat exchanger, and without air, the water isn't flowing properly. That would be my guess.

Hello Michael,

I believe that the cylinder head gasket (ZKD) is tighter when the engine is cold compared to when it's hot. I imagine it like this: When the engine is hot, compression bubbles can enter the coolant and swirl around as long as the water is moving. When the engine is turned off and the coolant cools down, the gas bubbles floating in the water settle in the cooling system and combine to form larger bubbles (something you can also observe in a kettle in the kitchen). Engine restarted: The large gas bubbles are pushed through the cooling system by the water flow, making bubbling sounds.

If that's the case, a possible explanation for the disappearance of my 'blubber' (presumably referring to something like insulation or a protective layer) is that the engine simply didn't get hot enough or wasn't under enough load. 'That would also make sense, because before the sputtering, I was driving on the highway for a while (at a moderate speed), and now it's just rush hour traffic again.'

Well, let's just wait and see!

Regards,
Thomas.

Hello,

Do all air bubbles in the cooling system eventually reach the reservoir, or is it possible that there is no circulation if only the small circuit with the heater is open and the radiator is not?
Most likely, in cold weather and during rush hour (approximately 60 km/h), the engine only operates in the closed loop without the radiator.
Sometimes I have very little water in the container, and other times I have three centimeters more.
However, there are no visible air bubbles in the container.

Michael.

Hello Michael,

For me, 'rush hour' also includes a stretch of highway. The radiator gets warm during that time. There's also a bit of pressure in the system, but it's all within the normal range. I still have the same coolant level.

'Two years ago, it was similar. It ran for what felt like an eternity without any coolant loss or other issues, being driven daily. However, if I happened to drive on the highway for an extended period, the reservoir would be empty in no time.' At that time, the rattling sound had also been increasing continuously and eventually could be heard even when the engine was warm and idling.

Hello Thomas,

You seem to be a prolific writer.
My main concern is that there might be air trapped somewhere in the engine, which could prevent proper cooling.
So far, I haven't had to refill the water, but the level fluctuates quite a bit.
Are there any experiences available regarding whether the mechanics at ATU can properly perform a head gasket replacement?
They also offer a 6-month warranty, but I don't want to get stuck somewhere in Greece and then have to go to a VW dealership.

Michael.

Hello Michael,

I believe that the cylinder head gasket (ZKD) is tighter when the engine is cold compared to when it's hot. I imagine it like this: When the engine is hot, compression bubbles can enter the coolant and swirl around as long as the water is moving. When the engine is turned off and the coolant cools down, the gas bubbles floating in the water settle in the cooling system and combine to form larger bubbles (something you can also observe in a kettle in the kitchen). Engine restarted: The large gas bubbles are pushed through the cooling system by the water flow, making bubbling sounds.

If that's the case, a possible explanation for the disappearance of my 'blubber' (presumably referring to something like insulation or a protective layer) is that the engine simply didn't get hot enough or wasn't under enough load. 'That would also make sense, because before the sputtering, I was driving on the highway for a while (at a moderate speed), and now it's just rush hour traffic again.'

Well, let's just wait and see!

Regards,
Thomas

Hello Thomas,
Hot liquids can only absorb a small amount of 'gas.' The ability of a gas to dissolve decreases significantly with temperature; therefore, a bottle of mineral water will also have a higher pressure when it is warm.
After repeatedly warming the cooling water, these 'air bubbles' are all gone; the gas absorption through the small surface area of the expansion tank is negligible.
It's crucial for the proper functioning of the cooling system that it's properly bled according to the instructions.
Best regards,
Günther.