AFN ZKD broken again?

Hello everyone,

Approximately 2 years and 50,000 km ago, I had to replace the head gasket on my A4 AFN. It started with gurgling and hissing sounds coming from the dashboard, progressed to coolant gurgling under load, and ended with a massive pressure buildup in the radiator itself, even without load.

I then replaced the cylinder head gasket and noticed that some of the head gasket bolts were suspiciously loose.

Now, I can hear a slight gurgling sound again. Otherwise, there are no other symptoms. I'm afraid that it's likely happening again. Now, I'd like to hear your opinions.

Could it be worth tightening the screws now? If so, how and how much?

Sure, here's the translation:

'Thank you in advance.'

Regards,
Thomas.

Did you tighten the screws properly back then, and did you also retighten them?

I don't believe that the screws should be tightened, or that they are allowed to be tightened.

Hello,

Properly tightened: yes. Tightened further: no, because according to... Repair manuals and information from the 'friendly' support team are no longer available for current ZKD materials.

Regards,
Thomas.

Hello,

It's probably going to be too late anyway. Why not try tightening it to the specified torque again? Was your torque wrench from back then the reliable kind?

Sincerely,
Christian

Hello Chris,

'Reliable tool? That's a tough one! But even the original screws came loose.' However, they were recruited in Ingolstadt, and they definitely have reliable equipment.

Regards,
Thomas.

Hello,

I carried out the repair back then together with Meyert.
Individual screws weren't actually loose, but rather, they were much easier to loosen compared to the others.

The torque wrench is made by Belzer.
However, I don't think this is significant, because the screws are only pre-tightened with the torque wrench, and then tightened an additional two times, each by 90 degrees (if I remember correctly). Here, they really get stretched out.
It seems that the correct pre-tension is more or less automatically adjusted by the stretching itself.

The remnants of the sealant were carefully removed, and after that, the engine ran perfectly again.

I would also like to mention that another ZKD (Zeitkapseldefekt - time capsule defect) has not yet been definitively confirmed.

Perhaps the question should be phrased as: Are there any known quality issues regarding the replacement seals and expansion bolts, or have such problems occurred in the past?

km124 wrote:

The torque wrench is made by Belzer. However, I don't think this is significant, because the screws are only pre-tightened with the torque wrench, and then tightened an additional two times, each by 90 degrees (if I remember correctly). Here, they really get stretched out. In this process, the correct pre-tension seems to be adjusted more or less automatically by the stretching itself.

That sounds good so far. I've had bad experiences with cheap socket wrenches from hardware stores myself, and almost lost my rear wheel because of one. It turned out that the key only delivered 50% of the set torque.

Even though a total of 180 degrees of tightening is specified, the stress state in the screws is already responsible for the final stress state even before the half-turn mark.

It's an interesting question why set screws can lose some of their preload over time.

Sincerely,
Christian

While I'm not a mechanical engineer, I believe the use of expansion bolts is problematic.
The material of the screws is extremely important, and the only advantage I see is a more even distribution of forces and a less critical tightening torque.
Apparently, the long-term stability of the screws' modulus of elasticity is questionable...

Hello,

Does the AFN engine not have a hose connecting to the turbocharger pressure sensor in the engine control unit?
If there's a leak, the control unit might measure, for example, 1 bar, while the actual boost pressure would be higher.
The turbocharger pressure sensor could also be defective. When the vehicle is stationary, the pressure reading from the turbocharger pressure sensor should be approximately the same as the reading from the altitude sensor.
It would put a significant strain on the charger and the cylinder head gasket over time.

Have you ever measured the boost pressure directly at the EGR valve (using a hose connection for a pressure gauge before introducing any cleaning agents)?
"Is there anything in the error memory (regarding the fuel injection start control)?"

Are ZK screws tightened in the correct sequence and in multiple steps?

Quote:

It is, however, an interesting question as to why set screws can lose a portion of their preload.

"In my case, when I had a major engine failure, the bolts were at 147,000 km, and they weren't in great condition either, especially considering the head gasket was leaking." It could be solved without much effort.
For highly modified engines, I would personally prefer to install high-quality head studs. Unfortunately, I only found very expensive options at the time, and I ended up rejecting the idea.

Best regards, Rainer.

Hello Rainer,

The AGR is out of order . During the last timing chain replacement, the nipple on the solenoid valve for the hose to the AGR valve broke off. I carefully sealed the hole in the solenoid valve with silicone.

I can read the error codes, I did that a few weeks ago when I was having a problem with the cruise control. But there was nothing there then. Unfortunately, I don't have a pressure gauge available.

I can't really imagine that the cylinder head gasket defect has anything to do with excessive strain. I haven't done any modifications or tuning, nor do I stress the car in any other way. I just drive it regularly through rush hour traffic every day, and I'm happy when I can occasionally reach 100 km/h. I can't even remember the last time I drove full throttle.

Regards,

Thomas.

I'm not sure if it's relevant, but my ZK screws were loosened using a one-meter extension. I was standing there when the master mechanic (a Turkish backyard mechanic) personally said that it had to be done that way. I didn't look any further after that... By the way, since he worked on it, I subjectively feel a lot more power and about 10 km/h more top speed. Okay, but there wasn't that much smoke before either.

The EGR valve is out of order:oops:. During the last timing chain replacement, the nipple on the solenoid valve for the hose to the EGR valve broke off. I carefully sealed the hole in the solenoid valve with silicone.

Okay, the AGR (Abgasrückführung) probably shouldn't have anything to do with the turbocharger boost pressure.

I can read the error memory, I did that a few weeks ago in connection with a cruise control problem. There was nothing then. Unfortunately, a pressure gauge is not available to me.

A missing AGR (Abgasrückführung valve) doesn't always have to be obvious.
'A missing signal for the turbocharger pressure regulation, that's the problem.'

I can hardly imagine that the head gasket defect has anything to do with overstressing. I haven't taken any tuning measures, nor do I stress the car in any other way. I just drive it every day through the rush hour traffic, and I'm happy when it occasionally goes up to 100 km/h. I can't even remember the last time I drove full throttle.


I can easily imagine that the cylinder head gasket would fail quickly if the engine is constantly overloaded.

So, we already have 2 things that we can check:
1. The turbocharger pressure control system.
2. Tightening torques for cylinder head bolts.

Hello,

Quote:

It is an interesting question why set screws can lose a portion of their preload during operation.

It's simply due to the different materials involved and their respective coefficients of thermal expansion.

First, the screws are tightened according to the specified procedure. This procedure is designed to ensure that the screws have a defined preload. When the engine heats up, the steel screws expand less than the aluminum cylinder head. Consequently, the preload increases. However, this warm/cold cycle is subject to hysteresis for various reasons, which means that as the number of temperature changes (i.e., engine cold/warm) increases, the preload on the screws continues to decrease.
The worst-case scenario for the head gasket is an older engine with lower sealing pressure that is immediately subjected to high stress upon starting in a cold state.
Modern multi-layer steel gaskets can compensate for this quite well, which is why it is no longer necessary to retighten the screws these days, and the head gasket also lasts almost forever under normal stress.
Instead of replacing the part, I would loosen the head screws and then re-tighten them according to the specifications, especially if the damage is still in its early stages. It requires little effort, and the chances of success are good.

Best regards,
Albrecht.

Hello Albrecht,

Am I understanding correctly that you're suggesting completely unscrewing the bolts and then tightening them again according to the appropriate repair manual procedure?

Do you mean the old screws are sufficient, or do they need to be new? Once, I replaced the head gasket with a fellow forum member, km124, on his old Audi 100 TD (87 horsepower, I think). It was completely shot at the time. For cost reasons, we reused the old screws back then. After that, it ran great again, but unfortunately, km124 had to park the car in oncoming traffic a short time later , so there are no real long-term experiences with it.

I assume that the coolant needs to be drained beforehand, right?

Have you actually had any practical experience with re-seating the old gasket, or is that more of a theoretical suggestion?

Hello,

It is necessary to completely loosen all the screws in order to achieve the correct preload again. A specific torque must be applied, and then it is required to tighten it an additional 90 degrees twice.

You might be able to skip loosening the coolant entirely if you follow these steps. Loosen each bolt individually (leaving all others tightened), tighten them to the specified torque, and then turn them another 90 degrees. Repeat this process for all the coolant bolts, one after another. Then, finally, tighten all the bolts again, this time in a criss-cross pattern, for the remaining 90 degrees. This should restore the original tension. If the cylinder head gasket is still leaking, it means you haven't put in much work.

Sincerely,
Christian

Hello Meyert,

Christian has essentially already written the answer. But draining the coolant isn't much work either. The head gasket and screws can withstand being tightened again. I can also confirm that from my own experience.

Best regards,
Albrecht.