Low Compression: VW 2.5 TDI V6 AFB Engine

Hello,

This is the type of engine:

VAG 2.5 TDI V6
MKZ: AFB
Mileage: 91,000 km.
Cartronic Powerbox - www.dieselchip.de

I have insufficient compression pressure in all six cylinders and overpressure in the cooling system.
Here are the data for all the cylinders.
1. 18.4 bar
2. 17.5 bar
3. 17.7 bar
4. 18.2 bar
5. 17.6 bar
6. 17.8 bar
measured multiple times using VAG 1381 and VAG 1763.
Crankcase pressure: 271 mbar - normal: 5 mbar.

My question:
My dealer told me that the pistons are damaged and a new engine is needed.
Could this low pressure be caused by wear on the piston, and is that possible with this mileage, or could there be another cause for the low pressure?
It should be noted that the back pressure in the exhaust manifold is excessively high, and there is oil mist in the exhaust gases.
see this thread for more information:

/viewtopic.php?t=2924&highlight=print+crankshaft_housing

Thank you for your help.
v6Motion

The piston rings will likely be worn (assuming the head gasket is okay). It would also be possible to install new, oversized piston rings along with new pistons, but who manufactures those for your engine?
Normally, wear and tear isn't related to mileage, unless the engine has been abused. Another possible cause could be the ignition coil; it's best to discard it and get a new one with the new engine, that's a better solution.

Hi,

v6Motion wrote:

... I have insufficient compression pressure in all six cylinders and overpressure in the cooling system. Here are the data for all the cylinders. 1. 18.4 bar 2. 17.5 bar 3. 17.7 bar 4. 18.2 bar 5. 17.6 bar 6. 17.8 bar ...

What are the wear values for this engine (I unfortunately can't find them)? The engine has a compression ratio of 19:1 (as far as I remember). I would describe these values as nearly normal.
A difference of just under 1 bar is not critical.
Crankcase overpressure can also have other causes.

As Julian already mentioned, I believe that excessive wear and tear for the mileage is not normal.

So, the new pressure is approximately 27-33 bar above atmospheric pressure. The wear limit is 24 bar, with a maximum difference of 5 bar per cylinder.

Hi Julian,

Julian wrote:

So, the new pressure is approximately 27-33 bar above atmospheric pressure. The wear limit is 24 bar, with a maximum difference of 5 bar per cylinder.

Are you sure about that?
That seems a bit high for a turbocharged diesel. Those are the exact values I was getting from my old naturally aspirated diesel engine.

That's really surprising that the wear was supposed to be so even. I haven't encountered that before. Well, you never stop learning .

Hi,

What are the wear values for this engine (I unfortunately don't have access to them)? The engine has a compression ratio of 19:1 (as far as I remember). I would describe these values as nearly normal.
A difference of just under 1 bar is not critical.
Crankcase overpressure can also have other causes.

As Julian already said, I believe that wear and tear at the mileage level is not normal.


Hello Bertil,

You need to explain to me now how you establish a relationship between the compression ratio and the pressure values, and especially how you account for the difference of 1 bar.

Best, Ben.

Hi Ben,

Ben1972 wrote:

... You need to explain to me now how you establish a relationship between the compression ratio and the pressure values, and especially how you account for the difference of 1 bar.

I didn't see a connection between the compression ratio and the (cold) compression pressure, so I asked about the wear limits for the compression pressure. Unfortunately, I only had the value for the compression ratio in mind, which isn't very helpful for determining the compression pressure.

With the difference of 1 bar, I meant the difference between the maximum and minimum readings of each cylinder (so 18.4 bar to 17.5 bar) - that probably didn't come across correctly - sorry. However, Julian probably understood me correctly .

Julian probably understood me correctly .

If I didn't exist...

The values are listed here in a workshop manual, for example, a PD_130 model has a new pressure of 25-31 bar, with a wear limit of 19 bar.
The PD engine has a compression ratio of 19.1:1, while the V6 TDI engine has a compression ratio of 18.5:1.

@Everyone

Thank you for your explanations. Since the pressure is uniformly low in all cylinders, I suspect wear, which shouldn't be severe at this mileage. It's possible that the engine has actually run much more than the odometer indicates.
Do you see it that way too?
Is there a way to find the actual mileage of a vehicle? Is this information stored anywhere in a way that it cannot be erased?

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

'Hello'
v6Motion

Hello,

To my knowledge, the total mileage is not saved anywhere else.
You could try contacting VW, because recently, service data for Audi vehicles has also been stored centrally at the factory (although I'm not sure if that's guaranteed, I heard it somewhere).
However, considering the compression values, the machine must have at least 300,000 km (or even more) on it. And you would definitely notice that in other components as well (suspension, seats, etc.).

Greetings.

The operating hours of the control unit are also counted and stored. Assuming an average speed of 50 km/h, this can be correlated quite well with the total distance traveled.

Hi,

Okay, let me chime in: a potential source of error could be an unsuitable pressure testing device with a volume that is too large in the measuring connection.

Best regards, Rainer.

It's possible, but he also measured the overpressure in the gearbox, which is far too high (using a different pressure gauge). Two measurement errors at the same time? That would be quite a coincidence, wouldn't it?

Hi Rainer,

Rainer K. wrote:

... Okay, let me chime in: a potential source of error could be an unsuitable pressure testing device with a volume that is too large in the measuring connection. ...

It somehow seems suspicious to me. With such small deviations between the cylinders, I also can't believe it's due to wear and tear. My thought is that there might be a measurement error.
High KG pressures can also have other causes. However, in conjunction with this compression test, they hardly allow for any other conclusion.


A diesel engine that supposedly has such catastrophically low compression would also be extremely difficult to start, especially in cold weather. Perhaps v6Motion could comment on this.

Hello,

According to the workshop, the measurements were taken using VAG 1381 and VAG 1763, and this was likely done multiple times.
Furthermore, I was told that the intake valve for cylinder 2 and the intake valve for cylinder 4 were worn.

The engine still pulls well, and I haven't experienced any cold starting problems. The only issue is a rather long preheating time; once it's preheated, it starts immediately and runs, although it's a bit weak in power when cold.

According to VAG, the engine should not be running with these values. They are simply saying that everything needs to be replaced, with a total cost of €10,707.00.

Do you know of any wear values for specific mileage ranges that could be used to infer the actual mileage?

'I'm just trying to measure the piston clearance now. Is it possible to do this from the outside? I've heard it might be possible without removing the cylinder head, but I'm not sure if that applies to this model.'

I have no idea how to proceed from here.

The high wear (if it actually exists) could be caused by 40,000 km of driving with a powerbox, but that's probably not the case.

Thank you.
v6Motion

Hello V6Motion,

First, I would have the compression checked again, by a different mechanic in a different workshop.
Even though the values were low, the engine would have to have an incredibly long lifespan and experience serious problems (e.g., difficulty starting in cold weather)!
Nowadays, very little wear can be detected in the cylinder bores after 100,000 kilometers, usually still within the tolerances of a new part.
I would argue that if there are leaky valves or similar issues, the compression pattern would not be as uniform.

Good luck!