Vintage Tin Can

My Caddy TDI (AHU) experiences a loss of power above 120 km/h.
Nach erneuten Starten war die Leistung wieder da, bis 120 oder Vollgas im 2. oder 3. Gang.

The error code reported in the fault memory is 'Intake manifold control differential.'

While checking, I noticed that the vacuum sensor for the VTG (Variable Turbine Geometry) is not working.
(Thank you to the author of the post for explaining how to check the VTG.)
The throttle shaft does not move when starting the engine or when pressing the accelerator.
After removing the rear nut from the adjustment rod (and measuring the exposed thread), I was able to easily move the VTG lever --> This means the VTG on the turbocharger is likely working correctly.
The test using a 50 ml spray applied directly to the can also showed no effect on the control rod.
(Vacuum). When I tried it with pressure, the control rod moved away from the container.

Is the can broken?


[/b]

Hello.
The AHU has a waste gas turbocharger that only operates under positive pressure in the can. Therefore, the can itself is likely fine. The problem is probably located elsewhere. To avoid costly parts replacements based on suspicion, it is recommended to read the error memory and measurement blocks.

Just type 'AHU' into the search function, and you'll find a lot of reading material.
Check out the section: 'Engine, Transmission, and Electrical' under the article: 'Intake Manifold Pressure Control Difference.' You'll find the usual suspects described there.

Best regards, Holger.

The AHU does not have a VTG!

As I mentioned before, measurements would be helpful. Without them, it will just be guesswork. But perhaps you can help yourself: use the search function and enter 'performance loss 1z'. You'll find plenty of suggestions there.

If I were in your position, I would carefully inspect the charging cable and its connection points. It should be airtight.

Thank you for the information.

I'm going to order my VAG-Com and connector today, otherwise it will probably just be a matter of luck to find the problem.

Today, while troubleshooting, I measured the boost pressure. I connected a pressure line from the intake manifold to the control unit, installed a T-connector, and attached a pressure gauge with a long hose.

The boost pressure increases up to 1.2 bar when fully accelerating in 2nd gear - after that, the engine goes into a limp mode, with no power.

The boost pressure will not exceed 0.9 bar.
I think I can rule out the intercooler circuit with this.

The boost pressure is far too high for a standard configuration.

It could be the boost control valve, or the pneumatic control system in general... or a stuck/difficult-to-move wastegate.


it is interesting that things like excessive boost pressure and emergency mode only become apparent later. this actually belongs in the original thread...

Hello,

If you already have an LDA (Lambda датчик), you can easily check the function of the wastegate. Simply connect the hose that runs from the intake manifold to the boost pressure control valve (BCV) – it should be red – directly to the pressure sensor of the wastegate – it should be blue. The boost pressure should then not exceed 0.6 bar. It's also possible that the thin control hoses to the BCV are leaking. Typically, the hose connecting to the intake manifold is red, the hose connecting to the pressure sensor is blue, and the hose connecting to the intake pipe between the MAF sensor and the throttle body is black. There's also a black hose that connects to the control unit, which is where you probably have your LDA connected. If you haven't replaced the hoses yet, it's possible that you haven't ruled out this source of error. Ideally, one of the hoses is disconnected or leaking. A new boost pressure control valve costs about 30 euros, but everything else could be more expensive or a matter of luck.

Best regards from Mulag.

Hello,

Thank you for the tips.

I still tested quite a few things today.

But before that, I adjusted the threaded rod on the waste gate to its original length.
(I changed it yesterday, but I didn't revert it.)

I also checked the mass airflow sensor (MAF): with the ignition on, the voltage is 0.27 volts, and it increases to 4.20 volts at full throttle --> the MAF should be working correctly.

I have pressure-tested all the hoses connected to the solenoid valve (for the wastegate) using a small pump with a pressure gauge, as well as the pressure line to the control unit. All pressures are stable.

I also tested the pressure vessel (Wastegte) using the pump. It starts to move at 0.8 bar, although I don't know the exact accuracy of the pressure gauge.

During the test drive, the boost pressure rose to 1.1 bar and then dropped to 0.9 bar, after which there was no more power. The boost pressure was still present; when releasing the accelerator, it was 0 bar, and when pressing the accelerator, it increased to 0.9 bar.

ABR - Hose connected to the can and clamped with a hose clamp --> Boost pressure increases up to a maximum of 0.8 bar. If the boost pressure remains at approximately 0.8 bar for more than 5 seconds, the engine enters a limp mode. However, the boost pressure is still present, or it builds up to 0.8 bar after acceleration.

What should the boost pressure be? (0.75 - 0.9 bar?)

Thank you in advance.

Bernd.

Have you checked if the hose to the pressure sensor (in the engine computer?) is okay and hasn't come loose?
Your symptoms remind me of something... I'll probably have to include it in the article about common mistakes.

Hello Ulf,

I also checked the hose connected to the control unit.
My pressure gauge is also connected to the hose.

I believe I won't be able to proceed without VAG-Com.

I will also check something tomorrow: I'll inspect the EGR valve to make sure it's closing properly.

Regards,

Bernd.