Passat AHU: Turbocharger Boost Pressure Too High

The following problem:

Passat '98, 125,000 km, AHU engine, no modifications, error: boost pressure control differential exceeded.

I have taken the following actions:

Replaced all vacuum hoses.

Solenoid valve tested (in good working order).

Boost pressure sensor (without vacuum hose to the control unit) checked (actual value measured with a pressure gauge) corresponds to the value displayed by the tester.

The turbocharger pressure is definitely too high (greater than 2.2 bar).

Wastegate pressure sensor checked, OK. Wastegate linkage checked (was slightly bent, but moves freely).

Turbocharger removed and sent to a turbocharger specialist company for overhaul, inspection, or replacement.
"Except for a small amount of oil in the pressure chamber, no issues were found, everything is OK. The charger has been cleaned and returned."

Error still present.

Wastegate is working, the pressure sensor is functioning correctly (is approximately 1 cm of travel enough?), the solenoid valve is working...

I then adjusted the connecting rod and pressure sensor/wastegate assembly by another 1 cm, making it 1 cm longer overall, so that the wastegate opens earlier and the boost pressure decreases. This allows me to delay the "limp home" mode, but not eliminate it. The error now occurs in 4th gear, previously in 3rd gear, after a single rev to 3,800 RPM.

LMM is deviating by 20 mbar from the target value, which is within acceptable limits.

Why is the boost pressure too high?

Turbo is working, wastegate is working, the control system is working, but the overall system is not functioning correctly. What else can I do without my own diagnostic tool?

Unfortunately, I don't have my own testing equipment, so I can't provide or quote any measurement logs.

Perhaps someone can tell me what data I should have read out at a workshop (Bosch) so that the error can be better identified.

I'm definitely stuck at the moment.

Thank you for the tips.

Tomy3break wrote:

The turbo is working, the wastegate is working, the regulation is working, the control system is functioning correctly. What else can I do without my own tester???

Check the hoses from the compressor outlet to the solenoid valve and further to the wastegate control unit for blockages, constrictions, or leaks, including the connection point at the compressor outlet.

Hello Ulf,

Thank you for your answer .
"The hoses are all new and leak-free, the bores are clear, the solenoid valve is working when I activate it separately. The pressure is the same at both the turbo outlet and the auxiliary compressor outlet (pressure hose to the solenoid valve). During operation, the boost pressure increases to 1.15 bar (measured with a pressure gauge), which is 2.2 bar absolute. The diagnostic tool then shows 2.2 bar, and after 5 seconds, the control unit switches to limp mode."

If the pressure chamber is sealed and reaches its maximum travel at approximately 1 bar of boost pressure, then it is acceptable for me. Am I understanding this correctly ?

What needs to be checked, and what values do I need to determine in order to pinpoint the exact cause of the problem? I don't want to start replacing random components based on guesswork. Everyone says it's the turbocharger, but that one was sent in for a rebuild and isn't faulty...

Tom.

Tomy3break wrote:

If the pressure chamber is sealed and reaches its maximum stroke at approximately 1 bar of boost pressure, then it is acceptable for me. Am I understanding this correctly

I might be wrong, but I'm not very familiar with the details of wastegate chargers.

Quote:

The pressure is the same both at the turbo outlet and at the auxiliary outlet (pressure hose to the solenoid valve).

Then, measure the pressure that reaches the charge air cooler under full load.

Quote:

What needs to be checked, and what values do I need to determine in order to precisely identify the error?

See the technical documentation for the TDI pneumatics.

On Tuesday, I have the opportunity to take the car for a test drive with a diagnostic tool. I hope that after that, I will be able to provide more specific information about the problem.

Tom.

Hello everyone,

I was able to solve the problem today with the help of a Bosch tester.
During operation, it was observed that the boost pressure deviated from the target value, and the wastegate only responded relatively late. I then readjusted the linkage of the pressure sensor for the wastegate and tested the effect through test drives.

Result:

The error is no longer occurring. The actual boost pressure deviates upwards from the target value by a maximum of 0.1 bar across the entire speed range, but this mainly occurs under full load on the highway. The maximum value was 1.947 bar, while the target value was 1.846 bar. It appears that the pressure sensor or its membrane has changed its properties. It is still sealed and operates smoothly, but the necessary preload of the pressure sensor or the wastegate seems to have changed.

I'm going to drive for a few days now and see if the problem occurs again.

Does anyone know if and where you can buy the printing cartridges individually????

Regards,
Tom.