High Boost Issues: What Else Could It Be?

Hello,

My problem car is an Alfa Romeo with a 2.4 JTD (common-rail) engine. It's the turbocharger with a VTG (Variable Turbine Geometry) system, which uses adjustable turbine vanes.

For some time now, the turbo has been producing too much boost pressure. So far, we have taken the following measures:

Turbo removed, VTG adjustment cleaned. During the process, it was discovered that the turbine blades are corroded. The blades were lightly re-ground.

The VTG valves were adjusted until the entire mechanism could be easily operated by hand.

Turbo reinstalled, but the turbocharger is still producing too much boost pressure. The measured boost pressure is 1.5 bar (pressure gauge not calibrated, accuracy approximately +/- 0.1 bar).

The pressure sensor and adjustment mechanism are working. (Pressure was applied to the sensor, and during a test drive, there was little power in the lower RPM range, and the engine electronics no longer shut off.) The adjustment pressure of the pressure sensor is not yet known.

Is it reasonable to adjust the lever arm of the adjustment lever, connecting the pressure sensor to the VTG adjustment mechanism?

Could it be that the pressure sensor is defective, causing the actuation pressure to be too high?

What else could it be?

Regards,

Ingo.

Hello Ingo,

You can check if the actuator for the VTG adjustment is working by using a bicycle pump.
I don't know whether you have a pressure or vacuum control system, but it's easy to check. When the engine is idling, simply disconnect the hose from the vacuum actuator. You should be able to feel whether it's drawing air in or pushing air out.

If you unscrew the bicycle pump and mount the piston upside down, you'll have a simple and inexpensive vacuum pump.

When you apply pressure or vacuum to the adjustment cylinder as a test, the VTG adjustment must move smoothly and without any jerking to any position between the mechanical stops.
Testing the airtightness of the membrane in your adjustable container is simple: using pliers, squeeze the tube in various positions, and the adjustment should remain in place.

Is the spring in the pen case not broken?

You probably unscrewed the adjustment housing from the adjustment lever, or from the loader, in order to clean the adjustment mechanism.
Here's a potential problem: If you didn't tighten the can exactly the same way (2 turns of the adjustment nut on the actuator rod can be crucial, as can any shifts during the can's mounting!), it's possible that the boost pressure becomes too high, even if the magnetic valve is being electrically controlled correctly for boost pressure regulation.
Reason: Despite the motor electronics adjusting to the maximum extent, the adjustment lever is mechanically unable to reach the necessary position.
To properly adjust the length of the actuator rod, you have several options.
Since I don't know your measurement equipment, I have a few questions about it:
- Do you have the possibility to display the timing relationship of the magnetic valve to the VTG (Variable Turbine Geometry) adjustment, for example, via the engine control unit's diagnostic system?
- Do you have an analog multimeter?

If you don't have either of those options, or if it's not easy to connect the multimeter directly to the electrical wires leading to the charge pressure control solenoid valve, here's a cumbersome method:
- Determine the length setting of the control rod at which the lowest boost pressure is achieved. To do this, simply try different positions, each approximately 3mm apart, and perform a test run each time.
- Once you've found this setting, the engine shouldn't be able to build up too much boost pressure during acceleration. Your car should feel relatively sluggish and lack power.
Now, adjust it in the opposite direction, making only 2 turns of the adjustment nut on the actuator rod each time.
- You're still doing a test drive every time, and you keep doing it until the engine, in 3rd gear, after at least 3 seconds of full throttle, starts to recognize the boost pressure as too high (entering a limp mode with reduced power).
- Then, you gradually adjust the accuracy to within one revolution, approaching the point where the motor starts running.
- With the current outdoor temperatures, you only need to turn it back about 1 1/2 rotations from this position. Your VTG (Variable Torque Geometry) actuator rod is now correctly adjusted.

The duty cycle of the voltage at the solenoid valve for turbocharger pressure regulation must now be set such that it does not reach the control limits at any operating point of the engine. If possible, you should check this.

Are you sure that the length of the actuator rod is correct, and that the position of the adjustment cylinder is also correct? If so, you can skip the procedure described above.
Because, depending on the type of VTG adjustment you are using, there may be other potential sources of error.
To do that, you would need to tell me whether:
- The VTG pressure relief valve, operated by vacuum.
- or is operated at a specific pressure.

Best regards, Rainer.

Hello Rainer,

Thank you in advance. I'll try that out. Is it possible that the pressure vessel is defective? We recently pressurized it, held the pressure with a shut-off valve, and after a test drive, the pressure was still there. That actually means the membrane must be intact.

Regards,

Ingo.

Hello Ingo,

Burgerking wrote:

Hello Rainer, Thank you in advance. I will try that. Can we assume that the pressure vessel might be defective? We recently applied pressure, held it with a shut-off valve, and after a test drive, there was still pressure. This actually means that the membrane must definitely be intact.

Yes, if it's a pressure sensor and not a vacuum sensor.
Are you sure the can is being activated by pressure?

Best regards, Rainer.

Yes, the supply hose is under pressure, and the adjustment mechanism is moving in the direction of the pressure. During the test drive with the shut-off valve open and under pressure, the engine also had very little power in the lower RPM range, but in that case, the warning light didn't come on.

I can say with a fair degree of certainty that it's a pressure vessel.

To create a vacuum, wouldn't you also need a nozzle in the intake hose to achieve a Venturi effect?

Regards,

Ingo.

Regarding your question: In diesel engines, vacuum is typically generated by a dedicated pump.

Great, now we have 2 possible causes for the excessively high boost pressure.
- The pressure regulator is not receiving enough pressure.
- The VTG adjustment rod is the wrong length, or the position of the vacuum sensor has been altered. Even a difference of 2mm can be critical here.

You can determine the cause as follows:
- Use a pressure pump and pressure gauge to determine the pressure at which the VTG (Variable Turbine Geometry) adjustment mechanism just touches the limit for the minimum pressure increase. Note this value down.
- Then, you take it for a test drive, and connect your pressure gauge directly to the pressure hose in front of the regulator using a T-connector.

Now you can check if the pressure value measured earlier at the control valve is consistently reached when driving with full throttle for several seconds. Please post your results.
If it is reached, your adjustment rod is set too short; if it is not reached, then not enough adjustment pressure is being applied.

Best regards, Rainer.

Hello Ingo,

Do you have a qualified mechanic working on it, or did you disassemble and clean the turbo yourself? Were the same screws reused?

Best regards,
Achim.

What's slightly confusing me right now is:

In our TDI engines, the turbocharger is set to the minimum boost pressure when it is not in operation. This makes the system inherently safe.

'It confuses me that if the turbocharger is now pressure-actuated, and you apply pressure to it, but the engine barely builds boost and power. Because if the turbocharger isn't activated, it should be at its maximum boost level, even in the event of a hose rupture... Is that really possible? That would be a huge mistake.'

CU Gremlin.

Hello Gremlin,

Of course, we disassembled the turbocharger ourselves .

'However, we preferred to leave the VTG adjustment mechanism in place and clean it while it was installed. The Allen screws that secure the VTG adjustment mechanism looked quite special. I don't think they need to hold very much, but the temperature stresses are quite significant...'

When I asked the Alfa dealer about the screws, he just stared at me with wide eyes and said they probably never opened up the turbo.

Do you know of a good repair shop where I can get this fixed at a reasonable price?

Regards,

Ingo.

Hello Ingo,

Do you have any results yet?

Best regards, Rainer.