W639 exhibits low power with no apparent issues.

[Rebecca_Taylor90]

Apologies for the forthcoming lengthy and elaborate article. I often excel at resolving my own issues, but I am nearing my limit in diagnosing the problem with my Vito. It is a 2007 W639 111 equipped with an OM646 engine.

The failure was abrupt; it ceased producing boost, with a maximum speed at around 60 mph, and it struggles to reach 30 mph on inclines. During my seven years of ownership, I have had identical issues twice: once owing to EGR failure and once due to a malfunctioning turbo actuator. Both triggered the check engine light and associated problem codes, hence were promptly resolved. Aside from the significant power loss, it operates properly.


In this instance, the check engine light did not illuminate. Nonetheless, it generated a code indicating the malfunction of the MAF sensor (more details to follow). The replacement MAF sensor did not fix the issue.

I own an Icarsoft scanner equipped with MB software. I examined the values of all interrogable sensor readings.
Ambient and charge air temps align with my expectations.
Fuel, coolant, and oil temperatures are as anticipated.
Fuel pressure significantly increases with RPM, as anticipated.
The voltage of the O2 sensor fluctuates with load, however I am uncertain about the expected values.
Several more elude my immediate recollection, however all seemed typical save for:

Two seemed incorrect in the MB mode on the scanner.
The MAF value exhibits little variation, with units measured in mg/stroke (about 425 mg/str). Given that airflow escalates with RPM, I anticipate a somewhat stable figure irrespective of load, but not fully uniform. It stays unchanged even when disconnected, which immediately seems suspicious.
-The charge air pressure consistently registers at 800 hPa, regardless of whether the device is connected or not (presumably referring to the sensor with a three-pin connector located on the lower intercooler pipe).
However, when I exit the MB mode and use the basic OBD mode, both the MAF and charge air pressure sensors function correctly and provide credible values.
-Mass Air Flow (MAF) about 16 grams per second at idle. 70 grams per second at maximum velocity. 600 g/s when I direct the air compressor towards the sensor. This device ceased operation upon disconnection, as expected.
-The charge air pressure sensor was evaluated within a range of 100 kPa to 250 kPa using a syringe and hose for pressurization. Displays around 120 kPa in the vehicle when operated at maximum velocity.
The MAF and charge pressure sensors function properly according to the OBD system. I am uncertain if the engine module is accurately detecting them.

I brought the vehicle to a dealer for scanning using the Star tool. The ECU is incompatible and originates from a 109 Vito with a different VIN. The star system would not advance beyond that point. They are certainly accurate, and it is probable that a prior owner has replaced or cloned the ECU, perhaps after an immobiliser malfunction. However, it has not posed any issues over the six years of my ownership, so this is not a new problem. They were nonetheless pleased to accept £125 from me for connecting it.

I suspected a boost leak and conducted a pressure test on the pipes and intercooler. I discovered two little holes in the intercooler; nevertheless, they are insufficient to account for this amount of power loss. I have temporarily sealed them until I can repair the intercooler. Subsequent testing indicates no leakage at 2 bar.

The turbo actuator operates across its whole range as directed by the scanning tool. However, when driving, it remains in the no boost position unless during deceleration. I detached the actuator from the arm and configured the arm in the maximum boost position. The system now generates boost, as shown by the charge pressure reading on the OBD at 205 kPa, accompanied by increased turbo noise; nevertheless, it still fails to produce power, likely due to maintaining the same fuel injection rate as before. This demonstrates that the turbo is effective.

This leads me to conclude that the mechanism is functioning properly, although it is reluctant to provide power owing to an assumed situation it perceives. It would be preferable to assume that the pedal position sensor indicates I am using just half throttle. My scanner provides three distinct pedal position measurements across three different modes.
In the cruise control test mode, the throttle displays a smooth progression from 0% to 100% as expected. In the engine control module mode, it reaches 84% when completely depressed.
In general OBD mode, it reaches a maximum of 64% when completely depressed.
If I accelerate downhill, engage the cruise control, and maintain it over the subsequent uphill, I still experience a loss of power; thus, I believe the issue is unrelated to the pedal sensor.

I am currently depleting my ideas; I replaced the EGR with a functional unit I had available. New air and fuel filters, naturally. I am uncertain about the procedure for testing the O2 sensor, but it may be prudent to replace it to rule out that possibility, although I would anticipate a diagnostic code if it were defective. I removed the DPF years ago, so there are no concerns in that regard.

Aside from that, the only component I can consider is the ECU itself; however, unless it is a prevalent failure site in these cars, I would be very shocked to identify it as the cause.

I believe the true answer will ultimately be straightforward and evident - does anybody possess any suggestions?

[Rebecca_Taylor90]

Resolved now. The MAF required recalibration, despite its proper functioning. It detected a calibration error and entered limp mode. This specific error can only be resolved with proprietary STAR software, however it was completed in five minutes after I located an individual to do the task.

[Rebecca_Taylor90]

[David_Smith2003]

I am glad you have sorted it, Luke, and I appreciate you sharing the conclusion for the benefit of others.

[mortimerPAS]

Although it may have been verbose, it was certainly intriguing. Thank you!

[matthewlightburn]

Excellent suggestion

[matthewlightburn]