Mercedes-Benz W639 boost pressure control and limp mode
My W639 lacks power at low revs or goes into limp mode, what should I check in the boost pressure control?
The turbocharger control works on vacuum, so first check that there is enough vacuum and that the turbo actuator rod moves. A seized rod, a worn or disconnected vacuum hose and a leaking charge air pipe are common causes.
Why does the turbo actuator rod not move?
The rod may be stuck or the turbo vanes may be sticking. Often, however, the cause is a lack of vacuum: the thin hoses may be cracked, chafed or disconnected. Once the leaks are fixed, the rod starts to move. Replacing only the vacuum valve is not always enough.
Can the boost pressure sensor cause limp mode?
Yes. A fault in the sensor or its wiring can make boost pressure drop at full throttle. A new sensor does not always help if the fault is in the signal wiring, so the wiring is worth checking. The sensor must be the correct type for the vehicle (Viano 3.0 CDI: 1.27 bar).
What does fault code P0069 mean in the W639?
P0069 refers to the manifold absolute pressure sensor, that is, the boost pressure measurement. The code does not usually turn on the engine warning light. If limp mode returns even after a new sensor, the cause may be a boost pressure overshoot, and a more detailed diagnosis is a job for a workshop.