BMW E46 diesel not boosting
Why does the E46 diesel not boost or lack pull at low revs?
The most common cause is the turbo's vacuum control. The turbo's guide vanes are open at rest and close with vacuum, so without vacuum the boost only rises at high revs. A disconnected or leaking vacuum hose, a faulty boost pressure control valve or a stuck vane mechanism causes this.
How do you check the turbo's vacuum control?
After starting, the vane adjusting rod should move, and disconnecting the vacuum hose makes the vanes close. The rod's travel is at most 10 mm: fully in at 650 mbar of vacuum (boost at its maximum) and fully out with no vacuum. The rod's movement can also be tried by hand. Replace all cracked vacuum hoses.
What is the normal boost pressure on the E46 diesel?
Normal boost pressure varies with the engine and the gauge. When interpreting a reading, remember that diagnostic software often shows absolute pressure: atmospheric pressure (about 1000 mbar) plus boost. For example, 2100 mbar means about 1.1 bar of boost. The boost pressure sensor is directly in the intake manifold, so it cannot be read from a hose.
How can you tell the turbo itself is broken?
A whistle from the turbo under acceleration is one of the first signs. First check the boost pipes and the exhaust manifold gaskets, because leaks there whistle and cost power. There must be no hand-detectable lengthwise play in the turbo shaft; a little sideways play is normal.