[BMW7676]

Model: BMW 740xd F01
Year: 2010
Mileage: 289,000 km

Main issues: Slow acceleration and significant power loss, especially in first gear, with partial improvement in second gear, but only feeling like 20-40% of power. The power drops drastically even on slight uphill grades.

Initially, about a week ago, I noticed a power loss of about 20 percent. I then disconnected the mass air flow sensor (MAF) and nearly regained full power.

I then ordered a new MAF from Pierburg, as it was said that the original BMW MAF is also produced by them.
The MAF arrived the next day, so I did some experimenting and, following a colleague's advice, pulled the supposedly defective MAF while the engine was running to see if it still delivered values or was completely dead. When I did this, the engine certainly ran differently and went into limp mode, but after pulling the MAF, there has been no change since that moment, whether disconnected or connected, the same similarly poor performance as initially mentioned, about 20-40% of the power.

When the Pierburg MAF arrived the next day, I installed it immediately, reset all adaptation values with ISTA, and cleared the error memory. - No change whatsoever.

Since then, the following has been done.

A workshop known for tuning performed the following, according to the master there it would lead to 100% success: Deactivation of the EGR valve and removal of the DPF: These measures were taken to eliminate possible sources of power loss. - 10% more power as the DPF was completely clogged, and no (forced) regeneration was possible due to the errors stored.

Replacement of the pressure converters: Although new pressure converters were installed, this showed no significant improvement in performance.

Checking the boost pressure actuator: It was determined via ISTA that the boost pressure actuator only reaches 76% of its target position, instead of the expected 95%. This could indicate a problem with the boost pressure control valve.

Investigation of the turbine control flap and other components: These components were also checked via software, but without a clear finding regarding the performance issues.

Detection of a leak in the intake manifold: A leak was discovered through pressurization and smoke. A replacement of the intake manifold was suggested, although the workshop itself says that it can't be the cause.

The problem of power loss occurs regardless of temperature and weather, etc.
What I find strange: If I start the engine anew and then give full throttle for the first time, it struggles through the first gear with what feels like 60HP but then has much more power from the second gear on, I would even say about 70-80%.

Despite numerous tests and interventions, the cause of the significant power loss could not be clearly identified. The workshop suggests replacing the MAF again with an original part from BMW to eliminate possible compatibility issues. The situation is especially confusing since the performance improvement occurs after an engine restart and under certain conditions.

I have now reached a point where I'm not sure what steps to take next. Has anyone had similar experiences or has an idea what the cause could be? Could replacing the intake manifold be a solution despite the workshop's doubts? Are there specific tests or diagnostic procedures we might have overlooked?

Any hint, idea, or advice is welcome. Thank you in advance for your support.