BMW E60 530xi N52B30 (258 HP) – lack of power + random misfire cyl. 6 + unstable idle

[Daw2300]

Hi everyone,
I’m posting because I’ve been struggling for a while with a clear lack of power in my BMW E60 530xi (N52B30, 258 HP, automatic) and I feel the problem isn’t the engine itself, but somewhere around it: accessories, exhaust, drivetrain, or electronics.

Problem history – chronological
1️⃣ Old engine

Before the failure of the previous engine, the car lost noticeable power – subjectively maybe ~170 HP.

I was already scheduled to remove the catalytic converters, as they were the main suspect (cat error codes have been present since I bought the car).

A few days before the mechanic visit, the oil pump failed, so the engine needed replacement.

2️⃣ Comparison with Honda Accord VII 2.4

This test was done on the old engine, when it was still running well (before the power drop to ~170 HP).

Honda heavily modified, estimated ~210–220 HP

Result: cars were practically neck and neck, even though my BMW should have had an advantage

3️⃣ DIS and VANOS replacement (after Honda test)

After the Honda test, the DIS units were replaced with cheap ones → but since the test was done on originals, DIS likely didn’t affect the result

VANOS solenoids were also replaced with new original parts

4️⃣ Engine swap

Mechanic planned to remove the cats since the engine was already out

After inspection, he said the cats didn’t look too bad and didn’t remove them

After picking up the car, power felt restored (~220 HP?), but no measurement was done → it could be the same as now, just felt stronger after switching from the weakened engine

Within about a month after pickup, the car started stalling every 2–3 seconds → mechanic said it was the VVT sensor, after clearing the codes the problem didn’t return

5️⃣ Comparison with Mercedes W203 3.2

Test done on the new engine

Mercedes: 218 HP, 4MATIC, automatic, weight difference ~15 kg

Result: sometimes perfectly even, sometimes I would very slowly start pulling ahead

Conclusion: with ~40 HP difference, my BMW should clearly leave the Mercedes behind, not struggle alongside

Additional issues after engine swap
1️⃣ Misfire – cylinder 6

About a month after the swap: misfire cyl. 6

Swapped coil and spark plug → no effect

Suspected LPG: gas injector swapped → no effect

Gasoline injectors replaced → error persists

In ISTA: error occurs regularly every ~20–30 km

Mechanic then suggested possible mechanical issue → too low compression

2️⃣ Compression measurements

I bought a compression gauge and tried to measure myself

First attempt: didn’t disconnect VVT, got ~8 bar on all cylinders

Second attempt (following online instructions): disconnected Valvetronic motor, set it to 177° as ISTA recommended → readings the same or slightly worse, still around 8 bar

Note: in both measurements, I didn’t disconnect fuel supply, so readings may not be fully accurate

3️⃣ Unstable idle

Right after pickup: stable

Last month: fluctuations ±50–150 RPM, sometimes more, mostly on cold start but also occasionally when warm

My suspicions

Exhaust system / catalysts (clogged, partially melted?)

Some engine accessory

Torque limitation by ECU

Drivetrain / AWD / transmission

Mixture / adaptation problems

Intake leaks, crankcase ventilation, DISA, Valvetronic, VANOS

Possible mechanical issue with compression, though my measurements were inconclusive

Question to you

Has anyone experienced a similar case:

two different engines, yet same (low) performance

random misfire on a single cylinder despite swaps

increasing idle fluctuations

Where should I start looking before replacing half the car blindly?

Thanks in advance, because right now 258 HP seems to exist only on paper 😅