rough idle during DPF regen / low coolant temps?

[RobX5]

YouTube - rough idle and poor fuel economy?

2016 X5 F15 sdrive 25d. 77,600km.

This rough idle is driving me nuts. No codes on ISTA+, otherwise idles fine and car seems to have power avail up high. But when a DPF regeneration is in process (confrimed via BimmerLink OBD) and the car comes to stop, the engine without doubt idles roughly.

I also suspect poor fuel economy, poor torque down low rpm as it struggles to stay in 8th gear at times on level rd at 80-100km/h. Average fuel economy for highway straight line unable to get below 7.5-8L/100km which is well over the sticker claims for this 2L.

Soft failing Thermostat was suspected as coolant temps were sustained 80-82C on long drives. There’s plenty of forums that point to poor fuel economy and bad TSTAT. So I had the TSTAT changed this week but temps still coming up very slowly (10-15mins) to 88C, but can see rapid drops back down to 80C when accelerating and sustain 82-84C after diving for an hour. All the forums here suggest these temps need to get above 88+. I’ve attached a log of the x5 (pre thermostat change) vs my 2015 320d for the exact same drive (roughly 20mins).
With the new thermostat, coolant temps happily stabilise at 88-89 (after lots of stop/start heavy traffic) but will drop rapidly as soon as you begin accelerating. Idle = 88. Acceleration or sustained driving = 82-84. Not sure if this is related?

This car is only recent purchase. It presented with oil level too high and diluted fuel in oil. Drained oil and replaced filter. Suspected partially blocked DPF due to some back pressure faults. BMW dealership updated DDE ECU to latest. Likely previous owners were short trips.

Did a chemical clean of intake manifold and DPF 2 weeks prior to clean suspected CBU. Improved performance somewhat but idle / economy issues remain. Have also run numerous fuel / injector additives without success.

I asked the workshop when replacing thermostat to investigate for vacuum leaks, DPF differential pressure sensor and line checks, injector tests etc but gave clean bill of health. Have reset adaptions for most and ran all diagnostics on ISTA numerous times (smooth idle, MAF flow test, back pressure tests etc.) but nothing flags.

I’m starting to think EGR related but really after some ideas here. As a last resort I’ve upped the permanent idle by about 100rpm to see if that combats the issue as I think it’s odd the idle speed doesn’t increase during a regen.

I’ve also found a few other folk mention this same issue for this same model engine on forums here and on YouTube, but could never ever find a cause. There’s no way it’s normal.

Any help on this matter would be greatly appreciated as I’m losing sleep and $ over this.

[RED_Y_]

Did you do fuel injectors test in ISTA?