Fiesta 1.0 EcoBoost – turbo replaced,no boost, lack of power

Hi all,

I’m looking for help diagnosing a severe loss of power / no boost issue on my 2015 Ford Fiesta 1.0 EcoBoost.

Background / work done

• Turbo recently replaced (old turbo compressor nut came off)

• All boost pipes cleaned thoroughly and refitted and smoke-tested (no leaks found)

• Intercooler removed and thoroughly cleaned

• replaced oil and filter

• Vacuum line to wastegate confirmed connected and actuator moves

• Car idles smoothly, no knocking or abnormal engine noise

• Oil level is correct

• Worked perfectly for 15 - 20 mins on first drive

Current symptoms

• Same symptom of when my first turbo went, sounds like air noise "shuuup" and loses power - was happening around 3000rpm 

• After the issue came back I run a log with Forscan - [url=https://postimg.cc/fVRKBvPP][img]https://i.postimg.cc/8zh0kKmT/Screenshot_2026_01_02_172433.png[/img][/url]

• Car has very little power, struggles to accelerate

• Cannot rev past ~2000 rpm under load

• Juddering / hesitation when trying to pull away

• When slowing to let traffic pass, engine stalled and oil light came on briefly and stalled, but went out immediately after restart

• ChatGPT suggested disconnecting battery for 15 mins then doing a "relearn drive" - was even worse after i done this couldnt even get to 2000rpm in gear 1. Then i had to pull in to let a car pass, oil light came on and car stalled, turned it back on and managed to get home oil light didnt come back on engine is at normal temp and sounds perfect on idle

FORScan observations (screenshots attached)

• MAP stays around ~95–100 kPa (no boost)

• TIP pressure also ~100 kPa

• Wastegate duty cycles but boost never builds

• Throttle position and pedal position change, but ECU seems to limit torque

• Load stays low

• No active fault codes

What I’ve tried

• Battery disconnected to reset adaptations

• Throttle relearn procedure

• Gentle relearn drive (made symptoms worse — very low power)

• Verified vacuum supply to wastegate actuator

• Checked intake pipes and connections

• Engine idles fine after restart

Current behaviour

• Starts and idles normally

• Revving stationary feels a bit sluggish

• Under load it feels heavily restricted

• Feels like ECU is limiting torque / entering limp strategy

• No permanent warning lights

Question

What would cause ECU torque limiting / no boost with no DTCs on this engine?

Could this be:

• throttle correlation issue

• accelerator pedal / throttle disagreement

• misfire under load

• boost control plausibility

• protection mode after oil pressure event

• something else common on the 1.0 EcoBoost?

Any guidance on what to check next (specific sensors, relearn, or known faults) would be appreciated. It is driving me insane.