Hello, I hope the following is of assistance.
Regardless of what fault codes have or haven't been retrieved the engine management (PCM) is detecting a fault and switching to limited operating strategy (LOS) this is why the engine lacks power and won't rev above 3000 RPM. You'll find that when the concern happens and you return the ignition off then restart the car the engine will be back to normal until the PCM detects the fault again.
As the power loss is usually after accelerating then this would suggest either a turbo boost pressure concern or a fuel pressure concern.
If when accelerating the turbo boost goes too high and the PCM can't reduce it fast enough then the boost is set to zero and the PCM switches to LOS, once the ignition is turned off and the engine restarted the engine will run OK until the over boost is detected again.
The same applies to the fuel pressure, if the fuel pressure is too low or too high (usually too low) and the PCM can't correct it within a specified time then it will switch to LOS. In both cases the PCM switches to LOS to try to prevent any possible engine damage.
The reason for a turbo boost concern could be a sticking turbo actuator, sticking vanes in the turbo, a blocked pipe or faulty Sensor.
Fuel pressure concerns tend to be due to a partially blocked fuel filter, if the filter hasn't been replaced recently it would be good practice to replace this.
With regard to no fault codes been retrieved, a DTC has been set for the PCM to switch to LOS but not all code readers will retrieve all DTCs especially manufacturer specific codes relating to the DPF (if equipped) although DPF concerns tend to give a permanent lack of power.
If possible check live data on the engine when the fault occurs, this way you can check the boost pressure and fuel pressure when the fault is evident.
The only diagnostic tool 100% certain to pick up all associated DTCs is Ford IDS, most EOBD scan tools on the internet tend to only read set codes and no pending codes.
I hope this helps but let me know if you have further questions regarding this and I'll reply as soon as I'm able to.
Sean, thank you for the information. I will act on it and try to get the car fixed. Once it is fixed I will be able to rate you fairly.
no problem, let me know if you need further information.
The power loss problem occurred again today. I tried turning the engine off and on again like you suggested but the problem persisted. There was low power straight away, even without accelerating hard. The problem only seems to temporarily go away if I leave the engine off for, say, several hours. Does this narrow down what could be causing the problem?
I would still be inclined to suspect the fuel pressure, if there's a restriction anywhere in the fuel it may be the engine is being starved of fuel but once left for a while the restriction if debris has moved around allowing fuel to flow again.
You really need live data from when the fault happens and as it continues you should be able to see easier which sensor or actuator is not reading as expected.
If it is fuel pressure related then the rail pressure will be incorrect.
Let me know if I can assist further.
Hello Sean, thanks again. There's been another development. This morning, when I turned on the engine, the LCD display on the dashboard read 'Low Fuel Level' and there was a warning beep. This disappeared after a few seconds. I had only just filled the tank, and the fuel gage read as normal, so it appears as if that warning was faulty. Does this backup your suspicion that the problem is fuel-related? Martin