one that has been puzzling me for years on a 1.25 Fiesta

Could there be some sort of fault in the fuel system that is limiting the amount of fuel it can deliver? That is, it can only deliver enough fuel for full throttle up to 4400 RPM then it becomes too lean to develop full power.

it is a very sudden change, IME going weak would not usually be so abrupt and so exact. It appears to be something that is triggered at an exact engine speed rather than just load, so electrical something or other seems favourite, but as a test I could rig up a fuel pressure meter and see if it suddenly drops when the fault occurs. It would be interesting to see exactly what the ecu is meant to be doing at certain engine speeds, there are so many variables in these systems it is a pain in the proverbial, give me a carb and points ign. and diagnosis is much simpler.

Still at least the customer is happy enough!

I think Dervman had a nice solution for that.

It's not something dully old fashioned like the valve springs or hydraulic tappets gone soft & starting to bounce?

that is hard to judge, but since it is low mileage I tend to doubt it. it is very exact when it occurs and it can rev higher (valve bounce tends to limit the speed ), also there are no funny noises or anything. If I had spare coils and tdc sensors then they would be my first things to try, but random replacement at customer expense is not my style, especially since they are happy with the car, and probably one of them will drop dead before the car does, I would like to find the fault for my own satisfaction!

My Nephew has one and has complained that it too sometimes holds back until warm. How do I get the codes? Is there a connector I can short and watch lights blink on the dash or does it need a PC and USB code reader?

Going lean is very abrupt when it causes lean misfire. Before that leaning out just holds back as it reduces power. Unless you hit audible det you won't have much indication of going lean. Most EFi have a knock sensor so will pull the timing to stop det or keep it inaudible.

Could be the fuel pump, filter, tank vent, big unmetered air leak.

My case of going lean was down to the nicely toasted circuit board with dry joint on the 2A PC mounted relay and connector in the Foxguard alarm that some "professional alarm fitter" had wired the 5A fuel pump + air reg circuit though as the 2nd immobiliser circuit. I know it's 5A as that's what I got on the meter when I removed the fuse and stuck it across the fuse contacts. Drive around town just fine, get on a DC and go to overtake someone and it just didn't pickup, press the pedal harder and it cut.

In article , Mrcheerful writes

Some kind of software limiting in the ECU to protect the engine? (Since you say it's "very exact when it occurs")

that is what it feels like, usually these engines will rev to over 6k with no problems. I like someone else's idea of it going weak and the anti knock circuit coming in and limiting the speed, but once again the exactness of it ? I would have expected the weakness to have a variable effect depending on engine tempereature and loading, this is the same from a luke warm engine, hot engine, on the flat, up hill etc. What I need to get is an identical fiesta in and swap some parts around, if I can establish what is at fault, bob's my uncle.