Diagnostic help on S-Max 2.5T

[NDT]

Car hesitates slightly at part throttle.
Idles OK, runs on boost ok.

I have had a diagnostics specialist look at the car and his view is:
- The long term fuel trim is leaning the mixture out - i.e. it's running rich at some point and this is causing the trim to change.
- both lambdas working OK
- MAF and MAP working OK
- fuel rail pressure OK and injectors OK

He now thinks it's the variable vanes on the throttle which have stuck open, causing a lack of boost at part throttle.
I'm slightly baffled by this - I can see why might cause a part-throttle hesitation but not why it would make the mixture rich. I would assume that the MAF would correct for the lack of air flow? Or is there a transient fuelling thing going on which would send it rich?

Does anyone have any thoughts?

[NDT]

Heaven help us, now he reckons it's the wideband lambda after all...

[NDT]

Just in case it ever helps anyone on here - eventually I changed the MAF and all was well again.
Despite the idiot technician insisting the MAF was OK...

[anonymous-user]

Please tell me it didn't take over a year to fix it!!

[crossy67]

Fairly common problem with MAF equipped cars. As I see it (might well be wrong) they run on closed loop up to a point round about full boost is developed then they switch to map ignoring the MAF.

Tell me if I'm wrong please.

[NDT]

Dave_ST220 said:

Please tell me it didn't take over a year to fix it!!

No (not quite!).
Was a niggle rather than a massive problem.

[NDT]

crossy67 said:

Fairly common problem with MAF equipped cars. As I see it (might well be wrong) they run on closed loop up to a point round about full boost is developed then they switch to map ignoring the MAF.

Tell me if I'm wrong please.

I thought MAF was used in both open and closed loop?

[crossy67]

That's why I said tell me if I'm wrong.;) I suppose every vehicle is different.