hi guys, i want to share because i fixed my car in a way i haven't seen come up here yet:
i've had several MCSF and less-than-optimal running in the 4 years i've owned my 156 1.9 JTD 16V 140hp (220.000kms), lately a P1235 continuously on (which didn't alter the running apparently), and another boost failure when booting it up a hill in 4th, 5th, or 6th (long boost), making me loose the boost (ignition on/off and voila, boost is back).
after my mech changed the clutch a couple of weeks ago, i lost all power down low all the time and i finally decided to get in there and check everything bit by bit...cleaning EGR, MAP, MAF... I even did the Mr Muscle turbo clean (but the vanes were free anyway).
Anyway, actuator still wasn't moving...i wasn't getting vacuum to the turbo...checked the lines, all seemed OK, checked further, didn't even get vacuum to the VNT...checked the lines, all seemed OK, traced it back to the vacuum reservoir...no vacuum there either! a-ha.
I started tracing the lines going to the reservoir, and i found a cracked little hose at the first T-junction coming from the vacuum pump (right of engine, behind/under the oil filler cap). This hose looked like 10y old (which it probably was). The vacuum reservoir not only supplies the turbo actuator, but also a valve very close to the EGR and the big top turbo pipe, so that's working now too i guess.
Just went for a little drive, and all power is restored (wholy cow, goes like stink now), permanent MCSF went away, and even when booting in 4th for 10 seconds or more (on my private track of course), no more boost failure. All is well. finally.
so in short: do not only change the 3 little hoses in the front, but check the vacuum reservoir also if you still have issues. You'll have bleeding knuckles but it's doable to change all the lines there as well.