Just a quick update on this.
I had thought that the problem was fixed. The Patrol was behaving fine - as performance is great now (for a Patrol). It's also doing better for fuel consumption, so fitting the new MAF sensor was worth doing whatever.
We went away over Easter to Scotland - towed the caravan nearly 600 miles round trip. No problem on the way up, though I was really very gentle with it. On the way back, somewhere on the M74, I had a shot at overtaking 3 trucks. The 'Troll was in 5th and doing around 58mph-60mph on the flat, so it was going to just be a rolling overtake - should be no problem. It was no problem until I approached the 3rd truck, the motorway started to incline a bit and speed started to fall slightly. I changed down to 4th and pressed on the accelerator a little. Speed continued to fall. I pressed down some more and then got the dreaded overboost fault. It feels just like your foot has slipped off the throttle pedal (but in reality, the ECU has just reduced the amount of fuel supplied to the engine). Press the thottle to the floor - nothing happens. Lift off and press down again, the fault clears and performance is back to normal. We still have the overboost fault.
So, summarising where we are up to .........
* I know we have an ECU overboost condition but don't known whether we have a real overboost or a fictional one due to a duff sensor. I've already changed the MAF for a new one and have no reason to suppose that the throttle pedal sensor is faulty
* The VNT actuator seems to be working OK. The actuator rod is at the bottom with the engine switched off and at the top on tickover. The lever works smoothly and doesn't seem to be sticking. The turbo vanes are probably not sticking and the vac hoses are OK.
* As the'Troll has only done 43000m from new; performance is great; fuel consumption fine and the motor doesn't make smoke, common sense (and Occam's Razor) suggests that we don't really have excessive boost and the problem *probably* results from the "Charge Air Sensor" (aka MAP sensor, boost sensor, manifold pressure sensor or whatever) telling fibs.
This morning, I had Westway Nissan in Oldham fit a new "boost sensor" (I'd have done it myself, but can't find a source for the sensor other than Nissan). They charged £20 for fitting it and did a free wash & vac for the 'Troll - which I thought was might fine of them. What wasn't quite so fine was they they charged £186.96 +VAT for the sensor itself!!!! You can get them off Fleabay for most motors for less than £50 (sometimes a LOT less than £50). Ho hum .......... beggars can't be choosers, I suppose. (it's a Hitachi 22365-VC10A).
I'm 80%+ convinced that the fault is now really cured, but won't know for sure without an extended test drive. Unfortunately, I don't really have a strategy for deciding on the next step if it proves not to be fixed.
I'll keep you posted .............
(Actually that wasn't really a *quick* update, was it?)
Andrew