Three different mechanics are at their witts' end. Everything is new (turbo, tubes, etc) and working just fine (both, turbo charger and engine), but the engine still doesn't receive the boost from the turbocharger, leaving it with a serious lack of power, especially uphill.

It used to be a schoolbus, so there's to assume it should be strong enough to get up some hills carrying a literal bus load of kids (it's now much lighter than that since all 11 seats in the back are out).

Also, the van has an unusual low fuel consumption for such a big vehicle (which I'm certainly not complaining about, but may this be somehow connected to the problem?).

Anyone knows what may be possible reasons for such a lack of performance, even though everything was thoroughly checked and is supposedly working correctly (with the exception of that one "small detail")?