Sounds like they've made a mistake. You get the overrun valve on MFi cars - it sits behind the fuel distributor and when it goes wrong its normally a dodgy throttle switch...

EFi cars don't have one - the injectors just inject less fuel.

Get a £15 Gunson fault code reader from Halfords and plug that in. It should give you a good idea of what's going wrong.

Have you checked that you've not got a split air hose? Sounds a bit like a MAP sensor problem...