strange problem (please help)

[binman jim]

ive got ford puma 1.7vct, and whenever i take my foot off the accelerator between 1600-1700rpm she judders continuosly until its not between 1600-1700rpm. also whenever im at any revs above 1700rpm and take my foot off she judders, but only once as soon as my foot is off.
it went in to have a full diagnostic check and had no faults except the fan wasnt kicking in (fitted new thermostat still no luck). and i cant see how thats related.

please, any help would be really appreciated.

[Pumaracing]

Usually on fuel injected cars when you lift off above a certain rpm it cuts the fuel supply to improve economy. Sounds as though this isn't working properly under some conditions. Maybe coming in too abruptly or flicking in and out when it should be either on or off. Why though is another matter and it's not an area I ever get involved in.

I imagine the main signals to trigger this will be from crank speed sensor and throttle position sensor so you'd have to suspect those first. Could be a faulty or leaking injector or maybe there's a known ecu upgrade to adjust the settings on problem cars. If no codes are being reported then I'm afraid all a dealer will probably do is swap things at random at your great expense. I'd be tempted to live with it unless you can find an expert who's come across the problem before.