Engine won't hold a fast idle but car still drives ok!

[DerekHa]

Has anyone had similar problems with the Toyota Aygo 1000cc engine?

This relates to my partners 56 plate Citroen C1, the car drives fine on the road but when the throttle is opened when parked up the engine accelerates fine from tickover but it's impossible to hold the rpm at any point between 1000 and 2000 rpm. Rises normally to 2000 rpm and above but as the accelerator pedal is eased back slowly the engine just returns to tickover at around 1000 rpm. The engine still pulls well and revs freely on the road, a normal drive really and there's been no noticable drop in mpg that we're aware of.

There is a slight shudder or rough spot as the revs rise through this range at around 1800 rpm. Perhaps a slight misfire but obd2 reader doesn't show any codes or misfire but can't hold that engine speed with the thottle pedal.

Tried the same on a friends C1 but that revs fine and any point in the rev range can be held with ease just as expected. Both vehicles have fairly low milage at around 45k.

Thinking that maybe there's a failing spark plug (new plugs already purchased) or even a suspect coil pack. I'm going to take a look at this over the weekend but have read that a fuel injector issue may cause similar symptoms. Not sure of the service history, only had the vehicle for 3 months but it's in good condition and been well looked after.

Any other ideas or suggestions would be appreciated.