Any ideas what it might be?

I have 2.0 non turbo engine in an MY99 Impreza Sport , which after say a motorway drive of 1/2 hour feels lumpy at idle for a short time afterwards, before settling back down to a steady idle.

I.E trip down motorway at 70 or so, take slip road off come to a halt at junction off motorway and the idle feels lumpy, minute or two later after driving around at say 40 next junction stop at the idle is fine

Plugs and leads have been replaced.
Engine has a MAP sensor, so guess not a MAF issue

 

**** fuel?

 

Try some STP fuel injector cleaner at £5 per bottle it is a good and fairly cheap option t try. I have used it on a few cars to good effect.

 

Dodgy fuel filter ?

 

I would expect dodgy fuel or fuel filter to affect the car when driving not at idle, when we test diesel powered generators that are unstable at the upper limits it is normally due to the fuel filter being F'ed. They hold light and moderate load okay, that is why I am thinking clogged injectors as the Fuel pressure regulator will increase the line pressure when driving compared to at idle.

 

Don't think it is fuel as it has done it for some months now, and have put fuel sytem cleaner through the system. Will take a look at the service record to see how recently it was replaced.

The idle speed is stable when it is lumpy but the engine feels almost as if it is not firing on one cylinder a knd of out of balance lumpy. But plug leads have been replaced within last couple of months and plugs within last 10000 miles. and it was lumpy before then.