Unstable idle — how to find the cause
Four readings on one screen show what exactly is swinging.
Put four readings on one screen
Revs, load, intake pressure and throttle position. Together they show who leads: if load swings after the revs, something is disturbing the engine; if the throttle twitches first, the throttle is the cause.
Check the throttle at closed pedal
With the pedal released the position must sit on one number. Wandering by a couple of per cent makes the unit recompute fuelling each time, and the revs swing after it. Often it is dirt, not a fault.
Look at the intake pressure
At idle it sits well below atmospheric and stays flat. Jumps mean air arrives outside the calculation: a leak, a stuck recirculation valve, a loose hose.
Compare cold and hot behaviour
Swinging only when cold points to the warm-up idle and temperature; only when hot — to the recirculation valve and the cooling fan that adds load. Different behaviour is half the answer.
Do not rush to clean everything
Cleaning the throttle without resetting adaptation sometimes makes things worse: the unit keeps correcting from old memory. If the idle gets worse after cleaning, that is not «a second fault surfacing» but expected behaviour.