Ford Focus Mk1. 2001, 1.6 petrol, 140K miles
Once warmed up the idle will regularly pulse with 2 or 3 second period between 800 and 900 rpm. It sits at 800rpm and "blips" to 900 rpm before immediately falling back to 800 rpm. This pulsing is more pronounced if the lights or heated screen is on nd the blip may just reach 1000 rpm. It's almost as if its detecting a lower battery voltage and increasing the engine revs but as soon as the alternator supplies more power it senses this and drops the revs back to 800 rpm.
There is no noticeable drop in performance when driving. Car starts on first turn of key and when cold the idle is at approx 1200 rpm which drops back to nominally 800 rpm after warming up for 10 minutes.
Any ideas?