Focus ST stumble from engine with A/C on

[Tame Technician]

When driving on very light throttle, say 30 - 50 mph just cruising along, you can feel the engine stumble as the A/C compressor cuts in, feels almost like a misfire, certainly holding back for a split second. With the A/C switch off the car drive fine, have looked at live data for airmass and ignition advance, and you can see the ECU inputs a spike of more Airmass (and injection duration) for a seconds as the compressor cuts in. Surely you shouldnt be able to feel this.

Further investigation showed

The A/C does blow cold, but the air con compressor seems to be cutting in and out far to much. If I hold a fast idle, say 2000rpm with the A/C on the compressor cuts in, runs for about 30 secs, then starts to cycle in and out (nothing unusual about that you might think) but its cutting in for 10 secs, then out for 10 secs, then in for 10. My other car (jag) with same type magnetic clutch A/C runs almost constantly, maybe cutting out after nearly a minute for a few seconds then back in.

Does anyone have any ideas what this could be.????

The A/C gas was recovered and measured at 380grams, recharge to 600 grams. I've reshimmed the compressor clutch as the gap was to big, is now approx 20thou (0.5mm) but this has made no difference. Cooling fans are running ok, condensor doesnt appear to be blocked and get warm but no too warm.

Help

[LawrenceF]

I changed the boost control solenoid on mine after similar symptoms, and made it a lot nicer to drive. Poor running with the AC on seems to be common with them from a lot of googling when I had mine.