[MG ZS STE]

Mine has started to do this since changing the thermostat. It never did it before and the only thing i removed during the thermostat change was the TB which makes me believe it is related to this. potentially a vacuum leak? ive cleaned (and yes properly cleaned) the IACV. Next i will probably remove the TB and make sure both mating faces are completely clean.

If that does nothiing i'll be investigating the possibility of the piggy back ECU may have packed in?

It passed fast idle test for the mot emissions but failed natural idle test. The examiner told me that the fast idle one was the important one so was happy to pass it.

Thing is, i would have thought that this would have been handled by the piggyback ECU. I think it was put on originally to overcome lean issues at top end but would they also not have looked into where the car was fueling rich?

I dont know, good luck anyway and if i do get to the bottom of it will let you know.