Some interesting posts there. My IACV has always chattered a bit (learnt the correct term now) but usually settles down. Yesterday I was fitting new wiper blades as part of my LM Classic prep and the IACV was chattering away non-stop. I reseated the plug but no difference. If, as said above, the IACV doesn't feed back to the ECU then the chattering has to be either because the ECU keeps changing its mind, or possibly because the IACV can't set itself as requested - maybe it keeps overshooting or is gummed up? I guess that depends on how it actually works, what the ECU is acting on and whether the IACV knows if it's moved correctly. Revilla - you're right; if it ain't broke and all that. But I can't believe a K16 in a Rover 25 or MGF or whatever does this, and a few people above have said their caterhams don't so it does seem a bit odd. Fingers crossed it goes back to a slight chatter and gets me to France and back but if anyone has any technical info about how it actually works and what the ECU does I'm all ears. Edited: John Milner - thanks for those links to detailed info.. just reading them now. Double edit: From one of John's links: S2 (PWM) The S2 ECU cant measure where the IACV is so has to count the drive pulses it sends and remember the position, then tries to return to a datum every ignition on/off. If it loses track e.g. battery disconnected the ignition needs to be switched on to position 2 and off several times to run the valve back to the datum. So.. perhaps this explains why not everyone has this and why production rovers didn't: if you have a battery isolator it will hunt on start-up and if you don't it won't.