Originally Posted by tabetha

Hi,
No probs matey, you are hardly stupid, you are intelligent enough to ask when you don't know!!
The CTS is coolant temperature sensor, this senses the water temperature and sends this info to the ecu, electronic control unit, the box of electrickery that controls the fuelling and ignition on your vehicle.
This is the one item that has more bearing on the mixture than anything else, the mixture means the ratio or air to fuel, commonly expressed as AFR, air/fuel/ratio.
When the engine is cold(cold water) it needs a richer mixture(like a choke does) so it has more fuel added to the same amount of air to keep running nice, obviously if the CTS is misreading it doesn't know where it is.
A normal culprit for idle problems is the iscv, idle speed control valve, but the fact it raises to 3000rpm shows this to be working IF you are 100% certain it has no air leaks.
The engine needs air to run and with the throttle closed this is supplied through the iscv, OR any air leak, also any air leak if on a "maf" system is not accounted for, not known by the ecu so it doesn't account for it, and the mixture becomes weaker than it needs to be.
This is all measured once burnt in the engine by the lambda, this simply measures the amount of oxygen present in the exhaust gas which is directly proportional to the mixture, sends this info to the ecu, which then makes adjustements as necessary, this happens many many times a second.
Just take to local mot centre and ask them to do a gas reading on it.
tabetha