I would look for air leaks first, no such thing as a cold start valve on a zetec, not needed as ecu control fuelling, don't put petrol in maf, use carb cleaner to spray the wires and shake out excess fluid, one wire will be grotty the other clean, especially if you use a K&N filter.
Have a good look at the "T" piece of hiose from the PCV system that lives at the conrner of the head area under the inlet manifold, this often splits and causes air leaks that weaken mixture, another thing to do is to make sure the pcv valve is not stuck open at idle, by clamping hose to T piece whilst running as this will weaken mix if open.
tabetha