Fluctuating / bad idle

[AndyW]

@ David: the hose from the IAC valve Plenum Body which goes goes back into the air intake hose after the MAF sensor.
@ Dave: when the o2 sensors are bad, there should not be an Error Code?

[SCP440]

Not always, a small air leak and a lambda that is still giving an out put but wrong might not be enough to put the light on. This is why it is useful to look at live data.

An air leak will obviously lean off the mixture and this should show up as the compensation should be quite high trying to get it back to where it should be. Look at the swing on all of the lambdas, they should all have the same amount of swing. You can move them from side to side if you want to test them.

Steve

[David]

@ David: the hose from the IAC valve Plenum Body which goes goes back into the air intake hose after the MAF sensor?

Not that hose, the other one, rearwards of it and lower, the positive crankcase ventilation valve and hose. It goes from the top left cam cover (the valve itself) just underneath the air trunking, to the plenum right of the tail of the Mustang on the throttle plenum plaque (so between the IAC and the EGR).

Look at the pictures. Here is the hose on eBay at the moment:
https://www.ebay.co.uk/itm/333612093681

... and the PCV valve itself, (a service replacement item at 80k say Ford, 90k say MGR):
https://www.ebay.co.uk/itm/MG-ZT-260...wAAOSwKNpel4p6

Available cheaper from the States of course, under the numbers I quoted in the earlier post.
When mine went bad, I did indeed get lean-mixture error codes after a while, but not straight away.

David