So I've inherited a 2004 Civic 1.6 (Executive, oh yeah). It's a lovely motor, just been stood for about 8 months or so, needs a couple of bits for MOT before I can start to use it as my work wagon (leather, fuel economy, etc - perfect). It's suspected to have a blocked cat as the exhaust is VERY hot - that's getting looked at over the weekend.
However it's also got an unusual idle issue, in that when it's stone cold it sits perfectly stable at 1000rpm or so. As soon as the temperature gauge starts to move, all hell breaks loose. It starts revving up to 2500rpm, then sits there for a few seconds, drops to 1400rpm, then fluctuates between 1400 and 2200rpm or thereabouts, for a few seconds at a time.
If I turn the air con on it will sit at whereever it was when it kicked in, at least for a minute, no fluctuating, then starts revving itself again.
I've stripped and cleaned the IACV and EGR and their connections, and the throttle body, to no avail.
I've plugged it in and there's no codes and no historic codes. So far it's done maybe 40 miles since a new battery but I'd expect the ECU to have forgotten anything earlier than that.
I've also tried putting various graphs on the Torque app on my phone but I have literally no clue what I'd be looking for there.
My things to look at so far (and also suggested by a couple of other people) - blocked cat, throwing the lambda reading out as it gets warm, making the car go mad (but no codes?); physical idle control, like the idle screw on a carb, to set the base idle; coolant temp sensor misbehaving; air leak (I've checked and can't see any obvious missing or split pipes) ; faulty EGR or IACV.
Any further thoughts before I start throwing parts at it in a main dealer fashion?
