Rough idle when air-con engaged

[CouncillorRobFord]

Update: over the past couple of days I have tried to classic crapshoot approach of doing the following: filling up with 98, cleaning the throttle body, spraying in a can of upper engine cleaner into the intake, plus an oil and filter change.

One thing I noticed when doing the oil change was that only about five and half litres came out, a litre less than the six and a half that I had previously filled it up with. I suspect this may have been recent, as I don't recall noticing it that low the last time I checked the oil level. I have kept an eye out for leaks and I have never seen one.

Upon taking for a drive, things seemed more or less fine and the engine was responsive and seemed stable enough. I got to speed on the freeway and felt some roughness when getting up to 90-100 (probably around 4k RPM or so). It seemed to stabilise quickly and I continued on normally.

Back on the streets, I tested out the AC again and it definitely felt a lot better. Still felt a vibration or two here and there, but not the same level of frequency or the noticeably dip in RPM when idling. One thing I did notice was that the heat from the engine through the dashboard vents might have felt hotter than normal when the AC was off, however it's not something I've really noticed that much before so don't have that much of a reference point. Oil temperature gauge is in normal range.

Overall, this combination of approaches probably did help out a bit with things and are all routine maintenance things I would be doing anyway, but I can't say that it's really got me any closer to figuring out exactly what is causing the issue. Considering the potential increase in engine temperature and burning of oil I'm suspecting it may be coolant related, but I'm not sure as this isn't reflected in the oil temperature gauge. PCV issue maybe?

 

[CouncillorRobFord]

Update again for anyone playing along at home: seems to be running a lot better and smoother since the oil change and upper engine cleaner. I've since refilled with more 98 and poured in some injector/fuel system cleaner into the tank for the hell of it. Only thing I've noticed is that the engine is still experiencing some vibrations when accelerating to freeway speeds/gearing: was fine getting up to speed but had some roughness around 90kmh for a few moments but settled down once cruising.

Right now there doesn't seem to be any issue with the AC being on or off, engine appears to be responding the same either way.

 

[CouncillorRobFord]

Three week update for anyone interested, have been running exclusively on 98 and have topped up each fill with a bit more of the Nulon fuel injector cleaner. Engine is now running as smooth as ever at all gears and speeds and zero of the previously reported issues. AC is running fine and cold with no noticeable impact on performance. Still haven't really figured out what the issue originally was, but it seems like a heavier than usual round of maintenance seems to have fixed it.