I have a 2013 XF Supercharged 5.0, I’ve recently noticed a pretty rough idle, doesn’t matter if it’s cold or warm. After I replaced the water pump, I removed the oil cap to check for sludge and noticed that the idle got worse when I removed the oil cap. When I put it back on it returned to normal, does anyone know if that could be related to the rough idle issue that I’m having? The idle isn’t bad enough to move the RPM needle. Maybe some sort of Vacuum/PCV leak? Checked the motor mounts, installed new air filters, cleaned the MAF sensors. Let me know what you guys think, Thanks

 

The cap made an air leak but the car must not have any at all.

This is why people watch fuel trims.

 

Any Codes? Some stored codes won't turn on the CEL, You need a higher level scanner to see these though.
Has your gas mileage changed?
Are all of your vacuum hose connections tight
You can water mist the engine to see if a vacuum leak exist. Take a spray bottle of water remove the plastic engine cover and LIGHTLY mist around the engine vacuum tubes and intake and listen for a change in rpm.

Depending on your mileage it might be time for a plug change, a slow O2 sensor, the AJ133SC is also very sensitive to low fuel pressure and Battery voltage etc., but right now you need to narrow things down.