3, Vacuum hose leakage:
Next thing was to see if I can find any leakage in the vacuum system. I already noticed that there were oil leakage on the right valve cover top, if oil can get out, air can get in. Modern engines with it's emission control system are very sensitive to vacuum leaks.
Check all vacuum hoses for cracks, especially at the curved paths of the hoses.
Here is the the right valve cover, I have an oil leak here. I didn't take away the whole valve cover, just the upper inspection hatch. Cleaned it from old sealing, it took a while to do this and it was cold outside that day.
Also check the O-ring at the top of the oil stick, very common problem, replace it even if it looks fine!
Left hand side, check those hoses for vacuum leaks.
Front of engine, small control vacuum hose, check those for vacuum leaks.
Front of engine, small vacuum hose, check those for vacuum leaks.
Back of engine, check around the air inlet for vacuum leak, especially the big 3" O-ring between manifold and knee shaped air inlet, also the screws can be loose. You don't see it here, deep down behind the intake manifold.
Left hand side vacuum hoses.