Idle problems driving me nuts!

[DrewN209]

First off, Thanks in advance...I've spent hours and hours searching not only this forum, but other bmw forums and even as desperate as googling. I could find hundreds of people w/ similar problems but no particular fix.

I've got a 2006 4.4i w/ 61k miles on it. I bought it in june w/ 54 miles. It quickly developed a low rough idle that would studder under acceleration from a stop. I replaced the MAF and that solved the studdering and problems under acceleration.

But the idle is very rough and shakes the whole cabin. It happens in drive at a stop, in park, reverse and neutral. Once under way it seems fine, no sputtering or any noticeable problems. When the A/C is (no matter how low) the idle increases by about 100 rpm or so and the vibrations get MUCH better. Turn the ac off and it immediately drops back down to about 525 rpm. I've also noticed that I can give it a hint of gas to bring the rpms up by about 75-100 and it's much much smoother... this fix works in park, drive, whatever.

So in the past month or so I've replaced the MAF, replaced the air filter, changed engine oil and filter, ran several fuel system cleaners, always use chevron or shell 91....

Any ideas as to what else it could be ? is 525 rpm too low ? It seems to me it is simply idling too low per the computer.

How do I check for a bad engine mount(s) ? I don't think this is the problem though because a few rpm increase and the problem goes away ... can anyone advise ?

Spark plugs and fuel filter... I was going to change, but upon further research it is quite unlikely the plugs went bad at 55k miles. The fuel filter likely isn't the culprit at this mileage and if it was plugged it's effects would be felt on fuel throttle and higher fuel demands, which is not the case.

I've checked all the hose I can and don't see where there could be a vacuum leak...

The battery also seems to be in tip top shape and is an OEM bmw battery that looks like it is new.

I'm at a loss and it's getting really frustrating... any advice or insight from people with similar problems is much appreciated. I've found many incidents of similar problems but no culprit.