harsh idle/vibration only at 40 F and below
 

I've read with dismay the other threads of 3.0s owners dealing with harsh idle and vibration on an otherwise excellent running engine and really finding no direct cause.

However my idle/vibration scenario is a little unique that i hope some of the seasoned Gurus might be able to throw some good suggestions my way,

My 3.0 runs great but anytime the outside temperature gets 40 F or below she will always have a harsh idle/vibration while stopped in park, even after the engine has warmed. When I hit the accelerator --gone- -and the engine runs smooth as butter otherwise. Yesterday the temperature got up to a wonderful 68 F and while at a stop the engine idle is normal and no vibrations. This has been the scenario for the past two years. But come Spring and Summer the vibration is non-existent until the cooler temps of late Fall and winter.

There are no codes and I've done quite a bit of usual maintenance that one would think would have addressed this issue: rebuilt my DISA (with new O ring), new intake elbow, cleaned the ICV, new valve cover gasket w/ NKG plugs, new pre-cat O2 sensors, cleaned MAF a few times. And checked, checked, and checked every hose i can find for a possible leak. But i don't think it would be a vacuum leak since this only happens with the outside temps are 40 F below.

As a side note a mutual friend of mine, who's rebuilt engines for 20 + years, says it sounds like a "weak" MAF as its not calibrating the fuel ratio properly for the colder air. If that were the case I'd surely get some fault codes? My X has 194,000.

I've always used 0w 40 do you think that might be a cause since its actually a bit more thicker than a 5w 30.

Thanks much in advance.