Hey, I'm back, sorry for the disappearance. Wonderful insight from everybody, thanks! Unfortunately, I don't have access to any diag software, and my Indy ir 90 minutes away. I ordered an OE thermostat from ECS, their website states it includes sensor. Is this the dual temp sensor you're talking about? Or is that the coolant temp sensor? I'm still leaning toward the thermostat because i took the X around the block today, and had no heat from vents at 106 KTMP. Temps seemed erratic today, normal for a bit then edging up to come rushing back down.

Still waiting on thermo to arrive.

At what temp does the aux fan kiclk in?

au contraire, BMW Scanner 1.4 will also give you info on a bad thermostat.

Not to hijack the thread but this here is one of the reasons why I switched to conventional thermostat on my 540 and X5.
http://www.xoutpost.com/bmw-sav-foru...-part-2-a.html

Well, I'm feeling a bit like a dunce! Cleaned the X up and let it set in the garage ever since it started acting up, since I wasn't about to change a T-stat and dual temp sensor in freezing temps. We've had mid 30 degree weather this week, so I decided to go for a drive and see how she's acting. VOILA..! Everything works as designed.

I get to thinking... Last fall not long after I picked up the X from my Indy after my prev maintenance, I developed a small leak in the coolant hose at the rear of the engine. Some coolant leaked out, I shortened the hose about an inch, added some distilled water, and kept trucking. I remembered all of this and decided to check my water/coolant ratio. Yep, it was only good till 5 degrees F! We've had quite a bit of sub-0 degree weather this winter. I thlnk my coolant would start to slush in the radiator, (since it only happened on the most frigid days) restricting fluid movement, thus causing it to overheat.

Now my question is, can i drain some fluid, and add coolant, or should I totally drain the whole system and refill, to know exactly what I got?

sure, drain some and add straight antifreeze. DO NOT add the premix 50/50 stuff!!!

I would probably drain close to a gallon or so out

Coolant is cheap enough that I would drain the lot and refill from scratch.

Regular coolant maybe, but per BMW you need their expensive coolant in order to not mess up the system.

I do have do have 2 gallons of undiluted BMW coolant in the garage. I'm thinking I'll drain a gallon, add a gallon of pure coolant, bleed the system, and see what I got. I can always buy another gallon and do a total flush if it doesn't work out...

I know a lot of people will argue with me but.....

as long as you do a complete flush not just a drain, you can refill with any coolant of your choice. I have been using regular stuff in my BMW's for over 10yrs and havent had a single problem

I could argue with you, but I won't. :) As long as you personally have no issues, that's what counts..!