BMW Repair: White smoke - 1999 BMW 740iL, valve seals, electronic thermostat

Question
I hope you can help me solve this problem with my car. It has been to the same BMW specialized dealership/repair store numerous times, but even their mechanics are not certain what the problem is. This has been going on since the beginning of August this year. When it starts (if it set for 3-4 hrs) my car blows a puff of white smoke out of the exhaust. It does not burn any oil, but it smokes only when it starts and not while driving. Based on the computer’s reading, so far they: resealed intake, cleaned throttle body, cleaned oil from intake plentum, removed and replaced air mass meter; replaced intake breather hose and cover; replaced electronic thermostat; replaced camshaft position sensor on bank; tested all cylinder heads to see if head gasket was blown and found nothing; and $1,674.23 later, white smoke is still there and getting worse.   

Thank you in advance,
1999 BMW 740IL  

Answer
Tatjana, I would suspect that oil is being burnt. That is usually the cause of the white smoke. I mainly see a faulty crankcase vent valve and oil/vapor seperator (back of the intake manifold) or valve seals that have failed. First try the oil seperator and backing plate to the manifold. Then look toward the valve seals.
-Evan