I have had one for awhile.
When the front transfer case starts falling apart (roughly 110 to 125,000) it will cost around $3K to fix. The problem is that the hydraulic fluid will start leaking into the front transfer case, causing the transfer case to overfill.
This may then overfill the transmision, which, when too full overflows into the exhaust making a noticeable white smoke.
You will notice the hydraulic fluid level dropping, and/or your 4matic warning light will come on. By the way the hydraulic fluid also autolevels your car.
I have a slightly unhappy temporary fix, i.e. I drain and refill the front transfer case about every three months (3,000 miles) and/or when I use the 4matic function due to slippery roads, ice etc.
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