I think the OP is getting good information out of this thread, just the kind of cautionary advice the DDs and CCs [as well as more sophisticated posters] can give from their own experience. As with any vehicle, its 'buyer beware,' and the OP is wise to come to this valuable resource for info before buying. Personally, I would not advise anyone buying an E53 [or any 9+ y/o bimmer, or most vehicles, for that matter] who is expecting bulletproof reliability/no problems or even the same problems you can expect from a 2-3 year old Honda Pilot/Toyota Highlander. At this stage, these are enthusiast vehicles, best owned by folks willing to accept frequent [and sometimes confusing/expensive] repair/maintenance issue in return for (1) a lower initial purchase price and (2) the aesthetic benefits [in terms of appearance/relative performance/handling] that an E53 gives. Most all the valuable contributors here [including in this thread] fit this category.

As for me, I'll spend most of my Sunday this weekend [having already invested some hours in research/diagnosis/parts search] replacing a secondary air pump valve, spark plugs, O2 sensors, valve cover gasket, and oil change while also inspecting for other issues. It will save maybe a thousand bucks or so over having a dealer/shop do the same, and can also be therapeutic! On Monday, I'll fight the traffic in an ultimate driving machine, making the commute a little more tolerable. I hope others here do the same!

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2006 X5 3.0 6-spd w/Evo UUC ssk
sport/premium pkgs
born Valentine's Day, 2006.