Thursday, April 30, 2009

Weekend Project: BMW Double VANOS rebuild

Double-wha? Simply put, VANOS (Variable Nockenwellen Steuerung) is BMW's name for their fancy oil-driven system for variable valve timing in their 6-cylinder engines. Oil is pumped into pistons which move the camshafts fore and aft to hit slightly different cam profiles, thus increasing engine performance and fuel economy. The problem? Due to a failure of materials design, basically every VANOS engine from 1992 to 2006 will eventually develop leaks that prevent the pistons from functioning correctly. If you're under warranty or willing to spend the money, they will happily fit a rebuilt VANOS assembly with the same design flaw and the same failure mode. A full explanation can be found at Beisan Systems, written by the clever bloke who figured out the problem and the solution for his own car, only to be told by BMW that "no further development" would be done on the design.

Luca, one of the members of the local San Diego BMW Z Car Club (SDBMWZCC) had done the research into this problem for his own stable of BMWs. Luca is an awesome shadetree mechanic, and I've never seen him do something half-assed, so if he was going to fix his wife's Z4, it was going to be done right, which meant fitting the upgraded O-rings and Teflon seals from Beisan Systems into the VANOS pistons. And in a display of car-club-generosity unmatched in many months, he also organized a group purchase to bring the price down the price of the parts, then hosted a clinic at his condo where several other Z-owners could turn their own wrenches under his guidance!

With a Sunday appointment, I readied my tools and headed off to the clinic. None of this work is terribly difficult, but quite a few parts have to be stripped back from the head in order to get the VANOS assembly off, including the valve cover, spark coils and harness, and fan. Since the cars are low, this isn't exactly easy, but this process is really good for getting familiar with the more intimate parts of the engine, and I got my first really good look at how the VANOS system interacts with the cam shafts.

Once removed, we drained the VANOS (onto a diaper for easy cleanup, seen here), disassembled and cleaned it, then reassembled it. The actual upgrade takes only a few minutes, its just too bad that you have to spend several hours pulling things off the engine to get to it. After the new seals were fitted to the pistons, everything gets reassembled and refitted, re-torqued, and replaced.

Sadly, getting this intimate with the Z did reveal some of her other flaws, namely some crusty guff near the thermostat housing that indicates a coolant leak. I'll be dealing with that next, but I'll be passing that one off to a professional because I lack the tools and patience to deal with draining, refilling and correctly purging the cooling system on a BMW.

And finally, after a test drive, I can report that yes, the fix was worth the effort and makes a noticeable difference. There is a noticeable increase in "urgency" from the engine, especially at the lower revs. The transition into VANOS can be felt but is subtle, but overall everything feels more powerful now. Most importantly, I know I won't have to look forward to doing this again in 20k or 30k miles as I would with the OEM seals.

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