I change my oil every 6000 miles or once a year, whichever comes first. I have had one valve that lasted about 80,000 miles. If I had to guess, I would guess the one that lasted longer had a LITTLE more clearance on the piston and/or the transfer rod. The slight increase in clearance would allow a little more "contamination" before sticking. Cat did change the part number on the oil filter some years ago and I was told (by Cat engineers) that it was to address the IAPV failure issue. Supposedly the new filter has a smaller micron rating.
As I mentioned, when I disassembled the valves, both of them had sticking pistons. I could not SEE any contamination but after cleaning them, the pistons would literally fall out of the valve if held upside down.
Whatever the reason, the valve that did fail, now works fine after being cleaned. We'll see how long it works.
Steve
