It took forever on ours before we finally were able to see the evidence of the leak. I'm talking about like 6 months of believing that's what it was, and topping off coolant before we finally were able to see the dried-up residue of the coolant leaking out back there. I don't recall what the process was out posted out there for confirming, but we knew it wasn't a head gasket & we weren't getting any coolant in the oil or out of the tailpipe, so we eventually confirmed based on others expertise. When we finally took it in we were putting in a gallon every 5-600 miles, so it was due for sure. I toyed with trying it myself, but ended up farming it out due to the number of plastic bits that had to be removed & moved out of the way to do it all. I figured I'd end up with the truck down a year while trying to keep ordering small parts & then get back to it, etc.... We ended up taking it to Boulder Toyota (or whatever they call it now), based on the premise that they should also know the motor well enough & we didn't have to go all the way to a Lexus dealership. If I had to do it again, I'd go to the Lexus dealership. The boulder Toyota location was a huge pain in the ass to deal with, and the screwed things up & had to redo a bunch of stuff for free.
