Just a further update for those following. About a week ago I ordered all my under-intake sundries from Volvo and they arrived a couple of days ago. Then of course one needs a maintenance appointment (!). With the car under the sun shade this AM I started dis-assembly. Simply could not believe it. I checked this a couple of months ago. Not one but two missing vacuum caps on the thermo switch under the throttle cover.

This brought up an interesting discovery. I snag the big packages of vacuum caps and plugs on the HELP shelf at the auto parts whenever there is one to be snagged (not often). In digging through my collection, I find that the most common sizes are either just a little too small for these connections and have to be forced on a bit, or just a little too large, and don't really grab. I know I've replaced these before. Found one broken early in this whole mileage debacle. I would not have used the loose caps, I don't think. I'm starting to think the tighter ones are cracking, possibly causing a leak and then falling off, definitely causing a leak, at temp. I replaced both caps, checked the rest and we are going to run another mileage test. One missing cap was at the bottom of the switch, the other at the top, so I was getting vacuum at a range of temps. I was able to find two caps on an intake I have sitting in stock that are the correct size.

DS