I brought a 1998 manual transmission SW2 home last weekend. 164K miles, no bad odors, no smoke or vapor, solid and not irretrievably molested near as I can tell. However, gas mileage was not quite where I'd expected.

First mileage figure was 30mpg for highway driving. Based on reading in Saturn forums, I monitored the coolant temperature with an OBD2 reader and found the max while driving was 140F. This is way too cold. So thermostat and ECTS were replaced. The gas mileage and coolant temperature became much better. Although I got 26mpg in a day of stop-and-go traffic, the interstate and two-lane highway mileage increased to 38-45 mpg. Coolant temperature range was 195F-199F during highway driving. My driving was fairly steady, acceleration is mild, and I used engine braking rather than the service brakes where possible. The terrain is rolling hills in this area.

I did encounter a problem some other folks have had. The replacement thermostat, Carquest 14279, includes an o-ring seal. That o-ring was too large (OD=54mm) to fit into the thermostat housing groove (OD=51mm). A separately purchased Felpro 35087 o-ring does fit the groove. The Felpro 35087 OD=51mm (2"), thickness about 2.5mm (3/32"?). My measurements of the thermostat housing groove are: OD=51mm, ID=44.6mm, depth=2mm.