On Sat, 25 Apr 1998 14:37:13 -0700 you said:
>in place by a clip.  Is it possible to switch from old to new sensor
>fast enough to avoid spilling a lot of coolant, or is it necessary to drain
>the coolant from the block first?  Is any form of sealant or O ring

yes, you can do it quickly and lose only about a quart of coolant. but
the first time you do it, it's likely to lose more. so put a big pan under
the car (below the thermostat housing).

you want to wait until the bus has sat overnight ... so the coolant will
be nice and COLD. the coolant stays hot enough to hurt (if not burn) for
several hours after you shut off the engine (i waited six hours and it
was still hot enough to cause pain).

the trick to swapping it out is, i think, to do it sorta this way:
- have the new sender AND it's new o-ring (yes, there is an o-ring needed.
  do NOT reuse the old one) ready nearby handy. like RIGHT there on the
  cargo deck, beside the open engine hatch.
- pull the c-clip out.
- wiggle the wires on the old sender and pull the sender out.
- and here's where you lose coolant: stick your finger up in the hole
  and check to make sure the old o-ring came out. doesn't hurt to wipe
  round the inside of the hole with a rag.
- put the new sender/o-ring into the hole and push it down into place.
- put the c-clip back. no coolant should be leaking now.
- now disconnect the old sender from the wiring. you do this by pressing
  down firmly on the silver wire-thingie that is across the wide part of
  the connector. this pops the side parts of the wire outward and lets the
  connector come off. check out the fuel injector connections ... they are
  the same sort of thing.
- wipe/dry/blow off the connector so there's no coolant on or in it. and
  snap it onto the new sender.

now you need to refill the coolant tank at the back (left) to make up for
the coolant that spilled out (you DID remember to put that pan under there,
right?). disconnect the little hose going to the big pressure cap, then
unscrew the cap, and refill the tank. put the cap back on, reconnect the
hose (a pair of pliers works fine to squeeze that clip-thingie on the hose).
then make sure the overflow tank (right one, behind the license plate-door)
is filled up to MAX.

now, as far as i know, this only works on the 2.1 liter engines (1986-91)
cause for some reason they don't seem to get air bubbles from this. i do
NOT know if the 83-85 1.9 liters do or don't. i've never done one of them.

good luck!!
joel