For the archives: had an intermittant cooling problem on my 1990 GL which
was tricky to pin down.  Finally took it to bus ninja (Greg's Repair in
Natick, MA) once it became apparent that I couldn't fix this streetside in
fourteen degree fahrenheit weather.

Symptoms
--------
  Bus would heat up when idling, but rad fan didn't kick in.  Manually
engaging rad fan by turning on the (disconnected) AC would not reduce
temps.  Running the rear heater fan on full blast would.
  On the highway, bus ran fine -- temp gauge rock-solid, with needle right
over the warning light.  Cab heat worked fine.  After highway runs, rad
would feel warm-ish.
  Main coolant reservoir would get occasional bubble in it.

Attempted fixes by me
---------------------
  - bleeding via the Boston Bob method
  - replaced pressure cap
  - visually checking for leaks after cool-down
  - intense starving-student wishful thinking that rad didn't
    need replacement

Attempted fixes by Greg
-----------------------
  - coolant system bleed + pressure test
  - pressure cap test
  - tighten / replace a couple of tiny hose clamp leak points
  - compression test
  - pressure test when stone-cold, to check for head gasket leak

... no dice, all of the above: in Greg's shop, the bus ran fine -- fan
kicked on, everything hunky-dory.  Once he tried it OUTSIDE the shop in
the cold, though, the problem turned up.

The fix
--------
New thermostat.  Greg pulled the old one (~2 yrs old) and found it stuck
shut.  Replace thermostat, flush coolant system, refill, bleed, everything
works fine now.

Happy Friday, all.

d.
1990 GL -- "Mudskipper"