86 5ks question (kind of long)

[Randy Rock <Randolph.A.Rock-2@tc.umn.edu>]

Hello all:

I am new to this list, and have my first question about my fwd 86 5000s.

Background:  I was sitting in traffic today, and the coolant temp warning
came on.  I immediately put the heater control to 90F Econ mode, and pulled
off as soon as I could to analyze the problem.

There was no coolant in the overflow reservoir, and I could see no steam or
wet areas on the radiator.  I put in an antifreeze mixture (50/50), leaving
some room for overflow, and proceeded home.  No more warnings about coolant
temp, but I inspected the reservoir when I got home, and saw that it was
all wet on top of the reservoir, and no leaks or anything from the radiator
or hoses.  I went through the archives and saw a brief mention about a
loose reservoir cap, but I figured I had better check with the more
knowledgable types on the list to verify the problem.

Question:  Is is just the cap, or do I need to flush and fill, and replace
the thermostat?  If it is the cap, is this readily obtainable, or an
expensive "dealer only" part (as I have been told numerous times by parts
guys for other things on my late 1988 80).  

On a related note, the temp gauge is dead.  Is replacing the temp sensor
easy on this car (like I saw mentioned once in the archives)? The car has
163,000 miles, but runs excellent.

TIA!
Randy Rock
St. Paul, MN