Got a weird situation - 1992 Sidekick 2 door, stock 8v, 4x4.  I just got it out of mothballs for the spring: checked all the fluids, tires, belts, hoses etc....   Ended up replacing an upper radiator hose, but everything else was good.

Driving along everything was fine, had the fan off and the heat/temp switch all the way over on hot frrom the last time I drove it.  I moved it back over to to cold, and within about 5 minutes the needle was right up on red.  I turned the heat on and the needle dropped right back down to the middle, until I turned the temp selector back to cold again.

The rad is nice and hot, so it doesn't seem to be the thermostat.  I don't know much about the cooling system on these - is there a bypass that gets clogged or anything?  Going to go play with it some more... Really strange, she's running perfectly... No water in the oil, no oil in the water, exhaust is normal.  Any ideas?

Thx,
-Al.

Might be a problem with the cooling system not being purged of all the bubbles.  Turning on the heater allows the flow to go through.

Do you have an electric fan on the front of your radiator?  Check to see if it powers up when the heater is engaged.

Your radiator could be clogging up. If someone used hose water instead of distilled water when filliing it up, they'll get deposit build-up in the radiator. If it's JUST at the point where it can't cool the engine by itself, opening up the heater core for the heat in-cab might be JUST enough to keep the engine cooled off.

Could also be the regular junk: Timing, water pump, T-stat

drain it flush it,  its was mothballed so it is crusty.

cross fingers , its not clogged tubes, bad pump rusted impeller.

also
the 92 is subject to the wrong stat syndrome.
there are 2 stat for this engine
1 is deep pocket
other is shallow.
if you put in a new stat, make sure you get the correct gasket.
one is rubber ring deep, and the other is std paper gasket .
there is TSB that covers this, i have it.

i mention this as you might be changing STAT.

if the guage is fibbing that too can  make you chase tail.
http://carfix.stufftoread.com/overheating.html

its all here.

i have the guage test there,too.

Simple... Fan clutch is tired...

The fan clutch is a wear-out item and may need to be replaced.  It can be tricky to diagnose a bad one. When I replaced mine (93 Samurai), I took a moment to clean out the radiator as well, since it had to be removed anyway. 
I filled the radiator with about a quart and a half of hot (microwaved) vinegar and then let it sit for about 10 minutes before rinsing well and re-installing.  The hot vinegar reacts with the accumulated scale inside the radiator, and it all poured away down my kitchen sink.  It's cheap and gives you some confidence that your radiator is clean inside. It's reassuring to peek inside afterwards and see the clean copper/brass.  Vinegar reacts rather vigorously with aluminum, so I chose not to add it to the cooling system.
When refilling with fresh coolant, it IS best to use real distilled water if yours has any dissolved minerals (e.g. Colorado river water here).  It doesn't take that much, and can help a lot to reduce scale buildup over time.   

Was away for a while, so I haven't had a chance to play with it.  Thanks for all the replies - That overheating website is great jtgh.  I made sure the fan clutch is working... I can definitely feel the extra air being moved when it kicks in.

I had it out for a two-hour run today to see if I could jar something loose or at least get a useful symptom of what's going on...   Turns out I'm getting oil in the coolant... Not a whole bunch, but I can see it floating in the top of my resevoir.  The oil itself seems fine though.

My wife had it out once in the winter, that's when she noticed the upper rad hose leaking.... and refilled it with (all) the distilled water I keep behind the seat.  Thinking back, it was the coldest two weeks of the winter after that....  When I saw the oil, I was thinking head gasket, and now I'm wondering about a cracked block.

Anyone here had any luck with the 'block sealer' stuff that's out there?  For me, this is a $700 junker so I can't see putting a thousand bucks into the head gasket.  I read through the Haynes manual to see what's involved in doing it myself, and the 8 valve motor seems fairly simple to get around... I've never done anything that involved before, but it might be worth a shot.

I'm really interested in hearing about block sealer stories -- The store near me sells "Gunk Block Seal" for $9...  A friend mentioned Thermagasket, too... That one's over $100 though.

Thanks again for all your help guys.
-Al.

Think of it this way, if the Gunk Block Seal doesn't work the engine is still broken. 

Might just be a head gasket though.