Very odd over heating...

I drive a 90 civic, and for some reason, it will run cool and then spike up a while later in the red (almost). I will coast and run the heater...and all of a suddon will cool down below mid-point...it will constantly do this. The radiator is fine, and the water punp has no problems. I wonderif it's a sensor that isn't working right...or the thermostat is sticking. I am not a motor head, so any advice is appreciated.

I would guess that it's the thermostat thats sticking causing the fluxulation in your guage. It's a easy and cheap fix to find out. Someone makes a "fail safe" type of thermostat that if it fails, it only fails in the "open" position, therefore preventing an overheat condition. good luck....p.s. the name of the company that sells the "fail safe" thermostat is called MotoRad...

Check if fan is working. relay could have gone bad

Check your thermostat to make sure it is not stuck. Check your fan relay and fuse also make sure that you are not low on antifreeze.

I had the same problem. Take the car for a run and then feel the upper/lower radiator hoses. If the upper one is hot but the lower one cold its your thermostat for sure. Your temp guage fluctuations designate a stuck thermostat. To change it, follow the lower hose to the thermostat housing, undo the two 12mm (i think) bolts and pull off the cover. Theres no need to pull off the lower house from the housing thats just a pain in the ass. Let the coolant flow out and pull of your old thermostat. Install new one with air hole up and spring toward the engine. My fan wasnt working too when i was overheating...replaced the thermostat and now my fan kicks in. If your thermostat is old, you can't go wrong replacing it.

Thanks for all the advice fellas...I took the car to a shop and he said go ahead and replace the therm., so I will start with the cheapest first. This is the coldest it's been where I live (TX 30F), so weather permitting, I'll be working on it. I was told it could be bad bearings also. If the therm. doesn't solve the problem, then I'll install a new relay. I had a nice 3 day weekend planned on an island on the coast...funny how it all works out.

Hope its not a head gasket. :(

Yep...I am preparing to saw off my arm and leg for that one. I paid too much for this car anyway...and that would just take the cherry off the cake of my day. What sucks, is that this car is excellent shape aside from over heating. It was definately pampered. I'm expecting to pay $500+ to solve the problem.

The problem ended up being a thermostat, and a broken bolt.