98 S10 runs HOT!
 

My girlfriend was driving her Blazer on the interstate and detected that it was slowing down on its own. She then noticed that the temp gauge was at the 260 degree mark and then it swung rapidly back toward zero. This was accompanied by some knocking or rattling from the engine. She was at an exit and drove about half way down and stopped.

With help from a good samaritan was was able to get the radiator filled and drove the car home. As it turns out the heater core is leaking and had slowly dumped the coolant overboard.

Now the engine cranks readily but does not idle smoothly. If you increase the rpms to 15-1800 it seems to even out and run ok. One other problem is that the indicated coolant temp runs higher. Before the usual temp was about 210. Now it is between 240 and the upper range of 260, although it may drop back to 210 for a couple of minutes.

Suggestions?

Id check the thermostat and the waterpump and its gaskets. Ive had the gaskets go and it would just piss out of them like crazy when it was running. just a thought. I replaced the whole pump and everything whent back to normal. the thermostat should open at like 195 or so so temps should rise to there and then drop a bit and then steady out a little below. this is just my experience though. Im sure others can give you some other suggestions.

I replaced the thermostat yesterday and it didn't change anything as far as the fluctuating temp gauge.

Since the overheating "incident" the radiator has been low by a quart or two everytime I run it up to operating temp and then check it after it cools down. I've bypassed the leaking heater core, so that loss point has been eliminated. I know I'm getting some leakage around the intake manifold. That also may be allowing all the coolant to drain from the topside of the engine, although I'm not seeing any dripping underneath.

It has been suggested that there may be a cracked head although there aren't any of the symptoms such as contaminated oil or exhaust gasses bubbling out of the radiator.

Hopefully you did not warp or crack a head when it overheated. Have you ran a compression check or looked at the plugs?