Hey all,
So after I did a full tune on my '92 Z28 5.7L (plugs, wires, cap, rotor, O2 sensor, ICM), it ran like a champ.
Two days later I was headed to work and noticed a harsh idle at red lights (w/ A/C on). About half way there the temp gauge started creeping up from ~220. I pulled into a parking lot and shut her down before it red lined.
When I tuned it up I made sure to take note if the fan worked, and it did. Normally I could hear it from inside the cabin (kinda clicking sound), but didn't notice it yesterday - couldn't hear it over the A/C whistle.
I let the car sit for about an hour with the key in the ON position - fan was running, had the A/C on "fan" too. Temp started going down as I expected it to.
I ended up leaving the car there for a few hours, walked home, and went back to it 6-7hrs later. Checked up under the hood before restarting it, and the antifreeze reservoir (which I topped off during tune up) was bone dry. Radiator was still full though. Filled up the reservoir and drove it home. No overheating, but it was night time in the rain so...

Could this be a bad coolant sensor? I know the fan works as it should so I don't think it has to do with that...

that is what you need to diagnose. Where is the coolant going?

This is what I don't understand. The thing was full after I tuned it up. I always check my driveway, and occasionally check where I've parked when out to see if I notice any leaks (a little paranoid with a 20yr old car) and never saw a single stain or drop.
Even today, after I drove it home last night and let it sit, the reservoir was full...
I'm stumped.

if coolant does not continue to disappear then perhaps your sytem was not full and it just pulled in the coolant from the bottle and now it's full. Only time will tell

do you have your air dam underneath? If not, you'd best get one

If you were anywhere near sarasota i'd say drive over and I'd show you the Hayden fan controller I put in and it's adjustable