Coolant top-off gone bad?
 

My 2000 986 Boxster was running beautifully until earlier this week when I noticed the coolant level was a good three inches below the Min line and decided to top it off. The temp light had not been flashing but I did feel like it had been running a tad bit warmer than it should at moderate speeds. So I filled it up 50/50 water and antifreeze to just below the Max line and got the engine warmed up. I can only assume I added too much and the overflow kicked in though because as soon as it hit 180 degrees it started pouring out coolant by the back right wheel.

It continued to pour out far more coolant than I put in though and was then below the Min mark once again and now running warmer than before at 190 degrees @ 60mph+. I thought that odd and added some more this time only to just below the Min mark and took it for another drive. This time around the temp warning light was flashing and the car was still running right at 190 degrees but stayed right there.

Further driving showed that keeping the car wide open between 3000-5000RPMS the car runs right at 180 degrees and doesn't budge. As soon as I slowed down and let it cruise about 60mph @2100RPMS the car temp raised to 220degrees before I returned home. Then, more strangely, when idling in the driveway the car cooled down to 180 degrees again. I also noticed a small amount of coolant on the ground beneath the rear right wheel later that afternoon and the level is low once again with the temp warning light still flashing.

Any ideas about what my coolant top-off could have done to cause this strange behavior? Could it be the thermostat acting up? The weather outside this week has been an average 80 degrees. I am hesitant to continue driving the car for fear of causing a much bigger issue.

could be your coolant cap not sealing correctly. Caps ending with .00 have a design flaw and has problems pressuring the whole system. Whats the part number model at the end of the cap. If its ".00" then its time to upgrade. .04 seems to be the model that is mostly in stock online.

coolant cap
 

Had the same problem about a month ago.Replaced the cap, added coolant my mechanic had it run for a while to make sure the temp was ok and the fans went on. So far so good.