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Coolant top-off gone bad?

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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.

Edited by Savvymanak
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Two possibilities:

Your cap is bad (those ending in “00” are known to be a problem, “04” is the latest revision)

You have a trapped air pocket in the cooling system, which is not a good thing. You need to either run a vacuum or atmospheric bleed (run a search for the procedures).

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I agree with JFP that it is likely an air bubble somewhere. My question would be: How did it get there? and, How did the fluid drop below the min line in the first place?

I would say that you've now experienced the cracked reservoir syndrome. Lift the carpet from the trunk floor and look for coolant. You may also notice that the carpet on the floor of the trunk is not flat/smooth in the area below/near the coolant reservoir indicating that it has been moistened by leaking coolant.

I'm in the midst of coolant reservoir replacement myself. Since I'm replacing old coolant, I'm also replacing the waterpump and the thermostat.

Good luck and let us know how it turns out.

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