Weird one time overheat? Water pump efficiency when car is parked uphill - 986 Forum - The Community for Porsche Boxster & Cayman Owners

2003, 2.7, 5 speed, 118k miles. So here is the story. Per the previous owner, water pump was replaced about 5 years ago, 10k miles ago. Coolant needed to be added a few times as the system burped here and there. That was shortly after I got the car, that is 5 years ago. No problems with coolant whatsoever. A few times on the autocross, never had overheated even in ambient at 25 C or more. Fans would kick in and do the job. Fast forward to yesterday. I drove about 70-80 miles roundtrip. Going there. No problem. AC was on, car was fine. On the way back, same thing. It was warmer, no AC, as roof was down. Car was OK. No sign of cooling issues. Ambient about 22 C. I stopped to drop off my work friend and I parked on his driveway. The driveway points up. Not super steep, but significant. I left the car running. At some point I heard the fans to kick in. Normal, right? I thought I sniffed something sweet, like coolant, but everything seemed normal. I pulled out and I see a puddle. I see no leaks in front or back. It was all from the reservoir. Over the cap. It was not a lot. The level was low, but the light was not flashing. Temp was 190 F. I got back home - short drive, like 3 miles. Light started flashing. I gently added coolant and water (I have concentrate - so half water, half coolant). It took about 1.5 liter. Initially did not take much, but then when I opened the vent valve, it burped more. Fans would kick in. All seemed OK. Some little air here and there. I have not checked today (did not drive), but the coolant went down a little more, of course. No leaks. So, it begs a question - is the water pump "weak" and it does not like pumping "uphill"? Or it is not the best water pump? I am not sure what was put in there. Or maybe it is normal? The coolant lines are long and two radiators on the front car. I get it make take a little time. Oh, the radiators were hot. Circulation is there. Just not sure if "enough". Or maybe someone else has some other idea. The car was parked for several days. I do not drive it so often anymore.

You may still have air in the system. With the system cold, you can pull a vacuum on it with the vacuum fill tool and it will come out.

Very likely a bad radiator cap. Had the same symptoms in my car.

Quotes source posts: 670898, 670901

Thank you guys for responding quickly. One more question. How much 1.5-2 liters coolant is for the car? Or when would the light start flashing? In others words - is it very bad or nothing to worry about? I know the coolant capacity is about 18-20 liters, so 10% should not be a lot. I was always wondering on those. When system is empty - that is easy. Vacuum, refill, done. Filled system - well, pull the vacuum on running system or stationary? Then, how much will coolant will come out when evacuating it? There are many collapsible hoses so I can expect a lot of coolant coming out as well. Like the cap did not hold (leaked - permanently open cap) or would not release the pressure (stuck closed)?

If the coolant level in the tank is normal and you apply the vacuum gradually to allow the air to come out smoothly rather than in one big burst, you should be fine. We do this regularly in the shop with no issues. That said, it is a good idea to have spare coolant mix on hand as removing the air usually ends up lowering the level in the tank when you are finished.

Cap did not hold.

OK then, I will give it vacuum, a slow controllable pull (I have a vacuum pump, I do not use compressor) and see what else pops out. And a new cap. Thanks guys.

The new cap part number should end in -04, that is the revised version that should not leak.