Losing Coolant
 

2000 Boxster

I was out of town for a few months, went to start the car and the low coolant light was on. I refilled the coolant reservoir, drove for just a few miles. Went to start the next day and low coolant light was flashing.
I refilled the tank, opened the engine compartment and access to the coolant tank. I ran the car to normal operating temp & until the fan came on. Shortly after the fan came on the low coolant light also came on and the reservoir was empty. It’s losing about 1 gal in 20 min.
There was a slight gurgling & hissing sound from the area of the coolant tank, but no leaks, no moisture either under the car or in the engine compartment. There was no moisture on interior or trunk carpets.
Once the engine is shut off and cools or when the cap is removed some coolant returns.
The car runs well, no smoke except occasionally on start-up.
The coolant tank was replaced by the prior owner a few years ago.

Where is the coolant going?

On my way to go find a coolant system pressure tester--

Did you remove the trunk carpet? I had the same symptoms, but lost less coolant than you. Either way my carpet itself was never wet, but when I pulled the carpet and the hard foam underneath it up there was a pool of coolant. There's also a little drain plug type thing that will release that coolant when it starts sloshing around (when you drive).

Your previous owner may have replaced with a non OE unit, which are known to fail quicker.

I was losing small quantities of coolant and I also couldn’t find where it was going—until the tank split completely in the middle of a national park. $500 tow later, and $1700 put it back on the road. Just replace the tank. They are a wear item. Oh, as mentioned, get OE.

Consider adding a UV coolant dye to your system, it will leave a bright trail under UV light where ever the leak is.

I'm prety sure it's not the tank, again it was replaced recently and there is no liquid or moisture anywhere around it.

It's not a small amount of liquid- I've poured about 3 gal into the tank so far. Nothing appears wet. No drips on the ground.

I would think anyplace it might pool wouldn't be water tight.

Ok just to be clear though- you removed the trunk carpet?

Front carpet - No
Rear carpet & foam- Yes. All dry

Picking up a loaner pressure tester today, but the system is pressurized to 15-ish PSI when running so I would think any leaks would show themselves

Is it possible I'm not actually losing coolant, but just have a lot of air in the system??

This is absolutely possible and dangerous for the engine. I would drain the system and refill it (or have it refilled) using a vacuum as described, or alternatively choose the "manual" method for refilling. I cannot upload the PDF, feel free to drop me a PM for the procedure.

Check https://www.pelicanparts.com/techarticles/Boxster_Tech/29-WATER-Flush/29-WATER-Flush.htm as well

I think that's the problem.
I got a pressure tester on the system - it held pressure, so it appears I don't have a leak. If it isn't leaking, I don't understand how it could have gotten full of air just from sitting for a few months.

How is it dangerous?

I think it's time to take it to the shop. I don't have a vacuum or a lift.
And the manual procedure starts with 'find a steep hill''

If you have an easier way -- I'd appreciate the help

Not sure how it makes sense for there to be an airlock, where you added 3 gallons of coolant that went missing. The cooling system only takes what 5 or 6 gallons total?

Have you looked at the water pump? Sometimes coolant leaks from it and burns up, but with that much leaking coolant, I'd think you would smell it.

Airlocks are dangerous because of no cooling. If your pressure (1,1barg) holds continously, you should go to a shop as mentioned as quick as possible for a vacuum and fill with 50/50 G30 an destilled water. After that measure the content of anti freeze!