Losing Coolant - 986 Forum - The Community for Porsche Boxster & Cayman Owners

eje · 10-25-2025, 09:38 AM

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--

alexc0814 · 10-25-2025, 10:09 AM

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.

Newsguy · 10-25-2025, 12:27 PM

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.

JFP in PA · 10-25-2025, 01:01 PM

Quote:

Originally Posted by eje

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--

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

eje · 10-25-2025, 01:28 PM

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.

alexc0814 · 10-25-2025, 01:33 PM

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

eje · 10-26-2025, 06:07 AM

Quote:

Originally Posted by alexc0814

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

eje · 10-26-2025, 06:48 AM

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

997_986 · 10-26-2025, 08:05 AM

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

eje · 10-26-2025, 02:06 PM

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

alexc0814 · 10-26-2025, 03:29 PM

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.

997_986 · 10-27-2025, 01:55 AM

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!

eje · 10-28-2025, 07:15 AM

It appears I've sorted out the problem. I've filled & bled the system. It's running at normal temps, fluid level is stable.
Problem now is it's full of tap water.

I coolant & distilled water. Got it up on jack stands.
I just couldn't get it up high enough to get under it.
Figured it was time to quit f###ing with it. Dropped it off at the shop. They will check the system, flush & fill.

eje · 10-28-2025, 01:34 PM

So the saga continues--

Driving to the shop to get the coolant replaced- car ran cool. Coolant level was stable, so I must have successfully purged most all the air.

The shop checked the car- no leaks, no issues with the cooling system. Replaced the coolant. I picked up the car.

Eight miles on the way home the temp starts rising & coolant light starts flashing.

I pull over, call the shop - they said it was an anomaly. Should have been good. Let it cool off & put some water in the tank. Put in @ 3/4 gal.Got home, reservoir was a little low, put in the rest of the gallon.
This was a shop that specializes in German cars. Don't think I'll go to that shop again

alexc0814 · 10-28-2025, 02:17 PM

Have you tried popping off the engine access cover behind the seats and looking at the water pump and the top of the undertray beneath it?

997_986 · 10-28-2025, 11:34 PM

I thought of a problem with a cracked head etc. but I do not think it will be able to "burn" that amount of water without any indication. But: I think it is a temperature related issue.
Really strange, thermostat and water pump can be ruled out? Otherwise that would be my next approach (it is a 2000 Boxster) and use UV coolant dye as mentioned. Even if it cannot be tracked back to the pump/thermostat it is worth doing it anyway.