Coolant dump -- help narrowing down the source?

Hi all. Trying to play detective here with a coolant issue on my 2003 Boxster S w/90k miles

Timeline
- Night I arrived to BRBS in 2018, coolant light was flashing after 12 hour drive
- We filled it up, not sure on bleeder valve (assume it is closed)
- On first fun run next day, as soon as I stopped, full coolant dump #1

- We filled up, Alan followed me back to Swiss Inn, went about the day
- Stefan checked it out, we left bleeder valve open, car runs fine for 4 spirited days BRBS, I ship car to Seattle
- Car runs great, I order new coolant cap with 04 on it
- Put new cap on, I close the bleeder valve, go for a spirited drive, as soon as I stop, coolant dump #2

- I fill up, leave bleeder valve open, and (forgive me) drive for the next 2 years with no issues
- Bring in the car for 90k service, ask them to look into the issue. They say:
- Coolant issue appeared to have some air in system. Bled and filled. Pressure tested fine, no overheating or loss of coolant on test drive
- I pick up the car (bleeder valve is closed), spirited drive home, as soon as I stop, coolant dump #3

So my questions are ... before I bring it back to be looked at, has anyone had this and what is my lead?
- Is it the cap? As noted I replaced the cap and this has happened before and after
- Is it the tank? I had replaced the tank in 2012, there have been 0 leaks at any other time, and they said it pressure tested ok
- Indy was thinking it might be a thermostat?
- One message board described the exact situation and said " It turned out that I didn't have the driver's side fan running properly. There is a resistor that according to a local Indy mechanic goes bad somewhat regularly. When it goes bad you lose one of the fan speeds and it can cause the car to dump coolant. That's what happened to mine. They replaced the resistor and the cooling system works fine now." Does that seem feasible?
- Another board said it was the "bleeder valve o-rings." Maybe?

Thanks for your help

Generally these dumps are due to a trapped air bubble, generally in one of the radiators.
It's almost impossible to remove air in the radiators by using the bleeding valve because the bubble generally stays in place, allowing coolant to flow underneath it.
When the car gets very hot after a brisk drive (when the car is not moving anymore) or if you drive up the mountains to a lower air pressure the trapped bubble will expand and make the coolant overflow
The sure-way to solve this problem is to dump the coolant, draw a partial vacuum in the system (drawing all of the air out) the allow the vacuum to slowly pull in the coolant.
This technique will completely fill the system with coolant and will leave zero air.
Happy Boxstering,
Pedro

Pedro Bonilla
1998 Boxster 986 - 311,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

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Thanks so much for the info Pedro. Bringing it back this week and they are going to do your solution at no charge so I'll report back with how things went.
Appreciate it.

Hey Pedro and others

Just wanted to close the loop here to say the Indie followed Pedro's instructions and drained the coolant, drew a vacuum, closed the bleeder valve, took it on a brisk drive and everything held up fine, solving the problem!
They did put on a new cap as well, so technically we can't be 1,000% sure that wasn't a factor, but that is a remote possibility (since the cap was pretty new). They also verified the fan was working properly.

Thanks so much Pedro for helping to eliminate this pesky problem. Hoping others find the writeup helpful.

Jim



Edited 1 time(s). Last edit at 07/05/2020 10:36PM by hoppy44. (view changes)