Dumping Coolant from Overflow Hose

I have very recent issue where after I park the car, an '03 996 C2 manual, it is dumping coolant out of the overflow hose. It happened once before, and I took it for another drive this morning and checked the area immediately after shutting down. Coolant was actively coming out the bottom of the car, and I can see that coolant is getting past the expansion tank cap, running into the overflow hose, and out the bottom. I also noticed today: The expansion tank was FULL of coolant, like all the way to the top at the vent valve. When I went out this morning and the car was cold, the coolant level was at the normal "full" level. The expansion tank vent valve was not in the "up" position, as it normally is when the engine/coolant are hot. The car had a new water pump (Pierburg), LNE low temp thermostat, .04 expansion tank cap, and coolant about a year and a half or 4K miles ago. As far as I can see, there is no crack in the expansion tank as I don't see any moisture around or under it. Do I have a failed vent valve causing the system to not bleed excess pressure and causing coolant to vent past the expansion tank cap?

Quotes source post: 20278767

Little confused how the tank keeps filling up on its own as you haven't mentioned adding any coolant. And with a new water pump doubt its a water pump failure. Are the radiators plugged up with debris..? What is the coolant temp while running fully warmed up..? How does the coolant in the tank look..? is it a milkshake or showing signs of any intermix of oil/coolant..? How does the oil look on the dipstick..?

Quotes source post: 20278816

Coolant and oil are both clean, no intermix. Car is running normal operating temp, not running hot at all. The vent/bleeder valve on the expansion tank is supposed to rise/open on its own as the coolant warms. It also closes on its own as the coolant cools down after shutdown. See this thread on how it operates: https://rennlist.com/forums/996-foru...lve-works.html

Loosen the cap, check the seal, re-tighten the cap. Do several drive cycles with the bale up...heat then cool...rinse and repeat. Is there dried residue around the cap anywhere?

Might want to pressure test the cap, cap should be fully open at 20.3 psi and close at 14.5 psi.

You can't see a crack in the coolant tank. They only leak under full pressure and are pretty much invisible. When my tank went bad it exhibited the same symptoms. It would be fine and then when I parked the car after a decent drive, it would puke a fair amount of fluid out. My first replacement lasted only a year or two because it was "OEM" and not Genuine Porsche, If it turns out to be your coolant tank, don't skimp on the replacement.

Quotes source post: 20278979

This exactly.

Quotes source posts: 20279177, 20278979, 20278965

I've looked and confirmed it's coming from the overflow hose, pretty certain it's not the expansion tank, unless there's a crack in the housing where coolant collects to flow out of the overflow hose. The cap is practically brand new. It's Genuine Porsche part put on a year and a half ago. Would it fail that quickly? Update: I took a drive with the bale on the bleed valve up, which keeps the bleed valve open, and there was no overflow this time. Since I've confirmed my bleed valve is not opening on its own from the system pressure when hot, I suspect that the pressure is building up in the vent lines that lead to the bleed valve, which is backing up the system and causing the condition I noted earlier where the reservoir is way beyond max fill when the system is hot.

Is there dried whitish residue or liquid on top of the tank,near the cap? That's what would go through the tube. If not, it's your tank. If you can lift the bale manually, the pressure in the tank could easily lift it.

It sound like you are over filling the cooling system. Sincs you are not experiencing overheating I suspect there is too much coolant when cold. The coolant expands as it heats. At some point it has nowhere to go except past the pressure cap and onto the ground. Once the "extra" coolant gets push out the coolant level will stabilize at the correct level. On subsequent drives there should not be another overflow event.

Quotes source posts: 20279660, 20279811

Yes, and better than residue, I was able to see liquid coolant coming past the cap. On the bale, if the diaphragm inside is broken, then it wouldn�t raise on its own from the system pressure. I need to get it off and inspect. Coolant level is just below �full� when cold.

Sorry the posts are getting all jumbled up. I’m still learning how to post on here lol.