308 Coolant overflows only when engine is turned off

Very strange problem. I drive a 2002 XJ8 Sport. I can drive it all day long around Los Angeles freeways in traffic or fast moving without loosing any coolant. When I drive into the mountains to about 5000 feet altitude and turn off the engine about 10 seconds after shut down coolant pours out onto the ground from the overflow tank. Then I get the low coolant message. The car was not overheating. I replaced the pressure cap but the same thing happened on my next trip up the mountain. I’ve replaced thermostat, pressure tank, cap, and water pump. I test drove it up the mountain today with the same problem. I start the car back up and it stops leaking. I drove it home and turned off the motor with no sign of overflow. Can there be a problem having to do with the altitude? The only other thing about that altitude is that its cold and may be the only time the heater is activated. Any thoughts are greatly appreciated!

Have a look at the top of the tank. I think there are 2 pipe take off’s.
One for the overflow tank and One for supercharged cars, so I assume blanked off on yours.

The tank in the wheel arch should be connected to the TOP outlet

Thank you kindly Andy. I will investigate that and check back.

Jalal

There are two pipe take offs both going towards the front of the car. The top pipe connects to the top of the radiator. Have my pipes been switched? I started my car with the pressure cap off and immediately coolant was pumping back into the pressure tank from the top pipe. See picture.

Here is the photo including the pipes. Has my mechanic transposed the pipes? I can’t find an overflow tank. Is there one in this system? Where does the single pipe from the left side of the tank in this picture go? When the coolant does leak its from the center rear of the engine compartment.

I spilled the coolant during top up. Need to invest in a funnel lol

Here is a photo of the engine compartment I found online and the top pipe does go towards the wheel well as you said. Could this be the sole cause of my strange coolant loss from the center of the car?

Yes your pipes are swapped. And that will be the cause. It’s quite common mistake.

You have to squeeze the collar so the pipe comes off easily.

The overflow tank is in the wheel arch. Under the headlight.

your pipes are swapped

Looking at how they’re arranged currently, i.e. the wrong way, no wonder it’s a common mistake. Having them crossed over the correct way looks totally illogical. If I’d been signing-off that design, I’d have told them to find a way to make it logical, with no cross-over.

Sorry for the delay. I was out of town for a few weeks. I just got home and switched the lines. Still not sure how the pressure builds up so much and where its poring out from at altitude. I’ll take it for another test run up the mountain and see what happens. If it leaks I’ll take it right over to a friend who has a lift to investigate. Thanks for the quick and spot on diagnosis!