a quart of coolant leaks out every 7-14 days

[electech]

For others who know the S80: Is there a bleed screw on the S80? There could be just enough air in the system to cause overpressure during "heat soak". Even if there is not air in the system, a sticking thermostat will do the same thing.

After the engine is shut off, the water pump is no longer circulating coolant through the engine block (or turbo units, but those usually cool faster). The leftover heat in the engine block heats up the coolant to much higher than normal operating temperature- resulting in high pressure. If the thermostat sticks, the pressure is trapped in the engine block. That can force coolant out of the weakest point only under specific circumstances- a very hot engine and a stuck thermostat.

Putting it to numbers, just throwing out random values as an example:

Say the normal pressure is about 15 lbs, and the max pressure is 18 pounds. A pressure test is conducted up to max pressure. No leaks or loss of pressure. Then, the thermostat sticks during a heat soak, and the coolant passages in the block heat up and the coolant has no where to go. The pressure in that loop goes up to 24 pounds. If you are lucky, a pinhole leak forms at a seam under a clamp and coolant squirts out. If you are unlucky, the head gasket or freeze plugs fail.

There are two fixes. One is to find the leak (it will fail again). Use dye if needed. The other is to replace the thermostat. Both fixes are necessary. If you fix the leak, and the thermostat sticks, the next weak point could be your head gasket.

[boosted5cyl]

pressure can escape from the expansion tank even when the thermostat is closed. That said you might be onto something. System might be only under 10psi when running but get up to 17-18 when stopped which is enough for it to leak. Cap is meant to release at 18PSI if I recall correctly.

Maybe change the expansion tank cap in case its not releasing pressure? Less than $10 worth a shot.

[JDS60R]

Where did the leak dye show it leaking from?

[boosted5cyl]

Where did the leak dye show it leaking from?

I dont think he's had a dye test done yet....

[JDS60R]

I agree.

I thought we wanted to know where the leak was coming from? Lots of great guesses on probable leaks but we need some action on part of the OP.

Because of the size of the cooling system the leak could be coming from almost anywhere. I think this cheap effective test is a good next step for the OP. Then the board can help with guidance on the repair itself.


OP (Jnelissen)- please perform a leak dye test or pressure test if you have the equipment and let us know what you found.

[jnelissen]

I will do that guys. Thank you so much for this info. God bless you guys for taking the time to help me out. Do you know any good trusted Volvo mechanics in the Mission Viejo, Southern California area?

[electech]

pressure can escape from the expansion tank even when the thermostat is closed.

Thanks boosted5cyl. I have most of my experience on American cars. I am not as familiar with the Volvo pressure system, yet.