I’ve been dealing with a persistent cooling system issue on my G37 that started around March. The car ran perfectly fine for about two months after I replaced the water pump in January. Since March, the coolant has consistently expanded into the reservoir after each drive but does not return to the radiator once the car cools down. I’ve bled the system more than 10 times using a proper no-spill funnel, elevated the front end, turned the heater on, and followed every recommended procedure. Despite that, air bubbles still appear when I rev the engine, even after 45 minutes to over an hour of bleeding. I’ve replaced the radiator cap with a 20 psi unit, replaced a throttle body coolant hose, and confirmed that my water pump is functioning. There are no visible external leaks, and I’m not losing any coolant from the system—just experiencing constant coolant expansion into the reservoir. Occasionally I smell a faint hint of coolant after hard pulls. A pressure test showed a slight pressure drop, but a block test confirmed no combustion gases were present in the coolant. There’s also no white smoke, no milky oil, and the car does not overheat unless I go too long (about a week or more) without manually moving the coolant back into the radiator. When the radiator runs low, that’s when temps start to climb. At this point, I’m trying to figure out whether it’s a minor internal issue like a small head gasket leak, a vacuum issue, a clogged radiator, or something else in the system allowing air in and keeping pressure from equalizing properly.

 

Have you checked or replaced the thermostat?
​​​​​​Sill have coolant lines to your throttle bodies and oil filter?
If automatic, have you checked the trans fluid?
Plastic "burp" tee and connectors in good shape?
Checked uner the mats for a heater core leak?
Deleted your TB and oil filter coolant lines?
all those "extra" hoses and clamps are places air can get sucked in, or coolant leak out.

Coolant can even get sucked into the intake manifold if the TBs are leaking.

I thought the factory cap being so low was stupid.
So I replaced the burp T with a second inline radiator cap. Stock pressure rating at the T and a higher press cap on the radiator, so the T cap opens first.
It self burps now!
Ran the overflow to the stock reservoir.

Some guys will even run a swirl pot to help keep air out of the system.

But yeah,
check yer thermostat and maybe add an inline cap

 

Thanks for the suggestions — I’ve already installed a new OEM thermostat, and all coolant lines to the throttle bodies and oil filter housing are intact with tight, leak-free connections. Trans fluid (7AT) is clean and full, and all plastic tees and connectors look solid. No signs of a heater core leak — no sweet smell or damp carpets. I haven’t deleted the throttle body or oil filter coolant lines yet, but I might bypass them to test. No white smoke or signs of coolant entering the intake, and I’m running a new radiator cap but will try an OEM one to be safe. The system was vacuum filled, but after a week or two, coolant still rises in the reservoir when hot and doesn’t return when cold, even though there’s no overheating or coolant loss. I appreciate the tips about the inline cap and swirl pot, definitely keeping that in mind and might try this at this point