Vehicle is a 2003 Dodge Neon with 2.0 liter 4 cylinder engine and 40TE 4 speed automatic transaxle. Current odometer reading is 282,350 miles. Problem is the coolant recovery container mounted next to the firewall is being overfilled by coolant from the cooling system. When the engine is cold, you add the proper coolant thru the radiator cap until the system is full. Then you make sure the recovery container has coolant at the ADD mark. You drive the car for 1 hour at highway speeds and then stop. After letting the vehicle sit for several hours and cool down, the level in the coolant recovery container is almost at the top and ready to spill out the overflow. You remove the radiator cap and the coolant in the system is low. This has been a ongoing issue for several years with this vehicle. This is the link on this forum where I discussed in year 2013. Coolant filling up radiator overflow bottle I have replaced the radiator cap several times and the last new cap installed one month ago was a Mopar unit. Cap made no difference; coolant still being pushed into the recovery container. Engine does not overheat. Ample heat from heat at all times. Engine temperature gauge is steady and normal at highway speeds and when idling. One repair shop I recently had inspect the car said the heater core was leaking. This was bleeding off the pressure such that when the engine cooled vacuum in the cooling system is being lost and will not return coolant from the recovery container to the system. Sounds plausible but there are no drips under the car from a leaky heater core. And there is no sharp, sweet smell in the ventilation system which I would tend to expect if a heater core is leaking. So I decided to disconnect the inlet and outlet to the heater core and install a length of 5/8 inch heater hose and completely bypass the heater circuit. I filled the cooling system through the radiator cap and adjusted the coolant in the recovery container to the ADD mark. I drove the car at highway speeds for 60 miles and let it sit. Recovery container filled to the top. I used a suction gun and drained the coolant from the recovery container and filled the cooling system. System full; recovery container fluid level at the ADD mark. Drove car again at highway speeds for 60 miles and same results. Recovery container filled to the top and cooling system low on fluid. So bypassing the heater circuit disproved the theory that a leaky heater core is causing vacuum problems in the cooling system. I am not having much luck finding a repair facility that can go beyond guess work. At this point my guess is that the engine does have a minor head gasket leak. I would like to have some definitive proof that the head gasket is leaking. Would a test where an adapter is placed in spark plug hole, cylinder placed at top dead center so valves are closed and cylinder pressurized to 150 psi pinpoint a leaky head gasket? Would a head gasket leak only when engine hot and not when engine cold?