2007 S40 2.4, car overheated one day and after it cooled off I found the upper radiator hose leaking pretty bad where the plastic 4 way connector was cracked and leaking. Replelaced the upper and lower hose, thermostat and also added new coolant to the reservoir. Started the car cold and took off the reservoir cap to let air out and turned on the heater but no matter how long I let it run with the cap off, there is what seems like bubbles coming into the tank from the black hose in the back of the reservoir. The car can idle for as long as I want without overheating with the cap off, once I put the cap back on it starts overheating shortly after. If I drive it, it starts overheating within 5-10 minutes and temp gauge will go to normal once I stop and idle. When I run the engine with cap on, the coolant reservoir makes this loud whining sound as if air is trying to move. I can run the engine with cap off for an hour straight and it doesn't overheat and engine sounds normal and no leaks. I'm hoping its air in the system but am afraid it's a blown head gasket. Could my car idle for an hour without overheating if the head gasket is blown? Am I not bleeding the air properly? Someone please help me as I need this car for work and I'm sort of screwed if the head gasket is blown. If I put cap back on and tighten, the bubbles almost completely go away except for maybe a few every few minutes.

 

always possible the replacement thermostat is a wrong part or failed out of the box (particularly if you went budget after market). Air in the system is worth looking into - Not sure where it is but there may be a bleed valve to help burp the system - see if there's a Youtube vid. As to a blown head gasket, that also is worth looking into, particularly following an overheating event. Does the upper radiator hose get rock hard when the engine has warmed up? You may need to get a system pressure test done. Anything showing up in the coolant, oil or any change in level to suggest coolant loss and that something's amiss? You can also buy an HC kit to test the coolant for any exhaust gasses (measures the hydrocarbons) - they run about $40 at a big box parts store. As to the wandering temp gauge - you may also want to test the ECT sensor but that can be caused by an air bubble.

 

The oil has no sign of coolant in it. I can run the engine while idling for hours and it doesn't overheat. Once I go for a mile long drive, it starts overheating and stops once I stop and idle. Not sure what's causing this. If I have cap off and coolant is between minimum and max with engine idling, once I put cap back on the coolant completely empties from tank. If I slowly release pressure from tank the level comes back to normal. Does the coolant go empty while car is running?