My friend’s '04 Sebring is still overheating even after these steps have been taken: heads were warped – had a professional shave them down and rebuild engine; new radiator; new thermostat; then removed thermostat to try this way, and bled the system for air bubbles (more than once). The car can run about five miles - then bubbling/boiling in the water recovery tank. The mechanic said there is a small crack on the water reservoir (a small plastic box) near where the bolt connects to the engine. Could this crack affect the pressure of the system and cause this overheating?
The coolant reservoir is not part of the pressurized sid eof the system and a leak in it cannot cause overheating as long as there remains in the reservoir sufficient coolant to allow the engine to draw back enough when it cools to keep the pressurized side of the system full.
A pressure test is always prudent to eliminate the possibilities of leakage under pressure (which would cause the coolant to boil at too low a temperature), but since you’d had so much work done you also have the possibility of an incorrect headgasket or even water pump gasket installation restricting coolant flow.
There’s also the possibility of a bad radiator cap. Have you tested this?
And a collapsed inner radiator hose liner. Did you replace the hoses?
And now, last but not least, the bubbling in the reservoir. That suggests a(nother) headgasket problem. That may very well be where the bubbles are coming from. They may be combustion gasses being blown through a headgasket breech, past the radiator cap, and out through the return tube in the reservoir.
When you had the head done, did anyone examine its corresponding surface on the block for erosion?
The cooling system reservoir is pressurized on the 2.7 L, but not on the 4 cylinder engine. Replace the reservoir so system can be pressure tested.