Q: Showing signs of a blown head gasket.

If there is a head gasket leak, as the engine heats up and expands, that may be sealing the leak somewhat and that would explain your experience. Based on what you are describing, it certainly sounds like a break at the head gasket is likely and/or you could have even gotten cracks in the head or block. All the possibilities would be investigated on disassembly. Overheating will often ruin the thermostat, of course ensuring (or at least contributing to) continued overheating even if nothing else (like a head gasket) is wrong and the coolant level is restored. To get to the bottom of this there are a couple strategies. You can check for exhaust gas in the coolant, you can check for cylinder leakage using a leak down tester and you can see if there is coolant loss as you drive the car. If there are no external leaks, and yet you have coolant loss, that would be a sign of a head gasket leak.

As far as resolving this, if you do attempt a repair keep in mind that cylinder head gasket replacement requires strict adherence to Factory Service Manual specifications particularly regarding block and head flatness. Overheating tends to warp these surfaces and consequently the head and block have to be carefully measured before gasket replacement is even contemplated; these surfaces have to be flat within .002 inches, which is one-half the thickness of a human hair. Also, head bolt replacement and torque sequence, surface preparation (surfaces have to be perfectly smooth and mirror-like before gasket is applied), and so forth, are all critical. Unless the conditions are perfect, leaks are actually common. That’s why a lot of shops won’t do the work because it takes too much effort. If you desire that this problem be diagnosed by a certified Mechanic, dispatched by YourMechanic right to your location, please request an engine overheating diagnostic and the responding certified mechanic will identify the problem and estimate a repair strategy to get your car running again. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.