My 1999 Cadilac DeVille is overheating, & I have eliminated every cause but internal coolant leak. How do I check for a blown engine gasket?
Watch the overflow bottle as the cap is off. remove the cap, warm the engine and watch for bubbles. Those bubbles come from the combustion getting past the head gasket.
Pressure test the cooling system. Basically, a radiator cap with a gauge and air fill. If it won’t hold pressure, you have a leak somewhere. You might be able to rent one from the auto parts store.
Do a compression test. A blown head gasket won’t let that cylinder’s pressure rise to the same level as the others. Two cylinders, side by side, that won’t make pressure screams bad head gasket in the narrow region between them.