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You can perform a compression test of all cylinders to check for a failed head gasket. If the gasket has failed between two adjacent cylinders, those two cylinders will have low numbers. You also can use the compression tester hose (with the Shrader valve removed) to apply compressed air to each cylinder. When the air enters the cylinder, a leak to the cooling system or oil passages will produce pressurized air in those systems as well (check for bubbles in the radiator or air escaping the valve covers or oil dipstick).