How common is a head gasket failure at 155k miles that doesn't breach a coolant passage? The engine's running rough (so much so that the vehicle is undrivable) and throwing P0303 and P0304 codes. These are misfire codes for cylinders 3 and 4. There's no antifreeze in the oil, and no disappearing coolant, but the engine burns about 1 quart per 800 miles. I'm going to examine the plugs and swap fuel injectors to chase the problem. How do you do a compression check on a Gen 3?