[Superman]

When guides and/or valve stems are worn, the valve rocks and sometimes does not seat squarely. So, potentially worn guides could be found this way. When valves rock and fail to seat properly, they retain, instead of transfer, their heat. They get too hot and burn. Then they do not seal. Intake valve hiss is heard in the intake. Exhaust valve hiss is heard in the exhaust. Ring blow-by hiss is heard from the crankcase, which is easy to hear if you remove the oil tank cap. Before making conclusions, fire the car up, drive it a bit and repeat. Carbon can get stuck under the exhaust valves. YOu can have a pretty good idea of what's going on in a combustion chamber before you remove the engine.