The car will stumble very badly and idle like crap due to not being able to hold compression in that particular cylinder. The car will run like it is on 5.5 cylinders as opposed to 6. You will also get a gas smell in the exhaust due to all the unburnt air/gas that is being passed by the valve on the compression stroke. Compression test each cylinder and you will know which one(s) are the culprit.
If you are not holding compression ... one or more of three possible things ... shot intake valve, shot exhaust vavle, or shot rings. Almost forgot ... a cracked head gasket or cracked head or loose spark plug threads could also cause compression loss.
