Q: Blocked exhaust

Hi there - no, blocking the exhaust will not damage valves or lifter. It will, however, cause your engine to lose power quickly (minutes), depending on how severely the exhaust is blocked. Think of the engine as an air pump, if the outlet side is blocked, back pressure builds to the point where the engine cannot overcome that back pressure, and loses its efficiency. If an exhaust pipe, muffler or catalytic converter is crushed or otherwise damaged such that it becomes restricted, this should be repaired or replaced to remove the restriction. If there is any question about damage, an exhaust system inspection performed by a mobile, professional mechanic, such as one from YourMechanic, who will come to your location, diagnose this problem, give you an accurate assessment of damage and cost estimate for repairs, is a good idea.