Ok I have been fighting this problem for awhile now and I'm out of ideas. I have a 2007 Gmc Sierra 6.0 VMax, with a valve train problem. It has a dead cylinder on number six and on further investigation the rocker arms are not moving. Now I know about the horrible DOD lifters and I have already had to change this whole bank before so these are fairly new lifters I would say about 15,000 miles on them. I have found that while turning the motor over the two rockers for cylinder six go up and down like they should but about 2 sec after the motor starts the rockers go dead (no movement) they are still pretty tight not much slack. Every time before I have had a collapsed lifter the rocker has a lot of travel up and down as to not have any resistance from the dropped lifter. I don't have a clue what the problem is. Also changed the dod valley cover just in case it was the number 6 dod solenoid that was bad.....no luck PLEASE HELP