Help! I have a 2005 GTX 4 tech that has no compression in rear cylinder. the front two have 130 each. I have been told to check valve clearences as that often happens mptoe was quiet and just started to loose power and run ruff. All temps and oils are were good???

Definitely pull the head. You need to get in there and see what's wrong. Otherwise you'd just be guessing.

thanks so far.. but I would like to know how to check valve clearances or.. are they hydraulic with 0 lash? Before I pull the head I will do a leakdown test to see if it is rings or valves thanks in advance.