HI, I have a 2000 Expedition that quit sitting at a light. Would crank but not start. Checked spark, fuel and compression. Spark and fuel are good. Shop told us it was probably timing chain break and damaged motor and new motor will run about 3200. Husband took it apart...found timing chain slipped but didnt harm the valves. Replaced both timing chains and head gasket. Still no compression and wont start...any ideas?...its a 4x4 4.6 L