I don't know how to check the clearance, but we recently had the same symptoms as you, low power no smoke. Talked to mechanic at dealership and he gave us ideas on where to start looking. There is a hose from the intake that hooks to the wastegate and the android valve on the injection pump. When the turbo builds enough boost the pressure will activate the diaphram in the android valve and give more fuel. If there is a hole in this hose it will not activate the diaphram. We had a heater hose rub a hole it the cover that gives access to the andriod valve setscrew. Used J-B Weld to fix hole and it fixed our problem. If no leaks in the hose you need to check the diaphrams on the android valve and the wastegate. They use a vaccum pump to check them but said if you have a regulator on your air compressor set it to no more than 20psi, take the small hose loose between the turbo, intake, and injection pump and blow into each diaphram. If either one pops before 20psi, it is bad. The wastegate diaphram is over counter parts but the injection pump diaphram they have to get through a diesel shop. Had a neighbor that unhooked the wastegate and screwed the setscrew down on the andriod valve and said it acted like a different combine. Asked mechanic about it and he has heard of people screwing the setscrew down and it will give a lot of power but has also seen gearbox problems and bulges in the cage as a result, something has to give so be careful, count the turns if you do it. Also said newer motors are tamper-proof. Best of luck.