Ask yourself, what genuine mileage has the vehicle covered? What service history has it had? What oil(s) has it been run on?
A cylinder compression test will only give an indication as to the engine's health. The results of which can sometimes be misleading. The only true and accurate diagnostic method is the cylinder leakage test. It more accurately replicates real conditions and should be a must for any professional outfit diagnosing turbocharged engines.
If any engine problem is found, it is quite likely that the turbocharger will have been affected in some way. It is therefore prudent to have it checked over to avoid any further damage. Other specific diagnostic equipment is available from Turbo Dynamics.
Recommended Turbocharger Inspection Procedure (On Vehicle)
Inspecting the turbocharger on certain applications is simply not possible because of their location. If it is possible to get at the unit, certain checks can be carried out to eliminate the turbocharger.
CAUTION! Do not place hands or fingers near the turbocharger compressor inlet while the engine is running. The air pressure drop at this point can draw fingers into the revolving compressor wheel blades causing injury.
Remove the hose from the air filter to compressor inlet.
Inspect wheel for blade damage caused by foreign material. A torch may be required for close examination. Look at the leading edges for signs of impact damage or dust erosion. Examine the outer blade tip edges and all the way down the profile to check for wheel rub. Burred edges or scuffing on the profile of the housing are the usual signs. The turbine wheel ideally should be checked but it is virtually impossible to see the blade tips even with the exhaust down pipe removed.
Rotate the shaft and wheel by hand and feel for any drag or binding. Push the shaft to one side and rotate to feel for any blade rub.
Check the 'float' on the bearings, both side to side (radial play) and end to end (axial play). Tolerances for different units can be obtained from Turbo Dynamics' technical staff. The axial float is generally between 0.001" – 0.004" which can hardly be felt. The radial float is generally between 0.012" – 0.024" which can be felt as a definite rock. This can only be a guide as a dial gauge is required to measure this accurately. If either of these movements feel excessive then the unit will require immediate service.
If the shaft and wheel rotates freely, no wheel damage, binding or rubs have been noticed, it can be assumed that the turbocharger is probably serviceable.