Don't take the turbo apart. Too easy to mess with the balance, and the compressor wheel's nut is usually really tight, requires a special tool to get it off without bending the shaft. The shaft is tiny on these. Shaft speed maximum is up around 200,000 RPM, give or take a little so you sure don't want the balance to be off at all.
The compressor wheel wears. The edges wear down due to air. The air actually has enough friction-at the speeds that the compressor wheel turns-to wear the aluminum. This is normal and is mostly considered in the design of the wheel, so it hurts nothing.
The wastegate actuator ("solenoid") is controlled by intake manifold pressure. With the engine running and 0.0 psi boost, the WGA (wastegate actuator) holds the flapper valve (wastegate) SHUT against the turbine housing. As the engine load and engine speed increase, the turbine wheel is spun faster, which spins the compressor wheel faster, which creates a postive pressure in the intake manifold, which backs up into the compressor housing. Once the pressure gets up to somewhere around 5-7 psi, the WGA's diaphragm has enough force against it-from the little hose that's connected to the compressor housing-to overcome the spring inside the WGA, which then pushes the rod, which in turn moves the wastegate "puck" off of it's seat, which allows exhaust to bypass the turbine wheel and then sort of lets the turbo shaft speed to slow down a little, or equalize. The WGA hose doesn't have to be very big, it doesn't need to flow a lot of air, just enough to send some pressurized air to the WGA and no more. I've tried different sized hoses on my car and it makes no difference until you get really big, which slows the response of the WG, which leads to boost spikes (which can be dangerous on a gas engine). On a diesel, one could disconnect the hose altogether and it won't hurt the engine in the short term. EGT's may get a little high. A lot of Powerstroke diesels didn't even have a wastegate. My dad's doesn't. Mine is a newer PSD (7.3) and it does, and is disconnected until I can pull it off and fix it. The WGA sticks and sometimes sticks open, so the engine makes no power to speak of.
Sounds complicated but it's really simple. Turbo's are cute little devices that do an amazing job. As simple as they are, they need precision when handling. The Kubota stuff is real simple, but when you get into VGT and VNT stuff in trucks/cars, it gets more complex-and more "fun" to work on. And more stuff to potentially go wrong.
What I do if I suspect a turbo, is to do a visual. Is the wheel broken and laying in the housing? If not does it turn with a finger? If yes, put the tube back on it and connect a "tee" to the WGA, start and then load the engine. Does it make boost? Without smoke? If yes, the turbo is generally not the problem. Usually when a turbo dies, the exhaust smoke will be so severe that it'll eat up quarts of oil in minutes, most of the time you'll get liquid oil out of the tailpipe with generally a TON of smoke. Mosquito repellant!
One some engines, a failing turbo sometimes doesn't give any black smoke. Maybe a little but not much. Just had a MX5100 in last week with a failed turbo (air cleaner was washed multiple times by the owner trying to save a dollar, and the element came apart which then damaged the compressor wheel), and with zero boost made at any RPM and load, there was no appreciable exhaust smoke. New turbo makes 6.5 psi max, at full throttle and loading the engine. There was only two complaints with this deal. One, he said he thought the engine was a little weak and two, I heard noise from the turbo-which the owner didn't really complain about. The MX5100 isn't a noisy engine to speak of (unlike an L2501), so the turbo noise was pretty audible over the normal engine noises. On the newer stuff like the L01's, some of the M's, all M5's and M6's and M7's, the only way to properly diagnose a low power issue is with a dyno and diagmaster. That is the ONLY proper way.
I still say the turbo's not the problem, and I still say don't mess with it-especially based on what I'm reading. Keep looking.....
