I have a 2007 Mazda bt50 3000d 2x4. It seems to have limited power up to 2800rpm but it is as if a second turbo kicks in from there to 3500rpm. the turbo spools and boosts properly.

SOURCE: 2001 Audi TT - feels as if the turbo doesn't kick in???!!!

If you are shuddering, it's the MAF or bad spark.
If you have no boost, it can be many things.
-Vacuum Leak
-Bad MAF (ebay has them for us$90, just make sure you get the exact p/n)
-Bad wastegate controller (N75)
-Bad wastegate actuator (the actual wastegate valve, or the vacuum controlled plunger)
-Bad or stuck Blowoff/Diverter Valve

My TT just had the weirdest of "no boost problems". it would throw a code, saying the wastegate was stuck open, and i would have no boost. If I disconnect the battery and reconnect after 5 minutes, i would have full boost; and everything would work great for anywhere between 20 minutes-10 days!

It turned out to be a dirty Boost Pressure Sensor (aka Thrust sensor). It is a $130 part. I finally found the location of it on my 2001 TT Quattro (AMU), it's just below and in front of the battery between the driver's side intercooler and the intake manifold, connected to a hard black plastic intake pipe. It is held on with 2 silver-colored torx screws. It has a connector that gets plugged-in facing the passenger side of the car. Once you pull it off, you will see it has a funnel shaped tip. I took it off the car and poured isopropyl alcohol (99%) into the inlet hole on the funnel shape. I shook the alcohol around and then shook it out. I let it dry, put some silicone grease on the seal, and hooked it back up. The car has been working great so far. If it starts having problems again, i am just going to throw in the $130 new part, and hope for the best.

SOURCE: turbo dont boost

Check the pipes from the turbo to the intercooler and the intercooler to the inlet manifold, one if the hoses maybe not sealing 100%. let me know.

SOURCE: TURBO NOT BOOSTING

change air filter.check waste gate actuator make sure its open the waste gate.look at the actuator rod make sure its not stuck.there should be movement.

SOURCE: Subaru Legacy GT CEL code 66

There is a differential pressure sensor mounted beside the 2wd fuse link with a couple of 5mm pressure lines going in each end. These lines connect to each turbo output. The two pressures acting on each side of this sensor produces a voltage read by the ecm which tells it the differential pressure between both turbo boosts during acceleration. Normally the primary turbo leads the boost and the secondary turbo plays catch up until the pressures are even then air valves get opened and shut to bring the second turbo compression in parallel with the first turbo. The two turbos simply act in sequence but function in parallel. Its just like having one big turbo but operating down two smaller paths.
The sensor measures the changing boost pressures as the second turbo rises to equal the first.
If the generated voltage is not what it expects it shows as a code 66. As you can start to see there could be many different reasons why there might be an imbalance in the two pressures.
It could be either turbo causing the imbalance or a faulty bypass valve or pressure relief valve or a number of other issues. It doesnt necessarily have to be a turbo or a solenoid which is faulty. All it means is that the pressure difference between the two boosts is wrong.
The real question is why is it wrong.
There is a lot you can do yourself to diagnose the cause. If you are able to study the manual which is available free off the internet (do a search for the link) then you will see that you can test many of the component parts quite easily at home and also temporarily replumb the turbos to bypass aspects of the system control to figure it out for yourself. To do this you will have to gain an understanding of how the twin turbo system works. However its not hard if youi strip away the complex jargon hiding simple functions.. There is a mystique of fear concerning working on these motors which is not justified.Just use common sense and logic.
Or you take it to a shop and pay someone else to do it for you.

SOURCE: mazda 6 2o diesel replaced turbo new unit same

There is a oil drain return from the turbo charger to the sump. If the sump oil level covers the outlet into the sump, oil will build excess pressure tin the turbo causing the sealing rings to leak oil in to the intake manifold. This would cause smokey exhaust and loss of oil. Try this. Drain the sump completely, refill with the exact quantity of oil to Mazda spec. Dip the oil and if the new oil level is lower than the full mark on the dipstick, you have found the cause of the problem. Mark this new level with a scriber or file and fill only to this new level. Hope this is the solution.