Ever wonder what VGT stands for? It stands for Variable Geometry Turbo. It can also be called a VNT, Variable Nozzle Turbo. 

First why is a VGT required on a diesel engine? The answer is the older wastegated designs are too slow for today's very precise fuel injection systems. Current common rail fuel injection systems fire as many as five times in one combustion cycle. To maintain proper air fuel ratios the turbo must react more rapidly than an air/pneumatically controlled wastegate can manage. 

Secondly the VGT balances the air intake pressure with back pressure required for the EGR induction and it's constantly adjusting to maintain the balance.

So... boost will fluctuate depending on vehicle speed, load and required EGR flow. Even at a steady load and throttle control the boost may be altered.

Boost can very day to day. Ambient air pressure and temperature are monitored by the ecm so the boost pressures are optimized for proper engine requirements. 

VGT's will whistle loudly at certain times when the ecm requests more exhaust heat for regeneration of the diesel particulate filter. This whistle sound like a boost leak but actually completely normal. 

So... Maintenance. VGT turbo shafts rotate at speeds exceeding 100,000 RPM. The engine oil cools and lubricates the shaft bearings. To protect a VGT from seizing it's essential to maintain engine oil properly.

VGT failure can occur when you don't follow engine hours between oil changes (not mileage) Watch for engine oil pan and valve cover sludge build up. Gritty contaminated oil will lead to failure. Watch for the engine "making oil". The oil can dilute with fuel from excessive DPF regeneration attempts due to dirty turbo outlet pressure sensors. 

These VGT turbos are not cheap. If sludge or dilution is detected you should clean or replace the oil pan and valve cover. Find the root cause of the intrusion. Taking proper precautions to prevent VGT failures can save thousands of dollars in replacement and calibration of a VGT turbo or after treatment components.