If the turbos aren't working, the blades are restricting the
exhaust, causing more
fuel to be burned. To a lesser degree the intake side blades also restrict the air flow. Because I can, I usually
rebuild turbos when I take over a
boat. That's replacing the bearings and
seals. Turbos usually have
motor oil to lube the bearings. If an engine hasn't had frequent enough
oil changes the small oil
passage gets plugged or partially plugged and the bearings go bad. Then the
seals go bad, oil if it's still there either goes in the intake and gets burned or the
exhaust and smokes.
I've never had a turbo go bad because I
rebuild at half life. It's not that hard. Clean out the oil
passage, too. I recently did my Ford PU turbo and the kit was $120, but a rebuilt turbo is about $1200.