Oil and carbon tend to build up, creating restrictions in the intake manifold. Symptoms will be loss of power (sometimes severe) and poor fuel economy.
Car Problem Reports
Volkswagen Beetle TDI Loss of Power Due to Oil and Carbon Build Up in Intake Manifold
Volkswagen Beetle TDI Problem
Engine Affected: 1.9L 4 Cylinder Diesel Turbo
Average Mileage: 150,500 mi (140,000 mi - 161,000 mi)