Swirl flaps are small butterfly valves located inside the intake manifold runners of many diesel and some petrol engines. At low engine speeds, they close partially to force incoming air through a narrower path, creating a vortex or swirl effect. This turbulence mixes the air and fuel more thoroughly for better combustion and lower emissions. At higher engine speeds, they open fully to maximize airflow.
Check the intake manifold on that diesel, the swirl flaps are notorious for carboning up and sticking closed.
In older diesel engines, plastic swirl flaps were prone to snapping off and falling directly into the cylinders, causing catastrophic engine failure. Many mechanics recommend deleting them with blanking plates.
Related terms
Intake manifold
The engine component that distributes incoming air or air-fuel mixture evenly to each cylinder.
Cylinder head
This cast metal component bolts to the top of the engine block to seal the combustion chambers.
Throttle body
A housing containing a pivoting valve that controls the amount of air entering a fuel-injected engine.