If you currently own or are thinking of buying a diesel BMW, the chances are you have heard the term swirl flaps. Other manufacturers use different names such as tumble flaps but essentially they are very similar if not exactly the same.
So what are swirl flaps?
Many makes and models of diesel vehicles use a flap design within the inlet manifold which has the primary functions of reducing emissions and increasing power at lower RPM. The name comes from the way they cause the air and fuel to mix together, creating a better mix and ensuring the diesel is used in the most efficient manner.
Within the inlet manifold, there is one swirl flap per cylinder. Engines such as the M47 will have four flaps as it is a four-cylinder diesel engine, whereas the M57 six cylinder diesel engines will have six flaps.
They are ECU controlled and when functioning correctly they are fully closed at idle and gradually open with the RPM increase.
Introduced in 2000, they began as 22mm steel flaps and in 2004 the design was changed. The size was increased to 32mm and the screw diameter was increased. From the 2007 M57 engine, some in production no longer received swirl flaps before the design was phased out in 2008. Some BMW common-rail diesel engines as late as 2010 have been found with swirl flaps fitted.
Does my engine have swirl flaps?
Interestingly and somewhat obviously to those with the knowledge, some models such as the 318d M47 engine never had swirl flaps at all. This could be due to the fact that the emissions are already reduced adequately by the restrictions placed on the two-litre M47 engine, unlike the 320d 163bhp version. An inspection of the inlet manifold is really the only way to be certain whether your inlet manifold has swirl flaps fitted.
What are the issues?
So reduced emissions and better fuel delivery, what’s the issue? These flaps are prone to failure throughout the range, despite BMWs attempts to modify the swirl flap design. Failure of the swirl flaps usually happens gradually either from the spindle fracture, loose screws or when the plastic breaks. Another issue is that carbon build up through the inlet manifold can restrict the swirl flaps movement only increasing the likelihood of failure or some would say making imminent. Component breakdown can cause extensive damage at even low RPM which could render your engine uneconomical to repair. Debris falling into the engine can cause pistons to damage the valves and further, more severe engine damage if pieces make it into the combustion chamber. This could cause damage to the turbo and catalytic converter.
How do I solve the problem?
You have the option to replace your engine swirl flaps for new ones but with no known benefits from doing this, why take the risk that one day those parts may fail and cause irreversible engine damage?
So what can be done? The flaps can be removed and blanked and BMW diesel owners can rest knowing nothing can go wrong in this respect. Your BMW will still pass its mandatory MOT test including emissions and you should not see any noticeable difference in performance or fuel economy.
During the delete, carbon deposits can be cleaned from the inlet manifold and EGR system if desired. This is recommended whilst the mechanic has access to the area anyway to save labour time and money in the future when your BMW would need new gaskets and bolts just for this area to be inspected.
Absolute BM can only advise as to whether your inlet manifold should have had swirl flaps fitted at the time of production. Please be aware that the swirl flap delete could have been carried out by a previous owner or the inlet manifold changed and there is no way of knowing this prior to inspection.