Intake Flaps disable
Swirl flaps are a BMW system to aid cylinder combustion and efficiency on diesels as diesels do not have throttles plates to regulate air/fuel mix, so naturally tend to run between lean and rich conditions wildly because throttle is controlled solely by the fuel injection on diesels. Unfortunately swirl flaps are also a bit of a design abomination as well and responsible for countless destroyed engines and expensive repairs due the flaps disintegrating from fatigue and the metal parts of the swirl flap are ingested into the relevant cylinder of the engine causing catastrophic damage.
Swirl flaps are located in the inlet tracts of the intake manifold and are either vacuum (DDE 4,0) or electric actuator (DDE5.0/DDE6.3) controlled by the engine ECU of the car (note on vacuum controlled models failure of the vacuum system results in the swirl flaps defaulting to the open position).
Effects of faulty swirl flap operation:
Swirl flaps stuck in open position: Deterioration in exhaust gas characteristics in lower speed ranges otherwise no effect.
Swirl flaps stuck in closed position: Power loss of approx. 10 % at higher engine speeds.
Swirl flaps operational function basics:
Closed at low engine speeds and low injection volumes (map-controlled)
The swirl flaps generally open when:
- The coolant temperature < 14 °C OR
- The amount of fuel > 24 mg OR
- Engine speed > 2250 rpm OR
- The intake temperature < -5 °C.
BMW/Pierburg made a decision to manufacture diesel engine swirl flaps from steel. A Diesel engine’s piston cycles at least 60 revs per minute, so an ingested piece of the metal swirl flap will be slammed by the moving internal components.
Most often one or more piston ends up being severely damaged along with several valves, an injector and the cylinder head, with possible collateral damage spreading to the turbo as well - I'm sure you can work out what is the cost of BMW swirl flap failure - several thousand pounds! In around 2004, BMW addressed the BMW swirl flap fault; the swirl flap supporting spindle was increased in size along with the affixing screw diameter. However, there have been BMW vehicles registered up to 2007 with early type 22mm swirl flaps fitted) and despite the increased spindle diameter, there have been reports from BMW owners still having swirl flap failure on these cars so it's up to you if you choose to ignore the warnings if you own a later year car!
Failing swirl flaps
Swirl flaps can be removed safely and in most cases when removed properly there is no noticeable loss of performance, but if some performance change is noticed this can be overcome with a tailored remap to compensate for any noticed performance issues and is advisable to have the flaps removed anyway before proceeding or planning a remap of the car as the increased boost pressure will fatigue the swirl flaps even faster.
The last of the swirl flap engine cars had swirl flaps manufactured fully in plastic so any engine ingestion damage would be minimal but as of yet no decent data is around on this type of ingestion and personally I wouldn’t want any sort of plastic either flying around inside a cylinder bore which could end up exiting into the turbo area.