VW Passat inlet manifold issue
The inlet manifold is part of the engine that supplies the air/fuel mixture in to the cylinder. The primary function of the intake manifold is to evenly distribute the combustion mixture (or just air in a direct injection engine) to each intake port in the cylinder head(s). Even distribution is important to optimize the efficiency and performance of the engine.
The inlet manifold contains Swirl flaps, which are small butterfly valves fitted to the intake manifold. These are just before the cylinder head intake ports. They are fitted in many modern automotive diesel engines, including those from Audi, BMW, Vauxhall and Alfa Romeo. The flaps are smaller than the intake runners. Therefore allow air to pass around them even when “closed”. Swirl flap position is adjusted by an electrical or vacuum-activated servo mechanism which is under the control of the engine management system. In a typical implementation the flaps will be closed at idle speed, creating additional turbulence in the intake.
So What Does The Intake Do?
As engine speed increases, the flaps are gradually opened until, at around 2,000 rpm, they are parallel to the airflow and present virtually no resistance. Their purpose is to ensure that the air entering the cylinder is sufficiently turbulent for good fuel-air mixing even at low engine speeds. The disadvantages of swirl flaps are mainly associated with fouling by exhaust gas recirculation. This leaves tarry deposits on the flaps and the inside of the intake manifold. Over time the flaps can begin to stick in one position. The engine management system may report an error code if the correct flap position cannot be achieved within a few percent of the design specification.
More seriously, the flaps or their mountings can fracture under the strain and parts can enter the cylinder. This almost always causes major engine damage due to the very small clearances inside a diesel engine. Some owners choose to have the flaps removed as a precaution, with blanking plates fitted to maintain manifold sealing. In many cases the impact on driveability and fuel economy is negligible.
The Passat owner came to the workshop as his car was running badly and the engine management light illuminated on the dash. We plugged the car into the diagnostic machine and it came up with a code which we recognized immediately. The fault was for the Intake Manifold Flap Position Sensor. This sensor is only available with the whole intake manifold – a very expensive job. But we have experienced this issue before and knew that if we were able to source an earlier model inlet manifold (a non-flap inlet manifold) we would be able to blank the swirl flaps and seal the manifold completely.
Inside The Inlet Manifold
When we removed the inlet manifold we could easily see what had happened. The swirl flap in the first manifold hole had snapped causing the flap rail to vibrate sideways. This eventually caused a hole to appear at the far end of the flap rail which had led to a boost leak. We where very lucky to catch it at this stage as the worst case scenario would be engine damage.
With the engine built back up with the non swirl flapped inlet manifold in position we ran the car to check there where still no boost leaks. Then we reprogrammed the ecu so the car would run without the original inlet manifold on.
Unfortunately when you look more into this problem it isn’t just the VW Passat that has issues with swirl flaps and the inlet manifold. The problem also affects Audi, BMW, Vauxhall and Alfa Romeo.