What does it mean?
OBD II fault code P0400 is defined as “ Exhaust Gas Recirculation Flow Malfunction”, and is set when the PCM (Powertrain Control Module) detects a problem with the amount of exhaust gas being rerouted to the inlet tract. Such a problem will cause a trouble code to be stored, and a warning light to be illuminated. On some applications, a code will be stored only after several failure cycles, while on others a code will be stored on the first failure.
The function of the EGR (Exhaust Gas Recirculation) system on both gasoline and diesel engines is to redirect a percentage of the exhaust gas into the inlet tract to be included in the air/fuel mixture. Since exhaust gas is relatively oxygen poor, the recirculated exhaust gas dilutes the air/fuel mixture, thereby reducing combustion temperatures to below 1 500 0 C (2 800 0 F), which is the temperature at which nitrous oxide forms.
Nitrous oxide is a combination of oxygen and nitrogen, and the chief component in smog caused by car exhaust emissions. Design details of EGR systems vary rather widely, but the PCM generally calculates the actual amount of exhaust gas being recirculated by monitoring changes in the manifold pressure as the gas is being introduced, or prevented from entering the manifold.
When the changes in manifold pressure do not coincide with the signal the PCM expects to receive under any given engine load/speed, a code will be stored. Note that there are fundamental differences between how EGR systems work on gasoline (spark ignited) engines, and diesel (compression ignited) engines.
Below are some details on the specifics; Spark ignited engines: On these applications the amount of recirculated exhaust gas is limited to about 10% (or slightly more on some applications), since excessive amount of gas interferes with the flame front (combustion process) that in turn causes misfires and poor combustion. Therefore, the EGR system is deactivated at idling and during high engine loads to ensure optimum combustion, by not diluting the charge (air/fuel) mixture density.
An added advantage of EGR is the cooling effect it has on the exhaust valves, extending the service life of these components considerably. Note that many of the adverse affects on combustion by recirculating exhaust gas can be countered effectively by the PCM by means of advancing the ignition timing. Compression ignited engines: Unlike spark ignited engines, diesels always run with excess air.
Moreover, diesel combustion does not depend on smooth flame propagation during combustion to the same extent that gasoline engines do, which means that much larger percentages of exhaust gas can be recirculated without adverse affects. Diesel engines can comfortably cope with up to 50% of the exhaust being recirculated at idle, since on these engines, there is always a large excess of air.
On many diesel applications, the hot, incoming exhaust gas is passed through a heat exchanger to cool it down before being passed into the engine via a metering valve. However, large amounts of exhaust gas translate into large amounts of particulate matter being introduced into the engine. Particulate matter in diesel exhaust consists mainly of sooty carbon, which can increase engine wear, and particularly when the soot washes into the engine oil.
NOTE: Many, if not most, engines fitted with VVT/VCT systems do not need EGR systems, since the valve overlap can be adjusted so that a small amount of exhaust gas is retained in the cylinders at the exhaust stroke. The gas that remains behind has the same quenching effect as gas that is introduced by other means. The image below shows a typical EGR valve that is clogged up with carbon. Note that this problem is far more prevalent on diesels than on gasoline engines.
When this does occur on a gasoline engine, the cause is almost certain to be related to excessive oil consumption.