Common rail Euro-III engine controls fuel pressure with one control valve which controls inlet or outlet of fuel line. Inlet control type has disadvantage of activating fast increase of fuel pressure when initial starting or sudden acceleration because the procedure to get high fuel pressure takes many steps like low pressure pump → control valve → high pressure pump → common rail. However, outlet control type has advantage of quick increase of fuel pressure but there are weak point such as loss of engine power and fuel temperatures up. Therefore, CRDi Euro-IV has been adopted "Dual Pressure Control type" so that precise and quick increase of fuel pressure can be easily established with various engine condition. Dual Pressure Control" type means that CRDi Euro-IV engine has the inlet pressure from high pressure pump and outlet pressure of common rail simultaneously.

Fuel pressure gets high if fuel pressure control valve (P-PRV, Pump Pressure Regulating Valve) installed between low pressure pump and high pressure pump (inlet side) closes the fuel line with increased duty values controlled by ECM. With rail pressure control valve (PRV, Pressure Regulator Valve), the higher duty values from ECM the higher rail pressure is set because it closes outlet of common rail.

ECM controls operating current of PRV installed on common rail and P-PRV installed on high pressure pump with duty corresponding to current engine rpm and load condition in order to control optimum rail pressure. The lower operating current of P-PRV, the more fuel amount is supplied with common rail that makes fuel pressure high, the higher operating current of P-PRV, the lower fuel amount is supplied with common rail that makes rail pressure low. Due to this reason, If the operating current of P-PRV is "0" caused by open circuit or disconnected connector, pressure in the common rail is increased to maximum resulting from supplying maximum fuel amount.

P0254 is set when excessive current in power circuit of 'Fuel metering unit' (integrated with high pressure pump) is detected for more than 0.22 sec.. This code is due to short to battery in power circuit or 'Fuel metering unit' internal short.

Fig.1) Signal waveform of PRV and P-PRV at idle.

Fig.2) Signal waveform of PRV and P-PRV at 4000 rpm.

☞ As PRV duty(current) increases at acceleration, rail pressure is increased.