In this entry, I will explain the concept of HPFP pressure maintenance.
The HPFP pressure is maintained with a closed-loop system.
To measure the pressure, the pressure sensor, located in rail, is used.
In the picture: Rail pressure sensor is marked.
To regulate the pressure, an electromagnetic (bypass) valve, which is a part of the pump, is used.
In the picture: valve marked.
A turned-off valve (if no current is supplied; PWM = 0) is open, HPFP is turned off and in bypass mode allows the fuel from LPFP to pass.
Increasing the PWM, the valve closes, the pressure in rail raises.
At the ”full functionality” mode the pressure of HPFP pump is maintained:
a) 200.000 hPa (200 Bar) for N53 series engines;
b) 150 .. 200.000 hPa (150 .. 200 Bar) for N43 series engines; 150 Bar at idle RPM and reaches 200 Bar by around 2000 RPM.
The closed-loop system continuously maintains the necessary rail pressure, changing the PWM of the valve. The management of valve PWM happens:
a) according to the valve management map (multidimensional map, which contains information about the engine temperature, RPM, consumed fuel, proposed fuel consumption, the pressure of the LPFP system, etc.);
b) according to valve adaptation map, which takes into account the parameters of the real HPFP;
c) operative (quick, realtime) corrections of the closed-loop system.
If the closed-loop system detects the inability to maintain the rail pressure according to required (longer than 5 seconds), and DME thinks, that the cause of the problem is HPFP (LPFP system reports correct fuel consumption – no leakage is identified), DME will immediately:
a) record the error message regarding rail pressure, static: 29F2;
b) DME disconnects HPFP (warning about reduced power appear on CIC and KOMBI), after restart switch HPFP to ”safe mode”;
c) if abnormal fuel consumption is identified, HPFP is switched off immediately and the engine continues to work with a rail pressure around 5 Bar (provided by LPFP).
Note: DME will renew the ”full functionality” mode of HPFP only after re-adaptation of the engine (next driving session/sleep mode of DME, clearing of errors will not help). More info here.
If DME detects a drop of the rail pressure, but at the same time – also an incorrect fuel mixture (incorrect Lambda values; incorrect mechanical efficiency of cylinders/torque):
a) the error message regarding rail pressure, static, is recorded;
b) switches HPFP in the mode of reduced pressure (100 Bar) – it is done, to reduce the leaking problem of the injector. If this doesn’t help, at the next step the HPFP is switched off.
With HPFP switched off (excluding the moment of start) the injectors are opened with several long impulses (taking into account, that the rail pressure is 10 .. 40 times lower than in a regular mode).
If DME detects, that the PWM, supplied to the valve of the closed-loop system, drops out of range to maintain the required pressure (system has more effort to pay to maintain the required pressure), the error messages 2FCA and 2FDA are recorded (accordingly, for conditions – before and after the opening of the injectors). These error messages don’t mean the defects of HPFP (defect of HPFP – valve blockage – is quite rare), but typically – increased fuel consumption. Typical culprits: leaking injectors.
In any case the most important: DME is able to maintain a correct fuel mixture in case of ANY rail pressure. With “any” I mean range from 5 to 250 Bar. Exactly in the same range of rail pressure, DME can correctly start the engine. Yes, of course, if HPFP is turned off (and the rail pressure is around 5 Bar), the EML symbol in KOMBI will be lighted up and the power of the engine will be limited, but the fuel mixture will be correct, the engine has to run normally, without misfires and shivering.
 N53; full functionality mode;
 N43; full functionality mode;
 N53; safe mode;
 N43; safe mode;
 N43/N53; home limp mode (leakage identified);
 HPFP; switched off.
The most important:
a) if the rail pressure sensor “don’t lie”, in any mode (full pressure, 100 Bar or even switched off) HPFP cannot be the reason of incorrect fuel mixture, misfire problems, problems with starting the engine;
b) in 99% of cases, when the error messages regarding HPFP/rail pressure are recorded, the LPFP contour (pressure) or leaking injectors are to blame;
c) the only HPFP defect, which can unexpectedly paralyze its work: the defect of the valve of the pressure regulator (typically – mechanical jamming).
Cases, when the HPHP pressure has to be checked:
if BOTH (offset and multiplicative) LTFT are significantly different from 0, and both are shifted in one direction.
For example: offset is +1.80 mg/stk (max), multiplicative LTFT is +20 .. 25% fr both banks.
How to check HPFP pressure?
a) connect the manometer of 250 .. 300 Bar to the rail;
b) open INPA: ../F5/F2/F6
Compare the indications of the manometer to rail pressure, indicated by INPA.
Check the indications in idle and till 2000 .. 3000 RPM. If the indications are in the range of +/-5% – the rail pressure sensor indicates correct values and is not a cause of the problem.
If indications are suspicious, you can perform additional control:
a) open INPA ../F6/F2/F6;
b) manage the HPFP pressure and compare the required values with indications of the manometer.
a) HPFP doesn’t work with one/fixed pressure. Accordingly – the performance of HPFP cannot be evaluated, simply measuring its pressure with a manometer;
b) the performance of the rail pressure sensor has to be evaluated, comparing the rail pressure (measuring it with a manometer) with indications of INPA/ISTA D/+;
c) HPFP pressure can drop by low RPM (N43 series engines) and high loads (N43/N53 series engines) – if the pump is switched to safe mode;
d) HPFP can work with a reduced (100 Bar) pressure – also this is not a reason for HPFP defect, but typically – the problem of leaking injectors;
e) also the rail emergency mode (with 5 Bar pressure) typically is a result of leaking injectors and LPFP contour problems. Only in 1 .. 2% of all cases, HPFP problems are confirmed (and also then – typically these are the problems with jamming of the pressure management valve).
If the service specialist offers to immediately replace the HPFP by the first error messages, in which the HPFP is mentioned – I suggest replacing the service specialist. In 99% of cases, these error messages inform the diagnostics specialist regarding problems of the pump to maintain the pressure, not regarding the defects of the pump itself!