HPFP. A real problem

In several entries I have mentioned, that very often the HPFP has been replaced unjustifiably. For example, the error message 2FCA; 2FDA indicates the leaking of the injectors, 2AAE and 2AAF, instead – the problems of the low-pressure pump. Also, the error messages regarding the plausibility of the HP contour means the conditions of the maintenance of the HPFP pressure have changed, not the HPFP itself is definitely damaged.


In this entry, I will explain a case when the HPFP was really damaged.

First symptoms appeared after a mileage of approximately 420’000 km, they progressed swiftly. After approximately 10’000 km it was hard to drive normally with the car, HPFP was replaced.


First symptoms:

a) by an average cruise speed, for example, around 90 .. 100 km, when shifting from the 6-th gear to the 3-rd gear and swiftly increasing the required torque, for the first second or two the delay of the torque increase could be felt. Sometimes also in the 1-st and 2-nd gear, the car was mowing with jerks (the availability of the torque was delayed for the first moment);

b) the error messages regarding CO catalytic converters in the Homogeneous mode started to appear. They were recorded, but almost immediately the status of the error messages was changed to inactive. TI data indicated a suficient performance of the CO catalytic converters (CO below 0.015%; CH around 10 .. 15 ppm). So – the catalytic converters were not totally damaged, at ”zero loads” condition they burned at least 95% of CO/HC.


The most serious problems:

after several thousand of km, the following defect appeared – if by comparingly low RPM (for example, when driving with speed 60 .. 70 km in 6-th gear) the required torque was increased above 150 .. 200 Nm and accelerating steadily, unexpectedly a yellow EML symbol (not the engine icon, but a symbol in the central display of the KOMBI) in the KOMBI appeared together with an inscription in the central display of CCC: “Engine fault. Reduced power” (see the picture below).


At this moment always the error message 29F2 was recorded:

As we see, by low RPM (around 2200) but relatively high inquired torque (fuel consumption around 21 l/h), not taking into account closed backflow (pressure management) valve of HPFP (its PWM reached 66.8%), rail pressure has dropped below 200 Bar for a long time and was 144 Bar in the moment, when the error message was recorded.

The observation confirmed, that the error was recorded if the rail pressure is strongly below 200 Bar longer than for 5 seconds. If periodically, even for a very short moment, the accelerator pedal has been released and the rail pressure was allowed to reach 200 Bar, the error message was not recorded.

One more important nuance: till the moment of the recording of the error message the engine was working properly – the fuel mixture was correct (Lambda 1.0 in both banks). So – the leaking injector or inappropriate readings of the rail pressure sensor were not ones to blame. Maybe – the problem with LPFP?

To find out the cause of the problem, the performance of the LPFP and HPFP in real-life conditions were observed.


Here, a short video, how does LPFP and HPFP look alike when starting to accelerate in 2-nd and 3-rd gear:

As you see, LPFP pressure is quite sufficient to ensure the HPFP with a fuel supply; not taking it in the account HPFP is not able to maintain the pressure and the rail pressure drops (in short-term) even to 110 .. 115 Bar! In short-term, because when taking this video, I didn’t maintain the inquired torque too long.


In addition, at the moment when the pressure of HP contour was dropping swiftly and significantly, DME reduced the available torque (obviously, to reduce the complete crash of the HP contour). At this moment I got the confirmation, that the delay of the torque (more precisely – a short-term drop of the torque) was exactly due to the problems of HPFP.

When the required torque was maintained for a longer time (as mentioned before: longer than for 5 seconds), immediately the error message 29F2 was stored and HPFP was turned off (rail pressure dropped to 5 .. 6 Bar: LPFP supplies the fuel to HP system).


In such emergency mode the engine:

a) works only in Homogeneous mode;

b) available torque is reduced and swiftly drops above 3000 RPM (because there is not enough time for injectors to inject a necessary amount of fuel).

You can drive by car normally, even above 140 .. 160 km/h – you will reach the service station!


One more interesting nuance: if DME has identified such HP contour problem for one time and recorded the error message 29F2, since that moment (not taking in account – the error message has been deleted and won’t appear anymore) DME turns on a “safe” mode of the HPFP management:

a) the management of HPFP is managed “gentler”, it means – the closed loop feedback coefficient of HPFP is reduced (the requirements of rail pressure maintenance accuracy are reduced);

b) a wider corridor of the maintained rail pressure is allowed;

c) in separate segments (for example, in idle) a reduced rail pressure is maintained (around 150 Bar, as for N43 series engines).

So – the DME understands, that HPFP has some problems and takes it account, and accepts that it can have a reduced “power”.


Here, how the LPFP and HPFP pressures in similar driving conditions (as in the previous video) look-alike (29F2 error stored):

As you see, now the pressure of HP contour drops to the mark of 20 .. 25 Bar!


Note: “strict” requirements for HPFP management the DME restores only after re-adaptation of the engine.


Undoubtedly – HPFP has experienced technical problems. In addition – these problems were not connected with natural wear of the parts of the HPFP, because they progressed very swiftly.


One more exotic error message, which was recorded in the DME error message memory:

Attributes of the error message indicate, that the LPFP pressure has reached 9 Bar! At the same time, even setting 100% PWM in EKP module, LPFP developed a pressure of only around 7 Bar. How is that possible? Obviously, the HPFP pressure regulating valve has stuck in an open position, and the HP contour “dropped” the fuel back in the LP contour, in which the pressure raised swiftly. One more symptom, which confirms a defect of the HPFP.


At this stage, DME diligently recorded the error messages regarding the reduced performance of CO catalytic converters both in Homogeneous mode (29F4; 29F5) and Stratified charge (2A26; 2A27). For the error messages of the Homogeneous mode 29F4, 29F5, the status was usually very quick changed to the passive (DME detected, that the alarm was unjustified), the status of 29F4 and 29F5 was active for a longer time, the engine stopped to use the Stratified charge (after deleting these error messages – it was restored and for some time the Stratified charge worked correctly).


The reasons, why the error messages regarding CO catalytic converters are recorded, are the following:

a) in Homogeneous mode: the stability of the HP contour pressure was low, the pressure was shivering (HPFP had problems to maintain a stable pressure) much more than for correctly working HPFP (correctly working LPFP + HPFH system in the mode of even driving maintains the pressure of +/- 1 .. 1.5%). Every time, when the pressure dropped swiftly, Integrators were not able to correct the fuel mixture and during CO converters performance tests the fuel mixture becomes lean. This situation (presence of the oxygen in the exhaust) was detected by the control probes and DME made a wrong conclusion: CO catalytic converters don’t burn fuel/oxygen. In true – in the exhaust simply was too much oxygen, there was not enough fuel to burn it;

b) in Stratified charge mode, when performing the regeneration of the NOx catalytic converter, the fuel inquiry from the HPFP is swiftly increased. The fuel consumption grows for around 20%, because the engine switches from Stratified charge to Homogeneous mode and in addition – and fuel leftover of 20% is dropped in the exhaust in each bank sequently. During the time of the regeneration, DME manages HPFP according to the adaptation map, because there is not enough time to stabilize the pressure (regenerations session is around one second long for each bank, the engine work modes are changing swiftly). Accordingly, if the HPFP has problems with the flowrate and maintenance of the pressure, during the time of regeneration the required fuel amount actually is not dropped in the exhaust and CO catalytic converters have no fuel to burn. And again – a false decision of DME regarding the reduced performance of CO catalytic converters.


After replacement of the HPFP problems with maintenance of the HP contour pressure disappeared. HPFP maintained the pressure of 200 Bar without any problems with a deviance of several % even in kick down mode.

As you see, HPFP pressure is maintained without problems by 200 Bar even by completely open throttle (kick down) mode.


Note: pay attention – in first two videos with a damaged pump, the throttle valve (2nd column, 1st bar) opening doesn’t reach over 50 .. 60%. Actually, the accelerator pedal was pressed in kick down mode, but DME reduced the opening of the throttle, to avoid a complete crash of the HP contour.


At the end: as already mentioned in other entries, the car was running normally and the engine started correctly (without a long cranking) even with a completely turned-off HPFP. With this I just wanted to remark – a long cranking IS NOT a symptom of the HPFP damage!


What could happen to the HPFP?

In the image: simplified schematics of HPFP

As you see, the HPFP of N53 engine has three plungers, each of them has two valves; the pump has a emergency bypass valve and, of course, a pressure regulation valve. Usually, problems start with a pressure regulation valve – it starts to leak or jams. But, such problems of the pump can be caused by a defect of any valve.

Unfortunately, even the pressure regulation valve (not even talking regarding, for example, plungers) is not sold separately – in case of any problems, all HPFP has to be replaced.