In this entry – how the fuel is pumped from “the wrong side” to the side of the fuel pump.
Note: as we know, “normal” BMW is equipped with rear drive, which means – the drive shaft is placed to the rear axle. Due to the drive shaft, in the middle of the fuel tank is a significant narrowing (in a direction upwards) – the fuel tanks consists as from two separated fuel tanks. They are connected only with a narrow tunnel on the top of the fuel tank. To make fuel accessible to the fuel pump, which is located on the passenger’s side, the fuel has to be pumped from the driver’s side tank.
Attention: terms “passengers/drivers side” are used for vehicles, which are intended to drive on the right side of the road.
Here, how the LPFP system looks:
2: fuel level sensor, passenger’s side (meter 1)
3: fuel pipes
4: pipe curve, to fit in the tunnel
5: fuel level sensor, driver’s side (meter 2)
6: fuel filter and overpressure valve
Fuel pump (1) is placed in a plastic “cup” with a small volume. On the bottom of the cup, there is a valve, which allows the fuel to flow in from the fuel tank, but don’t allow to flow out. This valve is intended for the situation when an only small amount of fuel is left in the tank. Then primarily the cup is filled with the fuel, and when the car drives, fuel cannot “shake” and flow away – it will always be “around” the pump.
To identify the fuel amount correctly, there are two fuel level sensors in the fuel tank: on the passenger’s side meter 1 (2) and on driver’s side: meter 2 (5).
Wia fuel pipes (3) the fuel, pumped by the fuel pump, get to fuel filter (6) and is also pumped from “wrong” (driver’s) side back to the pump ‘s side (passenger’s side).
There is a special “elevation” (4) in the middle of the fuel tank to place the fuel pipes.
How does the system work?
Fuel is filled on the passenger’s side of fuel tank – in the part, where the fuel pump is located. Via valve on the bottom of pump’s (1) cup, the fuel gets in the cup, and the pump is ready to pump it – pass to HPFP. Turning on the pump, it pumps fuel and via one of the pipes the fuel under pressure gets to the fuel filter (6). Flowing via a filter, the fuel is further dispatched to the HPFP via the opening of the top part fo filter. Over-pressure valve works in the following cases:
- due to damaged EKP module (short-circuit of pumps management transistor), the pump is supplied with a full onboard voltage (12 .. 15V), its pressure 5000 +/- 30 hPa is not stabilized;
- EKP module works in emergency mode.
EKP, instead, works in emergency mode, if:
- communication with DME has been lost;
- data of LTFT pressure sensor are incorrect or not received at all.
Fuel, which flows from the overpressure valve, via pipe is directed back to the pump’s cup.
Fuel pressure, which is produced by the pump, is used to pump the fuel to the “right” (pump’s) side.
Fuel pipe, which pumps fuel from the pump to filter, is equipped with T type connection near from filter (on driver’s side). One connection branch (via over-pressure valve) goes to the filter – via it the fuel gets to the HPFP. Second branch gos to the back-pressure valve and after that – to the injector. The injector is a hole of around 0.5 mm diameter, via which the fuel flows out with high speed. The diameter of an injector is chosen so tiny, to maximally avoid loss of flow-rate for this overpumping. Fuel, which at high speed flows out the injector, is directed to the backflow pipe. Close to the injector, the fuel catcher is located – it’s an opening on the bottom part of driver’s side tank. Flow, created by the injector, is used to overpump fuel, which has got to the fuel catcher. A principle is very similar to the spray – simple, but efficient. Although the fuel amount, which floes out the injector, is small, due to a high speed of fuel flow, the fuel flow rate on the backflow pipe is remarkable. As my measurements show, such solution pumps 1 liter of fuel during 5 .. 7 seconds (around 150 .. 200 ml/second). Back valve before injector ensures, that in fuel pipe “pump – filter” all the time has fuel and no air bubble can form.
Here, all three fuel pipes, which connect both sides of the tank:
1. the thinner one, yellow: fuel pipe from pump to filter (T type connection by it) – to transport fuel to HPFP;
2. pipe in black color – pipe, via which the fuel get to the pump’s cup, if the overpressure valve works;
3. the thicker one, yellow: backflow pipe – via this pipe the fuel is pumped from driver’s side tank to the pump’s cup.
Problems of the system.
1. Most serious problem: fuel filter is located BEHIND pump, which means – if any dirt gets in the fuel tank, it will get also in the pump;
2. backflow injectors also are located BEFORE filter, which means – if any dirt gets to the fuel tank, it will get also in the injector. Clogged injector means – there will be no normal fuel pumping to the cup, the pump will suffer a lack of fuel, even if in the tank there will be 35 .. 36 liters of fuel;
3. if the backflow valve before injector is not airtight, in the pipe, which goes from pump to the filter, the air bubble can form. In such a case, the cold engine start will be difficult. Also, this valve is located before fuel filter, the dirt can easily get in it;
4. non-airtight backflow valve of the pump also can create the air bubble in the system, after the car has been standing for a long while. If LPFP pressure drops to 0 during a reasonable time (24 hours of car, being in sleeping mode), check the airtightness of the system and eliminate the problems;
5. non-airtight backflow valve by T type connection will cause similar problems as described in previous points;
6. non-airtight overpressure valve can cause problems when the car is started after a long break (similar as in previous points). This problem has following cases (which actually could be easily eliminated by BMW):
- the connection of overpressure valve/its backflow pipe (in black) is not made airtight, it means – if the valve has leakage, fuel will slowly leak to the driver’s side of the fuel tank, the air will get in the fuel filter, which can cause serious problems when starting the car;
- even, if the fuel leakage will not be in the connection, the fuel, leaked via a valve, will get in the black pipe, via it – to pump’s cup. Fuel, which has flown away, will be replaced by air, which will be “taken” from the area of the pump’s cup. The problem will be less pronounced in E6X cars because the fuel pipe of them is placed more correctly (see the pipe’s curve 4 in the 1-st picture). For E9X cars the problem is more pronounced because the pipes are formed as in the second picture (without such curve).
Note: LPFP pressure (its drop) have to be controlled, using manometer, while the car is in sleep mode (or with disconnected battery). Directly after waking up the car, EKP turns on LPFP and pumps (and later maintains) pressure of 5 bar in contour – diagnostics tools will indicate correct pressure, but its true value in sleep mode will not be visible. Correct pressure (after its pumping) don’t guarantees, that the air haven’t got in the system!
Unlike “old type” systems, N43/N53 LPFP contour has no large fuel flow, the air bubble has much larger (negative) impact – the car will start to shiver strongly and will “choke” not directly in the moment of start, but after some time, when the air bubble will “travel” till HPFP and injectors.