There seems to be very little information out there about how flow works. In fact in 9 years of ownership I've only ever come across one set of pictures of a dismantled high pressure pump, which is odd as these are used in a wide variety of applications (not even exclusive to VW). Because of that I've had to learn it hands on, by dismantling my pumps several times. Hopefully this will help people understand and troubleshoot. Even better if it saves paying for a new pump/installation - which I've now done several times. Pretty much every part of this pump is removable with the pump on the car except for the cam/lobe. I'm going to concentrate on just what happens in between the high pressure fuel pump intake and the return lines as different models have different low pressure setups. Whilst this is specific to the 1.6 and 2.0 TDI's I also believe it will be applicable to other EA189 and EA288 - AFAIK the EA288 uses different injectors and higher pressure but the process is the same.
Feel free to correct me, as I've said information is hard to comeby, so its almost all my own work.
Low Pressure side:
Through the filter and the LPFP the fuel is fed to the intake of the high pressure fuel pump [1]. Within the HPFP, fuel is swirled around to lubricate the cam [2]. There is a feed [3], through a fuel metering valve (N290)[4], which allows entry into the high pressure side [5] (we'll come back to that). Through the swirling around is a return [6] that is restricted by another check valve [7].
High Pressure side:
Through the flow metering valve [4], fuel is pumped into the head of the pump [8]. A check valve [9] allows flow from the low pressure side to the high pressure piston chamber [11] (either on the downstroke or when priming with the engine off). On the upstroke, high pressure is created, blocked by the check valve [9] and enters the fuel rail supply [10]. On the fuel rail itself is a pressure sensor [12] and a pressure regulator [13]. The pressure regulator works on a PWM type circuit, maintaining desired pressure by restricting return flow. After the pressure regulator fuel is fed back into the return line [14]. Blowby, or injector return, is also routed into the return. This is not a return in the same sense as a continuous flow back to the tank; in an ideal world none of the injectors should leak back, but age/wear/build tolerances inevitably lead to some.
Inside the pump is a cam [15] that acts on a roller [16] which in turn pushes the piston [17] up. A spring [18] is used to push the piston back down on the downstroke. The cam is timed to pulse the same time an injector needs to fire. The check valve can be removed/inspected easily enough in situ. The head can also be removed in situ but will need to be pressed back down, against the force of the spring, to bolt it back down (such as using longer bolts to get it started). While the roller/follower can be removed it's imperative to put it back in in the exact same orientation.
Both the fuel metering valve and pressure regulator are normally open. That is, when de-energised they sit at their fully open state. I think it's a misconception that you have to release the pressure before working on these but the moment the engine switches off, high pressure is vented, and only the low pressure remains (~40PSI) for a short duration, until the pressure leaks back. In addition, I'm unsure whether these are driven by 5v or 12v but applying a 9v battery to the terminals will fully close them with an audible click. Useful for indication (but not proof) that they are travelling freely.
When cranking:
When cranking, the pressure regulator on the rail closes up to allow pressure to build. Additionally, the injectors will not fire until sufficient pressure is reached. That is, cranking the engine is not a suitable way to bleed air as it's potentially got no where to go.
Overview:
So... hope that helps. Here every page there is a thread with someone with fuel issues, and more on other forums. Yes, these can grenade (where the cam and follower start arguing) but the first area to start collecting swarf is the N290 metering valve well. Which is two bolts to inspect. If you don't have swarf then it's unlikely to be catastrophic and every other component can be removed with the pump in situ. So while a new pump is commonly a fix, it's not always strictly required. This should hopefully give you something to study and work out potential failure points.
Feel free to correct me, as I've said information is hard to comeby, so its almost all my own work.
Low Pressure side:
Through the filter and the LPFP the fuel is fed to the intake of the high pressure fuel pump [1]. Within the HPFP, fuel is swirled around to lubricate the cam [2]. There is a feed [3], through a fuel metering valve (N290)[4], which allows entry into the high pressure side [5] (we'll come back to that). Through the swirling around is a return [6] that is restricted by another check valve [7].
High Pressure side:
Through the flow metering valve [4], fuel is pumped into the head of the pump [8]. A check valve [9] allows flow from the low pressure side to the high pressure piston chamber [11] (either on the downstroke or when priming with the engine off). On the upstroke, high pressure is created, blocked by the check valve [9] and enters the fuel rail supply [10]. On the fuel rail itself is a pressure sensor [12] and a pressure regulator [13]. The pressure regulator works on a PWM type circuit, maintaining desired pressure by restricting return flow. After the pressure regulator fuel is fed back into the return line [14]. Blowby, or injector return, is also routed into the return. This is not a return in the same sense as a continuous flow back to the tank; in an ideal world none of the injectors should leak back, but age/wear/build tolerances inevitably lead to some.
Inside the pump is a cam [15] that acts on a roller [16] which in turn pushes the piston [17] up. A spring [18] is used to push the piston back down on the downstroke. The cam is timed to pulse the same time an injector needs to fire. The check valve can be removed/inspected easily enough in situ. The head can also be removed in situ but will need to be pressed back down, against the force of the spring, to bolt it back down (such as using longer bolts to get it started). While the roller/follower can be removed it's imperative to put it back in in the exact same orientation.
Both the fuel metering valve and pressure regulator are normally open. That is, when de-energised they sit at their fully open state. I think it's a misconception that you have to release the pressure before working on these but the moment the engine switches off, high pressure is vented, and only the low pressure remains (~40PSI) for a short duration, until the pressure leaks back. In addition, I'm unsure whether these are driven by 5v or 12v but applying a 9v battery to the terminals will fully close them with an audible click. Useful for indication (but not proof) that they are travelling freely.
When cranking:
When cranking, the pressure regulator on the rail closes up to allow pressure to build. Additionally, the injectors will not fire until sufficient pressure is reached. That is, cranking the engine is not a suitable way to bleed air as it's potentially got no where to go.
Overview:
So... hope that helps. Here every page there is a thread with someone with fuel issues, and more on other forums. Yes, these can grenade (where the cam and follower start arguing) but the first area to start collecting swarf is the N290 metering valve well. Which is two bolts to inspect. If you don't have swarf then it's unlikely to be catastrophic and every other component can be removed with the pump in situ. So while a new pump is commonly a fix, it's not always strictly required. This should hopefully give you something to study and work out potential failure points.