HD Swap: Difficult Start, Recovery Tips

Hello, fellow car enthusiasts.

I had to replace the high-pressure pump (HDP) on my 2008 Phaeton 3.0 TDI (it was faulty), and I installed a used one. I bled the system according to the instructions in the service manual.

However, the car starts very poorly. Once it's running, there's no noticeable unevenness, and no error codes are stored in the vehicle's diagnostic system.
In my opinion, there are really only two possibilities as to what the problem might be.

Injector failure (but it should theoretically be recorded in the vehicle's diagnostic system?).
2. High-pressure pump failure (builds up pressure too slowly).

or?

Hello, if the HD pump has left debris behind, you should expect that other components, or even all of them, may be damaged. Perhaps the fuel is leaking through a dripping injector or a faulty fuel pressure regulator.

But since your high-pressure pump was defective, there might also be a problem with the low-pressure system. It's possible that the high-pressure pump was repeatedly running dry, which could have damaged it.

A diagnostic scan and some readings related to fuel pressure and the injectors, etc., would definitely be helpful.

Best regards,
Dago

Sure, setpoint, actual value, fuel pressure?

Chips in the pressure regulating valve?

Best regards, Rainer.

If a high-pressure pump fails, the entire fuel system, from the tank to the injectors, must be inspected for contamination. Anything less is not only grossly negligent but almost intentional, considering the consequential damage (which includes the installed high-pressure pump).

I don't necessarily agree with going as far as, for example, the Volkswagen Group does, which is to fundamentally replace everything. However, it's crucial to meticulously check whether and to what extent any potential debris may have already contaminated the system.