A 2012 Mercedes A-Class (W176) came into a Swedish workshop recently with a P008791 code and an engine knocking under acceleration. The combination of low fuel rail pressure and a physical knock points straight past simple sensors and into the mechanical health of the fuel system. The technician described the symptoms precisely as knocking in the engine and losing power when applying the throttle.
Contamination from the high-pressure pump
When the diagnostic platform we build analyzed this combination of the P008791 code and the knocking symptom, an internal failure of the high-pressure fuel pump (HPFP) ranked as the most probable cause at 90 percent.
This is not just about a pump failing to deliver pressure. Internal mechanical wear on the pump elements or the cam follower creates metal swarf. This debris then travels through the entire fuel system, contaminating the fuel rail, the lines, and the injectors. The knocking sound often stems directly from this, as the contaminated injectors fail to deliver a consistent spray pattern, causing uneven combustion. The immediate test here is a visual inspection of the fuel filter for metal shavings, followed by using a diagnostic tool to measure actual versus desired fuel rail pressure.
Restrictions in the low-pressure supply
If the HPFP is clean, the focus shifts to the low-pressure side, which ranked as an 80 percent probability. A clogged fuel filter restricts the volume of fuel reaching the high-pressure pump. Under heavy load, the HPFP simply starves. A weak in-tank low-pressure fuel pump will cause the exact same starvation.
On some Mercedes models, the fuel filter is integrated directly into the fuel pump unit inside the tank. The required tests are checking the vehicle service history to see when the filter was last replaced, and measuring the low-pressure fuel delivery right at the HPFP inlet.
Pressure regulation and injector leaks
The remaining probabilities involve components failing to hold the pressure that the pump generates. At a 70 percent likelihood, the fuel pressure regulating valve could be stuck open or leaking internally. If it cannot maintain rail pressure during acceleration, power drops out completely. Verifying this requires monitoring the duty cycle and control current of the valve.
Similarly, leaking fuel injectors can prevent the system from reaching target pressure. An internal leak or excessive return flow bleeds off the pressure required for acceleration, and the resulting poor atomization contributes to the knocking sound. A return flow test on the injectors will quickly identify if one or more are bypassing too much fuel.
Evaluating fuel system failures
Low fuel pressure codes are often treated as electrical faults, but physical symptoms like knocking demand a mechanical inspection first. Have you seen metal swarf take out the entire fuel system on these W176 engines in your own bays?
Reference pages for the codes in this article
- P0087, what we measure for this code across real diagnostics