Mercedes C43 Mystery: High-Performance Build Leads to Stubborn Fuel Pressure Failure

A 2019 Mercedes-AMG C43 owner thought he had solved a persistent low fuel pressure problem by returning the car to stock components. But after months of troubleshooting, the vehicle continues to stall under hard acceleration, leaving a trail of replaced parts and unanswered questions.

The saga began when the tuned 390-horsepower sedan threw a P008700 code for low fuel pressure in the rail, prompting the owner to swap back to original fuel system parts. For 67 months, everything ran smoothly, until a sudden highway failure at 140 km/h.

The driver was cruising with adaptive cruise control engaged when he switched it off and the engine died instantly. Though the dash showed a fuel level above 25%, diagnostic tools revealed the low-pressure fuel pump had burned out and the tank was effectively empty. As a temporary fix, he installed a Spool Stage 3 pump, which lacks an internal filter.

That likely pulled debris into the high-pressure pump, damaging it. After replacing both pumps, cleaning the injectors and rails, and swapping the HPFP cam follower, the car drove fine under 4,000 rpm. But repeated kickdowns past that threshold caused the high-pressure reading to crash to zero bars, cutting power to the pump entirely.

The owner’s latest steps included reinstalling the stock CPC, updating the ECU, and reapplying the OPF delete. Now the car pulls hard on kickdowns, but after four or five aggressive accelerations, the P008700 returns alongside new fault codes. Everything on the car is OEM except sport catalytic converters and a Wagner chargecooler.

Despite replacing the fuel rail pressure sensor, flexible fuel line, and cam follower, the high-pressure pump loses fuel pressure specifically under high-rpm demand. The owner is now scanning for live data to pinpoint the root cause, and hoping someone else has cracked this elusive issue.