W204 C250 CDI
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W204 C250 CDI
W204 C250 CDI
Hi everyone,
I'm currently working on upgrading my Mercedes C250 CDI Coupe 2014 (OM651 engine – 204hp / 500Nm) to Stage 2, aiming for around 300hp, but I'm dealing with a specific and frustrating issue.
The vehicle produced 202 hp and 480 Nm in stock condition, and with the first stage tune, it reached 241 hp and 520 Nm (with DPF and EGR still active) according to dyno measurements.
Currently doing a fuel pressure test with a mechanical gauge to check if pressure drops below 4 bar under load.
But the problem with the pressure gauge is that I can't monitor it under real load.
Thanks in advance for any help or guidance!
I'm currently working on upgrading my Mercedes C250 CDI Coupe 2014 (OM651 engine – 204hp / 500Nm) to Stage 2, aiming for around 300hp, but I'm dealing with a specific and frustrating issue.
The vehicle produced 202 hp and 480 Nm in stock condition, and with the first stage tune, it reached 241 hp and 520 Nm (with DPF and EGR still active) according to dyno measurements.
The Problem:
While driving with a proper Stage 1 remap (EGR + DPF OFF), the car shows the following symptoms during mid-range RPM (2500–3500) and under moderate to heavy load (for example, light throttle in 5th or 6th gear on the highway):- Heavy black smoke from the exhaust
- Noticeable power loss
- Occasional fault code: P0252 – Injection Pump Fuel Metering Control “A” Range/Performance
- At higher RPMs (4000+), the smoke completely disappears
- On the stock ECU map, the car runs fine even under load – no smoke, no faults
- The car can reach over 250 km/h on Stage 1, which suggests that the turbo is in excellent condition
Already checked / replaced:
- New fuel filter
- Injectors tested and fully working
- Turbo is building pressure without any leaks or issues
- The remap was done by a reputable tuner and confirmed working on other OM651 engines
- The tuner suspects CP4.1 high-pressure pump is failing under load, causing inaccurate injection timing and fueling errors
My suspicion:
The car was often driven low on fuel, so I also suspect a weak low-pressure fuel pump (LPFP)Currently doing a fuel pressure test with a mechanical gauge to check if pressure drops below 4 bar under load.
But the problem with the pressure gauge is that I can't monitor it under real load.
The Goal:
- Upgrade to Stage 2 + hybrid turbo + 200cpsi sport cat
- Achieve reliable 280–300hp without big smoke or drivability issues
Looking for recommendations:
- Best plug & play upgraded LPFP for OM651
- Anyone with experience on a CP3 pump swap for OM651?
- Where to find a complete CP3 conversion kit in Europe?
- Any suggestions or proven advice on diesel Stage 2 setups for OM651 engines
Thanks in advance for any help or guidance!
