Mercedes C250 CDI - Stage 2

[dukiii]

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.

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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

Interestingly:

• 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

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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

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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.

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The Goal:​

• Upgrade to Stage 2 + hybrid turbo + 200cpsi sport cat
• Achieve reliable 280–300hp without big smoke or drivability issues

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Looking for recommendations:​

1. Best plug & play upgraded LPFP for OM651
2. Anyone with experience on a CP3 pump swap for OM651?
3. Where to find a complete CP3 conversion kit in Europe?
4. Any suggestions or proven advice on diesel Stage 2 setups for OM651 engines

I would also like to point out that the car has 160,000km on it. It is unique and I would not trade it in because it is hard to find a car from those years that is a Coupe and also has the W204 Facelift line!
Thanks in advance for any help or guidance!

 

[ALFAitalia]

Deleted.

 

[Bellow]

• Heavy black smoke from the exhaust

The sign of overfuelling - or under boosting.

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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.

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​

But a weak pump would underfuel.

 

[dukiii]

Yes, I know you probably think that if the pump is weak, there is just less fuel and black smoke is not possible, but as the mapper told me, if the pressure is low or has weakened, before injection, the ECU tells the injector how much fuel to deliver, and if the HPFP has weakened, it takes more time and then the injectors stay open longer to deliver the given amount of fuel and poorly timed injections into the cylinder occur (black smoke, loss of performance)

 

[Bellow]

Yes, I know you probably think that if the pump is weak, there is just less fuel and black smoke is not possible, but as the mapper told me, if the pressure is low or has weakened, before injection, the ECU tells the injector how much fuel to deliver, and if the HPFP has weakened, it takes more time and then the injectors stay open longer to deliver the given amount of fuel and poorly timed injections into the cylinder occur (black smoke, loss of performance)

I can accept half of that - but not all of it.
Common rail diesels do not meter as per Bosch L systems ie, by varying injector duration with constant (wrt MAP) fuel pressure - though the world and their dog believes so. It uses fuel pressure alone for metering. Injector duration is all about 'pulse conditioning' ie, delivering fuel in stages to achieve the combustion event in a pre-defined manner and typically is a pilot injection followed by the main fuelling injection then as many as considered necessary post injections.
Low injection pressure will prevent correct penetration of the fuel into the air charge and inhibit clean combustion so I can see a source of black smoke there - but struggle to believe the ECU will permit running in such a scenario without entering 'limp-mode' and throwing codes - or is that where you are (PO252)?

 

[philnewmerc]

Can't help sorry, except to say it'll be a lot easier to sell and buy a 350 cdi - even stock it'll be far quicker, and sound nicer

 

[dukiii]

I can accept half of that - but not all of it.
Common rail diesels do not meter as per Bosch L systems ie, by varying injector duration with constant (wrt MAP) fuel pressure - though the world and their dog believes so. It uses fuel pressure alone for metering. Injector duration is all about 'pulse conditioning' ie, delivering fuel in stages to achieve the combustion event in a pre-defined manner and typically is a pilot injection followed by the main fuelling injection then as many as considered necessary post injections.
Low injection pressure will prevent correct penetration of the fuel into the air charge and inhibit clean combustion so I can see a source of black smoke there - but struggle to believe the ECU will permit running in such a scenario without entering 'limp-mode' and throwing codes - or is that where you are (PO252)?

Thanks for your insight – appreciate the detailed explanation.

Just to clarify:
The car behaves completely fine on the stock ECU map – no smoke, no fault codes, no issues at all.
The symptoms (black smoke under load, power loss, and occasional fault codes) only appear with the Stage 1 remap (DPF & EGR off).

Also worth mentioning:

• Sometimes, when I turn on the A/C, I notice a slight puff of black smoke from the exhaust, even at idle or very light load.
• Along with P0252, I occasionally get P0087 (Fuel Rail/System Pressure – Too Low).

That’s why I’m leaning towards low-pressure fuel delivery issues, possibly the LPFP or the CP4.1 under strain.
But your point is valid – in a stock scenario, the ECU would probably intervene earlier.
In this tuned setup, it seems that some of the safety thresholds have been raised or disabled, allowing the car to keep running even with suboptimal fuel pressure.

 

[Bellow]

Thanks for your insight – appreciate the detailed explanation.

Just to clarify:
The car behaves completely fine on the stock ECU map – no smoke, no fault codes, no issues at all.
The symptoms (black smoke under load, power loss, and occasional fault codes) only appear with the Stage 1 remap (DPF & EGR off).

Also worth mentioning:

• Sometimes, when I turn on the A/C, I notice a slight puff of black smoke from the exhaust, even at idle or very light load.
• Along with P0252, I occasionally get P0087 (Fuel Rail/System Pressure – Too Low).

That’s why I’m leaning towards low-pressure fuel delivery issues, possibly the LPFP or the CP4.1 under strain.
But your point is valid – in a stock scenario, the ECU would probably intervene earlier.
In this tuned setup, it seems that some of the safety thresholds have been raised or disabled, allowing the car to keep running even with suboptimal fuel pressure.

A couple of difficulties I'm having accepting the LPFP is at fault.
1) That the engine can deliver plenty - 204hp and 250km/h. Surely the LPFP has to be working properly for that?
2) If the HPFP is like any other underfed (ie, starved at inlet due to lazy LPFP) then I'd expect it be audible due to cavitation. No mention of that.