A 2021 Mercedes Sprinter (907/910) III limped into a shop blowing a mix of black and white smoke while shaking violently. The technician’s notes held the critical context: the engine had just been replaced. The vehicle was throwing two specific fault codes, P3100C9 and P3100B2. We see these situations in our diagnostic data frequently. A shop performs a major mechanical overhaul, only to be greeted by a vehicle that barely runs and logs internal control unit errors.
The instinct is often to blame the replacement engine or suspect a damaged control unit. But looking at the exact combination of symptoms and codes, the diagnostic platform we build ranked four distinct installation issues above any mechanical failure. Each one points back to the realities of installing a modern diesel engine.
When the ECU Resets Itself
Our system ranked poor engine grounding as the highest probability cause at 85 percent severity high. Code P3100B2 specifically defines a control unit reset. Code P3100C9 indicates an internal control unit error. When you combine these with a recent engine swap, the primary ground strap between the engine block, the gearbox, and the chassis becomes the prime suspect.
During an engine replacement, this heavy ground connection is completely removed. If it is reinstalled loosely, bolted over fresh paint, or attached to an oxidized surface, the circuit suffers severe voltage drops. The Delphi CRD3S2 engine control unit requires a perfectly stable ground reference to manage high speed injector events. When the voltage drops under load, the internal watchdog circuit in the ECU triggers. This forces an emergency restart, logging the reset and internal error codes.
The physical symptoms align perfectly with this electrical failure. An unstable power supply disrupts the injector drivers. The injectors begin firing erratically and dumping uncontrolled amounts of diesel into the cylinders. This massive fuel excess creates thick black smoke. The unburnt fuel droplets vaporize in the exhaust to create the white smoke. The uneven firing order causes the violent shaking the technician described.
To prove this out, a technician needs to perform a voltage drop test across the main ground between the engine block and the chassis ground point. This measurement must be taken during cranking and while idling under load. A static test will not reveal the high resistance failure. Logging live data for Terminal 30 and Terminal 15 at the ECU during the shaking will also expose the voltage dips forcing the reset. A visual and mechanical inspection of the chassis ground straps and the ground points at the engine block and transmission housing is also required.
The Missing Injector Data
The second most likely cause, ranked at 80 percent, involves the IMA injector coding. When a shop installs a replacement OM651 engine, it comes with a different set of Delphi injectors. Every single injector has a unique 20 character IMA or QR code stamped on the top.
This code is not just a part number. It defines the exact flow characteristics, the precise opening times, and the required pre-injection volumes for that specific injector across various fuel rail pressures. If the technician does not write these new codes into the control unit, the ECU operates the new injectors using the calibration data from the old engine. Furthermore, if the replacement engine uses a different generation of injector, such as moving from a 651.950 specification to a 651.958, the internal fuel map calculations will be entirely wrong.
This mismatch leads to a massive overdosage of fuel in certain cylinders and incomplete combustion in the rest. You get the same physical symptoms of heavy vibration and a combination of black and white exhaust smoke. Extreme deviations in the fuel calculation parameters can also trigger the internal calculation error code in the ECU.
Verifying this requires a direct comparison. The technician must read the stored IMA codes in the ECU using a scan tool and physically compare them against the printed codes on the new injector heads. They should also verify the Delphi and Mercedes part numbers against the chassis number specifications to ensure the replacement engine actually has compatible hardware. Reading the cylinder smoothness and correction quantity per cylinder via the diagnostic tool will highlight exactly which injectors are fighting the control unit map.
Pinched Wires in a Tight Engine Bay
Ranked closely behind at 75 percent was a damaged or pinched injector wiring harness. The engine bay in the W907 Sprinter is notoriously cramped, especially in the gap between the rear of the engine block, the firewall, and the bellhousing.
It is incredibly common for the engine harness to slip down and get crushed between the block and the transmission housing when mating the engine to the gearbox. The Delphi CRD3S2 injector circuits operate using high voltage pulses and rapid current surges. If an injector wire is pinched against the block and shorts to ground, it instantly overloads the internal output stages and voltage converters inside the ECU.
The ECU protects itself by shutting down the circuit, logging the P3100C9 internal error, and executing a protective reset resulting in P3100B2. Depending on which wire is crushed, the affected injector will either refuse to open at all or get stuck open, dumping fuel endlessly. This dead cylinder or flooded cylinder scenario generates the exact heavy shuddering and extreme smoke output seen in the bay.
Testing for a pinched harness requires tactile inspection. The technician needs to physically trace the harness routing around the bellhousing, the mounting brackets, and the valve cover. With the ECU disconnected, a resistance and insulation test to chassis ground on all injector lines will reveal the short. It is also worth inspecting the connectors at the ECU and the injectors for bent or pushed back pins, which often happen when wrestling a stiff harness back into place.
Unmetered Air and Boost Leaks
The final highly ranked cause, at 65 percent, moves away from the electrical system entirely. The P3100 fault code group on Mercedes Benz diesel systems can also relate to air mass and boost plausibility deviations.
Pulling an engine means disconnecting the entire charge air system. This includes the hoses between the turbocharger, the intercooler, the intake manifold, and the charge air resonator. If a technician fails to seat a quick connector fully until the locking clip clicks, or if they accidentally tear an O-ring during assembly, or if a hard pressure pipe cracks during handling, the system cannot hold pressure.
Under load, the boost pressure blows past the seal and escapes into the atmosphere. The mass air flow sensor at the intake reads the incoming volume accurately, and the ECU injects fuel based on that high air volume. But because the air leaks out before reaching the combustion chamber, the cylinders experience severe oxygen starvation. Burning that much fuel with no air results in constant black smoke from incomplete combustion, total power loss, and aggressive engine bucking.
Finding a leak this large requires smoke testing the charge air system from the compressor outlet all the way to the intake throttle. A visual and mechanical check of the locking clips and hose clamps is the first step, followed by logging the requested versus actual boost pressure under load to confirm the massive deviation.
Following the Evidence
When a vehicle runs this poorly immediately after a major component swap, the fault almost always lies in the reassembly process. Internal ECU error codes can trick a technician into ordering a costly replacement module, but tracing the voltage, the coding, the wiring, and the charge piping first keeps the job moving forward. Drop a comment or reach out if you see this pattern in your own bays.
