A 2019 Mercedes Vito came in with "olja i kylsystemet", oil in the cooling system, accompanied by three fault codes: P30093, P0090F, and P30083D. This combination of symptoms and codes points to a mechanical failure and an electrical issue occurring simultaneously. Here is how our diagnostic platform analyzed the faults and prioritized the diagnostic steps.
The Mechanical Failure: Oil and Coolant Mixing
The most critical symptom reported by the technician was oil in the cooling system. Our AI Diagnostic Guidance analyzed this and ranked an internal leak in the oil cooler as the most likely cause, at 90% probability with high severity.
On the OM654 engine, the oil cooler is integrated into the oil filter housing. The mixing of oil and coolant is typically caused by an internal crack in the heat exchanger plates or failed seals. Because oil pressure is higher than cooling system pressure (especially during a cold start), oil is forced into the cooling system. When the engine cools down, the local pressure in the cooling system can exceed the oil pressure, allowing coolant to leak back into the engine oil.
This cross-contamination damages engine bearings, rubber components in the cooling system, and the water pump. The recommended tests for this cause are a pressure test of the cooling system and a visual inspection of the oil cooler and filter housing.
Our system also considered a defective head gasket as a potential cause for the mixed fluids, ranking it at 50%. While a damaged head gasket can allow oil and coolant to mix, it is less common on the OM654 unless the engine has overheated. To rule this out, the platform suggests a CO2 test of the cooling system and a cylinder compression test.
The Electrical Issue: Fuel Pressure Control
The diagnostic trouble codes pointed to a separate electrical fault. P0090F indicates an open circuit in the control circuit for fuel pressure regulator 1. Our platform ranked this regulator fault at 85% probability with medium severity.
On a common rail diesel like the OM654, this regulator (often a quantity control valve or metering valve) adjusts the volume of fuel sent to the high-pressure pump to maintain the target rail pressure. An open circuit means the Engine Control Module (ECM) cannot command the regulator. This typically results in a limp-home mode or a crank-no-start condition. The other codes, P30093 and P30083D, are internal codes likely generated as a consequence of this primary fault.
To diagnose this, the recommended steps are measuring the resistance on the fuel pressure regulator and checking the wiring harness to the regulator.
Wiring Harness Damage
The platform also identified damage to the fuel pressure regulator wiring harness as a highly probable cause for P0090F, ranking it at 75%. An open circuit can easily stem from a broken wire, chafing, corrosion at the connector, or poor terminal tension.
Vibration in the Vito chassis is a known factor that can compromise wiring. A fault in the harness interrupts the circuit just like a failed regulator, preventing the ECM from controlling fuel pressure. The diagnostic path here focuses on a thorough inspection of the wiring harness leading to the regulator.
Next Steps for the Workshop
By analyzing the technician's symptom report alongside the specific fault codes, the diagnostic platform separated the mechanical fluid mixing from the electrical fuel pressure issue, providing a clear testing path for both. Have you encountered this combination of failures on the OM654 engine?
Reference pages for the codes in this article
- P0090, what we measure for this code across real diagnostics