Lost turbo power: DTC P0299 on BMW diesels (N47 and N57)

Your BMW diesel normally pulls hard from low rpm. Today, climbing a grade on the highway with a full load of passengers, it suddenly feels weak. You press the gas pedal further, but the car seems to be held back. The engine warning light comes on, and now and then you hear a hiss under acceleration. The scan result shows P0299.

Many people read this code as a failed turbo. In many cases, though, the turbo is still healthy and the problem is pressurized air leaking before it reaches the engine, or the system that controls the turbo. Replacing the turbo without finding that leak can end with the same code. This article explains what P0299 means, the specifics of the BMW N47 and N57 diesel engines, and a sensible order of checks.

P0299 is a generic OBD-II code with the title "Turbocharger/Supercharger A Underboost Condition", or a boost condition below target on turbo or supercharger A. The engine computer compares the requested boost pressure with the actual pressure from the sensor. If the actual pressure is lower than requested for a certain time and condition, this code is stored.

This code does not point to one component. The cause can be a boost leak, a problem in the turbo control system, a sensor that reads wrong, a restricted exhaust gas flow, or a turbo that really is worn. Many cars also limit engine power after this code appears, so the most noticeable symptom is lost power.

The two BMW diesel engine families most discussed for this era are the N47 and N57. The N47 is a four-cylinder common rail diesel used among others in the 318d and 320d (E90 and F30), 518d and 520d (F10), and X1 (E84). The N57 is a 3.0 inline six-cylinder diesel used among others in the 325d and 330d (E90), 530d (F10), 730d (F01) and X5 xDrive30d (E70).

Both use variable geometry turbos, and some of their versions use two or even three turbos. Both engines are also equipped with an EGR system and a diesel particulate filter (DPF). This means the path between the turbo and the engine is quite long: pressurized air pipes, the intercooler, joints and clamps, and control hoses for the turbo actuator. The longer that path, the more places there are for a leak or a sticking part. The variants sold differ by market and year, so confirm the engine code and number of turbos in your car before following advice specific to one version.

In the United States, BMW ran a recall for a number of diesel models, among others the 328d, 535d, 740Ld, X3 xDrive28d and X5 xDrive35d of certain years, which use an EGR module with an integrated cooler. According to that recall record, the EGR cooler can leak internally so coolant mixes with the diesel engine soot. Those particles can smolder and make the intake manifold melt, with a risk of fire. The first recall was later expanded with a new repair: the EGR cooler is replaced, the intake manifold is inspected and replaced if needed.

That recall record does not discuss code P0299. However, a damaged intake manifold is part of the air path into the engine, and a leak in the air path is the most often named cause of underboost. So on a BMW diesel that is one of those models and has stored P0299, it is worth checking the intake manifold and the recall history. This recall is recorded for the United States market. Cars in other markets may have different service campaigns or none at all, so ask an authorized dealer with your car's VIN.

• Boost leak: a cracked intercooler hose, a loose clamp, a split pressurized air pipe, or a leaking intercooler.
• A turbo actuator, wastegate or boost control solenoid that sticks or fails.
• Vacuum hoses to the turbo control components that leak, come off, or go soft.
• An air filter or intake duct that is clogged or collapsed.
• An exhaust leak before the turbo or a restricted exhaust, so the energy to spin the turbine drops.
• A boost pressure, MAP or MAF sensor that reads wrong.
• A worn turbo, for example shaft play or damaged compressor blades.

1. Read all codes and freeze frame data. Pressure sensor, EGR or DPF codes that are also stored can change the starting point of the inspection.
2. Confirm the engine version. N47 or N57, one turbo or more. Versions with two or three turbos have more pipes, control hoses and valves to check.
3. Check the pressurized air path from the turbo to the intake manifold. Look for cracks, loose clamps, and oil traces on the intercooler hoses and joints, then do a pressure test or smoke test.
4. Check the vacuum hoses and the variable geometry turbo actuator. A control hose that is soft or loose can keep the turbo from producing the requested boost.
5. On models on the EGR cooler recall list, check the recall history and the condition of the intake manifold.
6. Compare requested and actual boost during a controlled road test, then check for restrictions in the exhaust if the intake side is clean.
7. Only then inspect the turbo for shaft play, damaged blades, or oil leaks.

With P0299, the car can generally still be used gently for short distances, but its power can be limited and the problem can get worse quickly. Avoid hard acceleration, towing, and highway merging until the cause is found, because the car may have no power exactly when you need it. If power is lost on the road, turn on the hazard lights and pull over in a safe place.