GP Turbos
Information guide

Turbo losing boost or stuck in limp mode?

A turbo losing boost usually shows up as flat performance, a warning light and limp mode. The turbo often gets the blame first, but leaks, sensors, actuators and blocked exhausts can produce the same symptoms. This guide sets out the checks in a sensible order.

How the engine decides boost is wrong

The engine management constantly compares the boost it has asked for with the boost it actually measures. Underhood Service, a US trade magazine for technicians, describes the two fault codes that result: P0299 for underboost and P0234 for overboost, set when actual boost strays from the desired or commanded value by a specified amount for a specified time. It calls them some of the most common codes on turbocharged vehicles.

The symptoms it lists are a lack of power and entry into a limp, or derated, mode "that puts less stress on the engine and turbocharger", sometimes with surging, a low idle and fuel-trim codes. Limp mode is the engine protecting itself. It is a symptom, not the fault.

Start with the codes and the data

Read every stored code, not only the boost code. Underhood Service says codes for the actuators that control boost must be resolved before moving on to P0234 and P0299. The freeze-frame data saved with a code records when the event happened, with the boost pressure and engine speed at the time, which helps pin down an intermittent fault. Clearing the code and driving on throws that evidence away.

Checks before the turbo

Garrett's and Holset's lists for an engine that lacks power both leave the turbo until last.

STEP 01

Air intake

Check the air filter, hoses and pipes are clean and in good condition. Holset lists a dirty air cleaner and intake restriction first.

STEP 02

Boost leaks

Check every charge-air joint and the intercooler. MAHLE notes that reused, deformed sealing rings and stone-chipped coolers let charge air escape and cost power.

STEP 03

Exhaust

Look for leaks before the turbine, such as a cracked mounting flange or missing gasket, and for restrictions. Garrett's list includes checking the catalyst and diesel particulate filter are not blocked or damaged.

STEP 04

Fuel and engine

Garrett lists the condition and adjustment of the fuel injection system. Holset adds an incorrectly set fuel pump, injectors or valve timing, and burnt valves or pistons.

STEP 05

Boost control

Check the wastegate or vane actuator, its linkage and any pipe that feeds it. BorgWarner lists a wastegate valve that will not close, and a defective pipe to it, among the causes of low boost.

STEP 06

The turbo itself

Only then suspect the turbo. Holset puts a damaged turbocharger at the end of both its lists, for excess smoke and for lack of power.

Boost control faults

Boost is controlled, not just produced. On a wastegate turbo, Garrett explains that a pneumatic actuator may be powered by air pressure or vacuum, while an electric actuator responds to commands from the engine control unit. Underhood Service lists five ways a modern engine controls boost: the electronic throttle, variable cam timing, the wastegate, the compressor bypass (blow-off) valve, and variable-geometry or dual-scroll housings. Each has its own actuator, and each can log its own codes. On variable-geometry turbos, carbon can jam the vanes, which our guide to sticking turbo vanes covers in detail.

When the turbo is the cause

  • Oil starvation. MAHLE explains that when the bearings are short of oil, the rotor can no longer reach full speed, so boost falls and the exhaust can smoke black.
  • Damage and wear. BorgWarner's list of causes of low boost includes foreign body damage to the compressor or turbine, a damaged turbine housing or flap, and turbo bearing damage.
  • Bent vanes. On variable-geometry turbos, MAHLE says vanes bent by debris cause a significant loss of power.

Holset's advice is never to keep operating an engine with a suspect turbo. Our turbo noises guide describes what worn bearings sound like.

Overboost

BorgWarner lists three causes of boost pressure that is too high: a wastegate valve that does not open, a defective or badly adjusted fuel system, and a defective pipe to the wastegate valve. On a variable-geometry turbo, MAHLE says vanes jammed in their low-speed position can overspeed the turbo as the revs rise, and that a defective or leaking boost control can do the same. Our turbo overspeed guide explains the damage that follows.

Rebuild or replace

If the checks point to the turbo, our turbo rebuilds page explains how worn bearings and damaged wheels are dealt with, and turbo diagnostics covers how a boost fault is traced. Whatever the repair, any leak or control fault has to be put right as well.

Boost and limp mode questions

What does P0299 mean?

It is the fault code for underboost: the boost measured is lower than the engine management asked for, by more than a set amount for a set time. It says boost is low, not why, so it is the starting point for the checks above rather than proof of a failed turbo.

Can I keep driving in limp mode?

Limp mode cuts power to protect the engine and turbo. Holset advises never to keep operating an engine with a suspect turbo, so have the codes read and the cause found rather than driving on.

Can a boost leak cause limp mode?

Yes. A leak lowers the boost that reaches the engine, and once the gap between commanded and actual boost is large enough for long enough, an underboost code is set, and limp mode is one of its usual symptoms.

What causes overboost?

BorgWarner lists a wastegate valve that does not open, a defective or badly adjusted fuel system and a defective pipe to the wastegate valve. On variable-geometry turbos, vanes jammed by carbon are another possibility.

Sources

Quotations and figures on this page come from these published sources.

Down on boost and ruled out the leaks and sensors? Talk to GP Turbos in Scunthorpe about whether the turbo needs rebuilding.

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