Engine Turbocharger/Supercharger Overboost Condition
The PCM has detected a turbocharger or supercharger boost pressure rating that exceeds a predetermined limit.
Code Set Parameters
If the MAP sensor supplies the PCM with a data reading which indicates that turbocharger or supercharger pressure has reached a level that exceeds the manufacturer’s predetermined amount, then a trouble code will be stored and a malfunction indicator lamp will be illuminated.
Symptoms include a loss of power, noises from the engine that resemble those of a terrible spark knock, engine and transmission overheating, engine misfiring, a stored trouble code, and an illuminated service engine soon lamp.
The most common causes include an obstructed or faulty wastegate door mechanism, faulty boost pressure release valve, a broken, torn, or disconnected supply hose from the boost controller to the wastegate. There are also various wastegate door motors, actuator arms, diaphragms, and doors that can fail. The boost pressure release valves are often replaced with high-performance adjustable aftermarket valves. This is done to obtain greater turbocharger boost pressure and more power, however the higher boost readings may cause a P0234 to be stored and a malfunction indicator lamp to be illuminated.
The turbocharger and supercharger are often condemned in error when this code is presented. Wastegate failure is far and away the most common cause of overboost.
- In order to successfully diagnose the forced air induction engine, one must understand a brief overview of the forced air induction system and how overboost is prevented.
Forced air induction is a means of introducing excessive amounts of air into an engine in order to promote gains in horsepower
- Where a naturally aspirated engine utilizes vacuum created by downward piston movement to draw a controlled fuel/air mixture into the engine’s combustion chambers, the forced air induction engine has air and fuel forced into the combustion chambers using an alternately driven device
- Turbochargers and superchargers are simply engine driven air compressors, designed to accomplish this task
- These forced air induction devices are divided into three basic categories: turbochargers, roots type superchargers and centrifugal type superchargers
- Turbochargers use the pressure from engine exhaust to propel impellers in a two chambered housing
- The two chambers are totally separate one from another
- Engine exhaust pressure turns the impeller in chamber “A”, which in turn spins an impeller in chamber “B”
- The impeller in chamber “B” gathers fresh air through the turbocharger intake system (and intercoolers) and forces the cooler, denser air into the engine
- Superchargers (both types) are belt driven devices
- The roots-type supercharger sits on top of the engine and is bolted down in place of the intake manifold
- The centrifugal type supercharger is mounted on the face of the engine in much the same manner as an air conditioning compressor or an alternator
- Unlike the turbocharger, which harnesses engine exhaust for propulsion, the supercharger has one chamber
- Air is drawn into the device, compressed, and forced into engine combustion chambers, using a pair of intertwined independently spinning rotors in the roots type supercharger
- The centrifugal type supercharger utilizes a centrifugal vane type mechanism to draw air into the housing where it is compressed and reintroduced into the engine as cooler, denser air (in excessive amounts)
- Superchargers also use intercoolers to decrease air temperature prior to compressing it and forcing it into the engine
- The cooler that the air temperature can become prior to entering the forced air induction device, the denser it will be when it reaches the combustion chamber
- Denser air allows fuel to atomize more efficiently and promotes increased horsepower.
Obviously, as engine RPM levels rise, forced air induction devices spin faster as well
- The typical turbocharger doesn’t even begin to “spool up” until the engine reaches 1,700 to 2,500 RPMs and can operate at speeds of 250,000 RPMs under full boost pressure
- Extreme RPMs are necessary in order for the device to produce air pressure that is greater than that of the atmosphere
- These elevated air pressure levels are known as “boost pressure”.
As boost pressure rises, engine stress is also elevated
- Each engine manufacturer provides maximum recommended boost pressure specifications which are programmed into the PCM
- These specifications are calculated with the purpose of avoiding catastrophic engine failure due to excessive boost pressure (overboost) in engines that are equipped with factory forced air induction devices
- When the limits of these specifications are breached (on the high side) a code P0234 is stored in the PCM and a service engine soon lamp is illuminated
- When the code is set and the service engine illuminated, the overboost problem should be investigated immediately to prevent catastrophic engine damage from occurring.
To prevent overboost, most modern forced air induction engines use some form of a wastegate valve, or boost pressure release valve, to relieve boost pressure at high RPMs
- The wastegate is typically held in the closed position by a spring-loaded rod attached to the outside of the wastegate door
- As boost pressure increases at the wastegate door, it pushes against the spring-loaded rod until the door is opened and pressure is routed away from the compression device preventing further boost
- BMP sensors, MAP sensors, engine and transmission temperature sensors, and knock sensors are used by the PCM to calculate a safe level of boost that also yields optimum engine performance results
- Solenoids, stepper motors, and pulse modulators are used to effectively open and close the wastegate valve in order to provide the maximum safe level of boost pressure at different RPMs.
A scanner or code reader, a digital volt ohmmeter, and access to a manufacturer’s wiring schematic will be necessary to successfully diagnose this code
Begin your diagnosis with a visual inspection of all wiring and connectors
- Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
- Always retest the system after repairs are completed to ensure success.
If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
- This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
- After the codes are cleared, operate the vehicle to see if the code returns
- If the code fails to immediately return, you may have an intermittent condition
- Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made.
Most overboost malfunctions will be related to the wastegate actuator (rod)
- Always check for proper wastegate door operation first (assuming that no other codes are present besides P0234)
- If the wastegate is stuck closed, an overboost condition will occur
- Next, check for a cracked, broken, disconnected, or clogged vacuum supply hose from the boost controller to the wastegate actuator (if equipped)
- By connecting a hand-held vacuum pump to the wastegate controller, you can test for proper operation of the wastegate door
- Pump the vacuum pump while carefully observing the wastegate door and actuator rod
- Compare the inches of vacuum required to activate the wastegate door (if it opens at all) to the manufacturer’s specifications and repair as necessary.