What does it mean?
SPECIAL NOTES: Code P0234 is solely concerned with boost control issues on OEM turbochargers, and therefore this guide does NOT apply to stock applications that employ superchargers, which is an entirely different technology that requires boost control techniques and mechanisms that bear no relation to the boost control methods used on turbochargers.
Superchargers are also relatively rare on stock applications, being used mostly on Mercedes-Benz products and a few other imported European applications. END OF SPECIAL NOTES.
OBD II fault code P0234 is a generic trouble code that is defined as “Engine boost condition -limit exceeded”, and is set when the PCM (Powertrain Control Module) detects a level of boost pressure being delivered to the engine by a forced-induction device that matches, or exceeds the maximum boost pressure limit set by the manufacturer for that application.
Forced induction devices in the form of turbochargers are used by engine manufacturers to increase the performance of their engines by forcing compressed air into the inlet tract, and from there into the cylinders. The rationale behind the technology is the fact that more air can be mixed with more fuel, while still maintaining an air/fuel mixture that is close to the stoichiometric point for the fuel used on that application.
For instance, the stoichiometric ratio for gasoline is 14.7 parts of air to one part of fuel; at this ratio, all the fuel is combusted using all the available air. NOTE: For diesel engines, the issue is a little more complicated.
Since these engines are not throttled and almost always run with excess air, the ideal air/fuel ratio can vary from anywhere between about 14.6 parts of air to one part of fuel, to as much as 40 parts (or more) of air to one part of fuel, depending on the application, as well as the engine speed and load. However, even on stock applications that are designed for forced induction, the technology places extreme loads and stresses on engines.
Thus, to increase engine life car makers use devices known as “waste gates” to dump, or relieve excess drive pressure both as a means to extend engine life, and to strike a balance between the increased power delivery, and the overall durability, reliability, and running/maintenance costs of their engines. To accomplish this, most stock turbochargers are fitted with internal waste gates ( aka “dump valves”) to reduce the drive pressure, and therefore the speed of the turbine wheel.
In practice, turbochargers are driven by the exhaust gas that exits the engine, hence the term “drive pressure”. The exhaust gas drives a turbine wheel, which in its turn, drives a compressor wheel that is connected to the turbine wheel via a shaft that passes through the internal wall that divides the turbocharger assembly into two halves.
The compressor wheel is fed with air through the inlet ducting that starts at the air filter box: the intake air is then compressed by the rapidly spinning compressor wheel before being fed to the engine through the inlet manifold, sometimes passing through an intercooler on the way to the engine to reduce the compressed air’s temperature. NOTE : Since compressed air gains heat in the process of being compressed, it expands, which reduces the volume of air that is available to the engine.
Cooling the air by passing it through a heat exchanger ( aka “intercooler”) causes the air to contract, which increases its density, which means more cool air can be squeezed into the same volume.
As a practical matter though, the boost level that the turbocharger ultimately delivers to the engine depends on the design and diameter of the turbine and compressor wheels, the volume, flow rate, and pressure of the exhaust gas that drives the turbine wheel, the length and volume of both the inlet ducting and exhaust systems, as well as on whether or not the compressed air is cooled before being fed into the engine. If car engines always ran at constant speeds, forced induction