Poor boost is often a symptom of other problems. It is important to know why the boost is low before attempting corrections.
The DSM ECUs have partial control over the amount of turbo boost through the use of the boost control solenoid (BCS) a.k.a. thewastegate bypass solenoid. Under normal driving conditions, the BCS is open and allows the wastegate to open at the normal intake pressure. Should the ECU detect a serious problem with the engine, it will often close the BCS, causing the wastegate to open sooner and lowering the turbo boost produced.
The ECU uses the BCS to reduce turbo boost in several situations. Should the ECU detect a large amount of airflow into the engine, the BCS will be pulsed off and on to reduce the air intake to acceptable levels. This can lower the turbo boost significantly, and usually only occurs at high RPMs. This is usually a temporary problem, which disappears when the intake airflow drops to more normal levels.
The ECU will also close the BCS if it detects significant engine knock. Knock, also known as preignition or detonation, is a damaging condition brought on by excessively advanced ignition timing, lean air-fuel mixtures and/or low octane or poor quality gasoline. The BCS is the second and last line of defense against knock - the ECU will first retard the ignition timing in an attempt to prevent knock. If this fails, the BCS will close to reduce the intake air flow (and boost pressure) to a minimum value, hopefully eliminating the knock at the expense of engine power.
A simple LED monitor circuit can be constructed to check the operation of the BCS. If the BCS is pulsing, or remains closed during typical engine operation, it means that you may have some other problem that is making the ECU very nervous. This is often accompanied by retarded engine timing, resulting in a further power loss, all of which makes the car much slower than it should be. Note that the operation of the BCS monitor is not necessarily intuitive - study the Troubleshooting section, this issue of the Diagnostic Port, and the Boost Solenoid Details page very carefully before deciding you have a problem.
If the BCS is not operating as expected, suspects include poor quality gas, excessive turbo pressures, injector malfunctions, oxygen sensor malfunctions, and anything else that can lead to a low-octane, air-rich mixture inside the cylinders. Differences in mass airflow sensors from car to car will also affect the operation of the BCS.
If the BCS is fine, the timing may still be retarded due to airflow or knock problems. If you are certain this is not the case, it may be time for some modifications; see "What should I do to make my car faster, or handle better?" , above.
If you are certain this information does not relate to your problem, check your intercooler hoses. Often a hose has popped off, or is leaking air badly. Another spot to check is the wastegate actuator; make sure the actuator rod is still connected to the wastegate door. Otherwise, the wastegate may be flopping open and letting out all the boost air. Sometimes the arm has broken off the wastegate due to a rusted-out holding pin.
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