Overboost
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Overboost in simple terms is where the boost pressure produced by the Turbo exceeds a predefined maximum. Once the limit is reached the ECU cuts all power to the engine for a fraction of a second. The result of this when accelerating hard in second gear feels like running into a brick wall. |
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Or a more detailed explanation is: The wastegate is held shut so that boost is generated and when the desired boost level is achieved the wastegate is then opened. However this is mechanical stuff controlled by electronic stuff - electronics can react quickly whereas mechanics can not. |
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What you find is that when the wastegate is opened the turbo is still accelerating, so for a moment the boost continues to rise until the turbo speed falls. This moment of higher-than-desired boost is overboost. Complex boost controllers try to "guess" when the desired boost level will be reached and then open the wastegate in time, so as not to overshoot it. |
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Causes of overboot include the T piece clogging up with oil. Also if the orifice (brass hole restrictor) is placed in the wrong line, it will also overboost. Depending on which model it is, it should either be in the line from the turbo to the T-piece or the pipe from the solenoid on the right side of the struct tower to the inlet manifold. |
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97 98 cars seem to suffer this the most. The hoses from the wastegate can be cleaned and the problem still comes back. Also dealers have removed the restrictor that is in the line to the wastegate and fitted a larger holed one, this lets the car boost to a lower level. Problem solved. Some dealers are drilling the hole slightly larger. Beware they do not drill the hole to large or your boost will suffer. |