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Tech info on MP - VNT (turbo control)

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Tuning Technical Information

We are increasingly getting inquiries from customers having problems with standard vehicles. They are being frustrated with New Car Dealers who are not listening to their complaints about their vehicles poor Economy or Performance. Dealers are finding themselves caught in the middle as manufacturers dumb down the test procedures which the dealers are only allowed to carry out. They are only permitted to replace what their Diagnostic Equipment determines, so faulty sensors sending incorrectly calibrated signals, result in the Diagnostic Equipment not seeing the real faults. These Pages are here to give you a better understanding of how your vehicle works & help you diagnose problems you are having with your vehicle.

VNT Actuator

On vehicles with vacuum operated VNT, check that the actuator arm on the side of the Turbo moves when engine rpm is increased. Failure of this system renders the Turbo to operate as only a fixed vane Turbo, it will make boost but very slowly and results in excessive exhaust smoke at low revs. On Nissan 2.5L the vacuum hose runs near the oil filter, and can be damaged when oil filter is replaced. If the vacuum hose or plastic solenoid valve under inlet manifold is damaged, then no vacuum is available to operate the VNT on the side of the Turbo. On Toyota V8 D4D engines the VNT actuator is easily damaged or bent.
There are Tuning Systems which can adjust the VNT, but the easiest way is by manipulating the MAP Sensor.

Dawes Valve

Although these are still used as a tuning tool by many workshops, these shops fail to realize that the vacuum value they are changing also affects the VNT system & MAP Sensor. This causes the MAP Sensor to send an incorrect signal to the engine ECU. A lower vacuum signal results in retarded Injection Timing, & other related problems. Sometimes the issues they are trying to fix is simply a faulty MAP Sensor which is out of calibration or stuck at one voltage.

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