Tech Tip No. 6
Originally published in the quattro quarterly
LOSS OF BOOST PRESSURE, Part II
(See Tech Tip No. 5 for physical & mechanical reasons)
While the wastegate allows use of a larger turbo, it keeps the
boost pressure from becoming excessive by wasting or dumping some of the exhaust gas away from the turbo. What is excessive? When you hear the knocking, the pinging or the detonation that destroys the engine it may
already be too late.
In the early 80's boost control was simply the spring in the
wastegate. If the spring was made stronger there was more power, but engines could be damaged; therefore, a safety margin was allowed, and a spring was selected for engine durability not maximum horsepower.
In the late 80's engine computers were used to squeeze more safe
power from the engine. The computer controls the wastegate through a "Wastegate Frequency Valve". Of course when it is not safe to have the extra boost "the little man in the computer who runs things
pulls the plug on this control valve" and the boost pressure falls to minimum. Your loss of boost power in these instances is just the computer protecting the engine.
If you think things are O.K., but you loose power, what
happened? What do you need to check? Usually the boost control frequency valve is only receiving the "save the engine" signals from the computer.
THINGS TO CHECK:
Water temperature sensor
If the sensor is defective the computer will hold the boost at
minimum. The computer is programmed to do this when the water temperature exceeds 245 degrees. If high boost was allowed while the engine was this hot the engine could be damaged. Also, if the signal from the sensor
is bad or defective the computer reacts the same as if the engine is too hot. On some engines this sensor is part of the "Multi Function Temperature Sensor" on others it is a separate sensor.
Full throttle switch or Throttle valve potentiometer
If the switch is not closed only minimum power is allowed. On some
engines it is not a switch but a potentiometer but the same thinking applies. If the switch or potentiometer is defective only minimum power is allowed.
Low octane fuel will cause detonation, but this is detected by the
knock sensor system and boost is reduced. If you do not use premium fuel the engine may detonate so once again the computer keeps the power at minimum. Depending on your engine even putting in good fuel will not
immediately bring back the power. The newer computers have what is called "Adaptive charge pressure control". These computers remember what fuel you've been using and only gradually allows maximum power to
return after you put in good fuel. This may take up to 100 miles or 10 to 30 starts of the engine!
One job of the altitude sensor is to reduce boost pressure as the
altitude increases. As the air becomes less dense the turbo can spin with less restriction. This reduction in boost pressure is to avoid over-revving the turbo. If the altitude sensor is defective your power goes
Intake Air Temperature Sensor
As intake temperature increases, the boost pressure is reduced to
prevent detonation. If the sensor fails boost pressure is reduced.
One purpose of the bypass valve is to reduce boost pressure in the
air duct when the throttle is closed. Guess what happens when it sticks open. Reduced boost pressure all the time!
Boost Control Frequency Valve
This is the little item that controls the extra boost pressure. If
it is defective, you don't have the extra boost.
Not every computer and engine combination has all these controls
and sensors. If you don't know what you have, it is best to leave troubleshooting to a professional, or take your Audi to a dealer who has a mechanic who was trained on your model Audi.