Overboosting? - APC valve? [Archive] - SaabCentral Forums

: Overboosting? - APC valve?

Porsche Guy
2nd November 2002, 03:36 PM
Hey all -

I've been working at bringing my newly acquired '94 Aero up to snuff and have just re-read what the Saab shop had written down on my pre-purchase inspection

"Turbo overboosting - maybe apc valve $234"

What is overboosting and should I be concerned? I've driven a gentle 100 miles so far and have only seen the turbo needle in the beginning of the red 2 times so far.

What do you guys make of the comment and what should I do?

Many thanks, Aaron

2nd November 2002, 05:33 PM
"Turbo overboosting", to me, means that you will hit the overboost fuel cutout. This is a safety device. If the engine management system sees the boost pressure exceeding a certain threshold, it assumes it has lost control of the turbo (i.e. a fault is preventing it from controlling it properly) and it cuts the fuel to avoid any damage that uncontrolled boost might cause. In practice, this always happens under quite hard acceleration and the sudden loss of power and consequent sudden deceleration (amazing how much engine braking you can get with your foot to the floor and no power being generated) feels like someone threw a brick wall in your path.

This happens sometimes as the ECU adapts to new driving conditions and/or different fuel. If you drive the car enthusiastically for a while with your usual fuel and find it hits the cutout a couple of times but then doesn't do it again, I wouldn't worry.

You could also try the Trionic adaptation procedure at http://www.saab9000.com/procedures/powertrain/adaptation.html.

A faulty boost control valve could indeed cause overboost and fuel cutout, but try adapting th ECU first.