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Open the floodgates Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Thu, 14 Feb 2002 14:24:35
In Reply to: 9000 turbo not achieving full boost pressure, Mark Stoner, Thu, 14 Feb 2002 12:51:19 Members do not see ads below this line. - Help Keep This Site Online - Signup |
As in, a flood of possibilites.
Check back through the archives - not being able to get it all the way up (the boost gauge, that is) is a frequent item of discussion. Search especially for 'base boost'.
Some background - normally you get 'full' boost - right to the edge of the yellow/red. This level is controlled by the wastegate actuator - a piston and a spring, connected by a hose to the intake manifold. The higher the pressure, the more the piston moves against the spring. The other end of the piston is connected to the turbo - move the piston enough, and the turbo vanes get bypassed, limiting boost.
Next is 'base boost'. Base boost is less than full - typically about 1/2 into the yellow. This is controlled by the APC solenoid (also known as the BPC) mounted on your radiator - it's the little plastic cylinder with an electrical connector and three hoses. If the solenoid has NO power to it, it routes ALL of the pressure from the intake manifold to the wastegate actuator. This causes the actuator to move and limit boost to about 1/2 into the yellow. If the solenoid is ENERGIZED, it passes some of that pressure back into the intake before the turbo - this essentially tells the wastegate actuator that the pressure is less than it really is, so it doesn't limit boost, and the needle goes into the red.
The APC solenoid is controlled by the APC box. It will knock you back to base boost if any of the following things happen:
1- it senses knock
2 - it thinks cruise control is engaged
3 - the brake light switch is pressed (brake lights on).
With item 1, the APC actually modulates the solenoid to just keep you out of knock. So first thing to do is determine if you're at base boost, or in knock limiting.
Find a nice, long hill, some place you can get as much boost as she'll give. Charge up the hill, and when the needle is as far up as it'll go, LIGHTLY press the brake pedal - you don't need to apply the brakes, just get the brake lights on. Watch the boost gauge - does it drop down to middle yellow, or does it stay where it is?
If it drops down to middle yellow, then you know you aren't being limited to base boost. If it stays right where it is, then you have a base boost problem.
If the needle drops some when you press the brake pedal, it isn't base boost. What can it be? First off, the engine could be knocking. If you aren't already running high octane fuel (93 in the US), try a tankful - see if that helps. If it does, then the APC is detecting knock and reducing your boost. Check stuff like plug condition, look for vacuum leaks, check your timing - all stuff that causes knock. Or your engine may just need premium gas to get full boost.
Another thing could be that the wastegate actuator is sticky. Make sure it moves freely. Also, look for leaks in the air hoses between the turbo and the intake, and on the intake - perhaps you're blowing some of your pressure out a leak.
If you're at base boost, there's a lot there - check the archives. It could be a bad APC solenoid, wiring problem, ..... Much more than I can put into a post. But if you know it's base boost, between the archives and various websites, there is a LOT you can do.
Good luck!
posted by 140.157....
, Thu, 14 Feb 2002 14:24:35 <-- Viewing This Message, Fri, 15 Feb 2002 11:58:09
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