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Re: apc issues....HELP! Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Fri, 7 Nov 2008 22:06:40
In Reply to: apc issues....HELP!, eric, Fri, 7 Nov 2008 15:18:58 Members do not see ads below this line. - Help Keep This Site Online - Signup |
First off, it is normal for the valve to buzz all the time. The early cars had a APC solenoid that was either on or off - 2 wires. Your car has an Boost Pressure Control Valve (BPC) with a proportional control. It has two coils. The ECU drives electrical pulses to both coils. When the pulses to each coil are the same, the valve is in the center; make the pulses wider to one coil, and it pulls harder, and the valve moves in that direction.
The valve is always getting pulses, so it always buzzes - not very loudly, but it should buzz. Below 2500 rpm the valve gets 90 hz pulses; above 2500 rpm the frequency drops to 70 hz. The change in frequency is to prevent resonances in the air hoses.
So a valve that doesn't buzz is a bad sign. The ECU drives the coils with transistor switches to ground. A bad BPC valve can burn out one of those transistors; if that happens, only one coil is energized. You can replace BPCs 'till the cows come home and the ECU won't heal. That might not be the issue.
Can you be more specific about the boost issue, and the 'hesitation.' The engine should boost to the end of the yellow. A good way to test it is to find a nice uphill where you can put a good load on the engine and keep your foot in it. If sometimes you get less than full boost, is it under load (going uphill, versus a throttle stomp on a flat road)? When the engine is warm? What fuel are you using - does the problem go away with a tank of premium?
Do you have NGK spark plugs per the manual, gapped properly (just under 1 mm or 0.040 inches)? How do the plugs look - does it look like they're running lean (whitish) or a normal grey?
The BPC reduces boost if it detects knock. It detects knock using the spark plugs and the DI. The wrong spark plugs can mess up knock detection. Incorrect spark plug gap - even a little - can cause the engine to misfire under boost. And the engine could be knocking; obviously, low octane fuel can cause knock. So can running lean due to vacuum leaks or a weak FPR. Running too hot a spark plug, or just carbon buildup over the years in the cylinders.
You mentioned hesitation - by that do you mean the lack of boost, or does the engine feel like it is misfiring? Or does it stumble accelerating off the line at lower RPM?
posted by 76.231.24...
, Fri, 7 Nov 2008 22:06:40 <-- Viewing This Message
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