I pulled this from a member on another forum for you to look at.
the fc idle system at first glace seems complex and pointless but is actually very simple.
the parts:
throttle body, the throttle valves that are controlled by your foot.
double throttle system, on the na its simply hooked up to a thermoswitch that closes them when the thermowax is not up to temp.
the turbo does this too, but they additionanlly use the double throttle as some sort of damper.
thermowax: wax pellet in coolant, it literally bumps the throttle open cold and gradually lowers rpms as it gets warm
bac valve: meters air around the throttle body to raise engine speed
accelerated warmup system: like the bac, but its bigger, revs engine to 3000ish rpms on a cold start
air supply valve: same as bac, but this one kicks up the idle 100-200rpms when the ac is on, or you move the wheel in a ps car
tps: tells the ecu the throttle position
ecu: from the rpm, and tps, and afm it needs to calculate the bac duty, and the aws/air supply conditions, so it can deliver a good idle
afm: meters all of the total air in the system
how it works:
when you start the car cold, the ecu will open the aws and dump in a bunch of fuel. the car starts and revs up to 3000rpms for 30seconds, unless the throttle is blipped or the car is put into gear. the idle will come down to 1500-2000 rpms because the thermowax is pushing the throttle open slightly, and the double throttle should be closed. as the car warms up the thermowax also warms up, and starts lowering the idle until it eventually pops off (it is adjustable both for how long it stays on, and how high the rpms are). the double throttle opens here too. once the idle drops to spec (750 +/-50) the bac valve will take over, its job is to keep a steady 750rpm idle. they do this by duty cycling a solenoid (like a fuel injector) so it passes the correct amount of air. if you turn on the ac, the idle will kick up to 1000-1100 rpms due to the air supply valve turning on. its job is to keep the engine running under large loads like ac, or using the power steering at idle.
problems:
throttlebody: occasionally they will stick, this will keep the idle too high. also on the na's if you adjust the throttle stop for the secondaries and they hang open it will cause a large 1000-2000rpm surging
double throttle: on the na's failure leaves it open, its also possible to have a vacuum leak at the double throttle diaphram
thermowax: since this is in coolant its got a couple ways to be wrong.
if there's an air bubble or a blocked passage it can be stuck on, and idle high. it can also stick partway on intermittantly, and result in an intermittant high idle. or it can be bad and not come on at all. it is adjustable so if it sticks on this is usually fixable
bac valve: i have yet to see a dirty one on an fc, but theoretically they can get dirty and stick and you will get either a high or a low idle.
also on the turbo the hose from the bac to the intercooler pipe can split and cause a low idle due to the vacuum leak. the last failure mode is just plain failure, this will give you a low idle (500-600)
aws: the valve can fail stick shut, nobody will miss this if it doesn't work.
air supply valve: this is the same as the bac valve, only it wont give you a low idle, it will idle low/stall with the ac on, or when you move the steering wheel on a ps car
tps: causes all sorts of weirdness, its very important to the output of the ecu
afm: if there are vacuum leaks it will cause a low idle. the afm meters all of the air, if there's more coming in from a vacuum leak, it will be lean and idle low if at all
adjustments:
s4 na: on the top of the throttle body there is an idle speed screw. and by the airbox there is the idle mixture screw. the only important thing about the idle mixture screw is that if its fully rich or lean and you have a working air pump/acv then there is something else wrong (vacuum leaks)
s4 t2: the idle speed screw is on the bac valve on the right side of the intake above the turbo. idle mixture is like the na
s5's: same as the s4 except they don't have the idle mixture adjustment
As with any diagnostic routine, don't rule out other causes of rough idle, such as:
Low/ uneven compression
Fouled spark plugs/ arcing plug wires
Incorrect spark timing
Fuel pressure/ volume/ pump voltage/ regulator internal leaks
Poor grounds/ harness plugs
Loose intake boots/ clamps
Vacuum leaks
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