Just to confirm, here is a list of potentials...
Causes1 ) Warped plastic Throttle Body
2 ) Gunged up Idle Air Control Valve (IACV or Stepper Motor)
3 ) Broken IACV
4 ) Sticking throttle cable
5 ) Mounting screws to the potentiometer (throttle position sensor TPS) over tightened
6 ) Dirty or corroded contacts in the multiplugs to the TPS or IACV
or, my favourite...
7 ) Loose footwell mat jamming up the pedal!
Lastly...
8 ) Poor throttle body calibration.
Solutions1 ) Has been dealt with as you have an aluminium TB
2 ) Very very likely the cause, unclip the multiplug, use a T20 Torx bit and remove the 2 retaining screws (one above, one below - don't drop the lower one! - Unplug the hoses from the head to gain access), separate the 2 halves and use some Greygate or similar (plastic polish) to clean the gunk out from the female part and also the muck from the male part.
3 ) Whilst it is apart, plug in the multiplug and turn on the ignition - the metal tip should vibrate in and out (opening and closing the air way).
4 ) Unlikely, but it is worth disconnecting the cable from the throttle body and then feeling the cable as someone depresses the accelerator to check for a sticking cable, or lack of lubrication. If it feels 'gritty' then the cable is likely to be the cause (they can be lubricated but it is better to simply replace it - if it snaps then the car is imobilised) Likewise if it feels stiff then the cable is likely to be kinked somewhere - replacement is the way to go.
5 ) The TPS is mounted on the back of the throttle body with 2 screws. If these screws are overtightened they project too far out the back of the sensor and foul the return spring on the throttle body causing it to stick occasionally at high revs. Slacken the screws a little to cure.
6 ) Unplug them and use some contact cleaner.
7 ) Check your drivers footwell mat - is is secured properly? If not, is there wear on the top edge suggesting that it has 'walked' forwards and become trapped under the pedals? Secure it as this is potentially dangerous and could for instance jam the brake pedal - eeek!
8 ) Take the car to a dealer and ask them to calibrate the throttle body. This is done in around 20 minutes (most of which is waiting for the engine to warm up) and needs Testbook software (they plug the car in, run it up to temperature and then use Testbook to measure the air flow in the bypass circuit. They then simply wind the adjustment screw in/out until the air flow is in line with what the MEMS expects). Don't let them charge you a fortune for this, it is very simple and is less than an hour's labour.
NotesThe throttle body is a simple tube with a circular flap (or butterfly) inside it mounted on a horizontal spindle. When you accelerate the cable pulls on a cam that rotates the spindle and butterfly, opening the air way and allowing more air into the engine. A warped throttle body interferes with this process causing the high idle, as does projecting screws from the TPS and a sticky cable or jammed pedal. These are the 'mechanical' causes.
When the car is idling there is no pressure on the accelerator, therefore the butterfly is completely closed. However the engine needs some air in order to maintain the idle, so there is a bypass circuit built into the throttle body and the air flow in this is controlled by the MEMS, operating the IACV within set limits (limits are set by the adjustment screw). Sadly you get carbonisation within the IACV which can make the operation erratic. Also, if the signals are not getting back to the MEMS properly then it can leave the IACV open more than it should causing a high idle. Lastly, if the throttle body hasn't been calibrated and the adjustment screw is set a long way out, then it will flow more air than the MEMS exects, also causing a high idle (or sometimes a poor / lumpy idle if set too far out the other way). To be honest, the last one is pretty rare - the MEMS is pretty accomodating and the adjustment screw has to be set to either extreme to really be a problem.
This picture shows the IACV and the TPS. The IACV is the barrel shaped object immediately behind the oil filler cap on the left of the shot. The TPS is on the right hand side of the shot, mounted on the back of the throttle body (with a white circle on the back), below the breather hose.
Hope this helps (and maybe answers a few questions)
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