A
AJ4
Guest
There are loads of different things that can cause high or uneven idle -
1 : IAS Idle Air Screw. Its part of the IAA ( Idle Air Adjust unit which sits under the right hand side of the plenum chamber ). Used for setting the base idle speed. Is a slotted screw on the side of the IAA and can be seen under the plenum chamber.
2 : AAC Auxiliary Air Control valve. Also part of the IAA unit, this is the main solenoid that the ECU uses ( in conjunction with the Throttle Position Sensor ) to regulate the idle speed. Looks like a round tube fixed onto the IAA unit.
3 : FICD Fast Idle Control Device. Also part of the IAA unit, designed to raise the idle speed whenever the engine comes under heavy load, ie, when aircon is switched on etc. Looks like a small round tube with hex shaped sides, screws into the IAA right next the AAC.
4 : IAR Idle Air Regulator. Sits underneath plenum, works like an automatic choke ( but just for air ). Always switches on whenever you start the car and works on temperature, cold means open ( higher idle ) and gradually closes when intake heats up ( 3 minutes or so ? ). It has a heater inside so will always come on whether the engine is hot or cold and gradually close over the 3 minutes or so. Looks like a small cone with a disc stuck on the end with a pipe on each side.
5 : Worn throttle butterflies / throttle housing. Air leaking past butterflies or leaking in through the spindles.
6 : Throttle balance screws. One screw on each intake, used to balance the four butterlies at idle, can cause high idle if all are adjusted too far out. Can be found on top of the throttle body next to the injectors.
7 : Vacuum ports. One port on each intake that is used with a vacuum guage for balancing the throttle bodies ( see #6. ) Should have rubber boots blanking them off but have been known to disappear... Found next to the throttle balance screws on top of the throttle housing.
8: Igniton too far advanced at idle. Should be 20' +/- 2' for a standard car.
9 : Throttle butterfly stops incorrectly adjusted. Stops the butterflies from closing properly. One screw for each pair of butterflies, usually sealed in place and a right beach to adjust
10 : Accelerator pedal / cable not adjusted properly. Butterflies not closing fully, similar to #9.
11 : TPS Throttle Position Sensor fault or maladjusted. Gives the ECU wrong indication of whether the engine should be idling or not, see also # 2.
12 : Blow off valve leaking. Allowing air to blow past diaphram under vacuum.
13 : Air leak from any of the 3 billion pipes that connect everything above. The pipes are all rubber and do perish over time....
14 : Inlet manifold gasket or Plenum gasket leak.
15 : Faulty Lambda sensor. Used to set the mixture to approx 14.7:1 at low engine speeds / loads. Can cause mixture problems and uneven idle. Can be found on turbo exhaust ( dump ) pipe, bottom left hand side of the engine.
16 : Aftermarket fuel controller set incorrectly. Can be a problem on Apexi PowerFC ECU's where the 'electronic' base idle is set too low for the actual 'mechanical' setting. Causes the idle to hunt and jump around as it tries to achieve an impossible target.
Feel free to add your own thoughts and discoveries...
Ross.
1 : IAS Idle Air Screw. Its part of the IAA ( Idle Air Adjust unit which sits under the right hand side of the plenum chamber ). Used for setting the base idle speed. Is a slotted screw on the side of the IAA and can be seen under the plenum chamber.
2 : AAC Auxiliary Air Control valve. Also part of the IAA unit, this is the main solenoid that the ECU uses ( in conjunction with the Throttle Position Sensor ) to regulate the idle speed. Looks like a round tube fixed onto the IAA unit.
3 : FICD Fast Idle Control Device. Also part of the IAA unit, designed to raise the idle speed whenever the engine comes under heavy load, ie, when aircon is switched on etc. Looks like a small round tube with hex shaped sides, screws into the IAA right next the AAC.
4 : IAR Idle Air Regulator. Sits underneath plenum, works like an automatic choke ( but just for air ). Always switches on whenever you start the car and works on temperature, cold means open ( higher idle ) and gradually closes when intake heats up ( 3 minutes or so ? ). It has a heater inside so will always come on whether the engine is hot or cold and gradually close over the 3 minutes or so. Looks like a small cone with a disc stuck on the end with a pipe on each side.
5 : Worn throttle butterflies / throttle housing. Air leaking past butterflies or leaking in through the spindles.
6 : Throttle balance screws. One screw on each intake, used to balance the four butterlies at idle, can cause high idle if all are adjusted too far out. Can be found on top of the throttle body next to the injectors.
7 : Vacuum ports. One port on each intake that is used with a vacuum guage for balancing the throttle bodies ( see #6. ) Should have rubber boots blanking them off but have been known to disappear... Found next to the throttle balance screws on top of the throttle housing.
8: Igniton too far advanced at idle. Should be 20' +/- 2' for a standard car.
9 : Throttle butterfly stops incorrectly adjusted. Stops the butterflies from closing properly. One screw for each pair of butterflies, usually sealed in place and a right beach to adjust
10 : Accelerator pedal / cable not adjusted properly. Butterflies not closing fully, similar to #9.
11 : TPS Throttle Position Sensor fault or maladjusted. Gives the ECU wrong indication of whether the engine should be idling or not, see also # 2.
12 : Blow off valve leaking. Allowing air to blow past diaphram under vacuum.
13 : Air leak from any of the 3 billion pipes that connect everything above. The pipes are all rubber and do perish over time....
14 : Inlet manifold gasket or Plenum gasket leak.
15 : Faulty Lambda sensor. Used to set the mixture to approx 14.7:1 at low engine speeds / loads. Can cause mixture problems and uneven idle. Can be found on turbo exhaust ( dump ) pipe, bottom left hand side of the engine.
16 : Aftermarket fuel controller set incorrectly. Can be a problem on Apexi PowerFC ECU's where the 'electronic' base idle is set too low for the actual 'mechanical' setting. Causes the idle to hunt and jump around as it tries to achieve an impossible target.
Feel free to add your own thoughts and discoveries...
Ross.
