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Lotus Service Notes Section EMN
Page 14
EMN.11 - IDLE AIR CONTROL (IAC) VALVE
The idle air control (IAC) valve is mounted on the intake plenum, and controls an air passage which by-
passes the throttle valve. When the pintle of the valve is fully extended, the passage is closed off for a
minimum idle speed, but as the ECM commands the IAC stepper motor to withdraw the pintle, a progressively
greater amount of air is allowed to by-pass the throttle. In this way, the ECM is able to control engine idle speed
independently of the throttle butterfly.
NOTE: The closed setting of the throttle valve is preset during manufacture, and must NOT be ad-
justed.
During cold starts, the IAC is opened to provide a raised idle speed, dependent on coolant temperature,
with a gradual decay as the engine warms to normal operating temperature. The ECM also uses the fast
response of ignition timing variation to maintain idle stabilisation, such that at idle, the ignition timing will be
constantly changing. When loads are placed onto or removed from the engine, the ECM senses the change in
engine speed and uses both ignition timing and IAC position to control idle speed.
The position of the idle air control valve should be within the range of 20 to 40 steps (as displayed by a
'Lotus Check' scanner) when the engine is idling at normal engine temperature. This ensures that the idle ari
control valve is able to supply varying amounts of by-pass air to compensate for all loads and temperature
conditions. If outside of this range, there is likely to be a mechanical fault, such as an intake manifold air leak.
Using the 'Lotus Check' scanner, 'IDLE HOTDBPOS' indicates the number of IAC valve steps from fully
closed (0) which the ECM has learned as the position required to maintain the specified hot idle speed. 'STEPS'
shows the position of the IAC valve as calculated by the ECM based on the steps moved since the last ignition
switch off. This value will normally be changing during idle as the ECM continuously monitors idle speed.
When the ignition is switched off, the ECM will keep the main relay energised for a short period whilst the IAC
valve is driven to its power down position, ready for the next engine start.
The IAC valve position may be reset using the 'Lotus Check' tool, or, with the ignition on and mobilised,
by pressing the throttle fully and slowly 5 times.
To remove the valve, unplug the harness connector and remove the two Torx screws securing the valve
body to the plenum adaptor. Withdraw the valve and discard the 'O' ring.
To fit the valve:
- Ensure the mating faces of the plenum adaptor and valve are clean.
- Lubricate a new 'O' ring with silicone grease, and fit to the valve.
- Fit the valve into the adaptor, and tighten the two Torx screws to 1.5 Nm.
- Mate the electrical connector plug.
Idle Air Control
(IAC) valve
Hose to intake
side of throttle
plate
Intake plenum chamber
em200