Idle Speed Control Valve (ISCV)
1800 RS/GSR, Evo 1-7
Unstable idle and possible stalling or higher than normal idle speed especially when the engine loading changes (e.g. when fans or A/C comes on).
Problem and cure:
Check to make sure the connection to the ISCV is secure. Remove the connector and with the ignition on
measure the voltage between pin 2 (top centre) and earth on the harness side of the connector. Do the same with
with pin 5 (bottom centre). Both these pins should be at system voltage. If they are not then there is a problem
with the harness between the connector and the Engine Control Relay. If system voltage is present then the next
step is to check the ISCV stepper motor coils. Measure the resistance between pins 1&2, 2&3, 4&5 and 5&6 on the ISCV body connector (see figure 2).
All these should measure between 28 and 33 Ohms, if any of the coils do not fall between these values then the ISCV needs to be replaced.
Unstable idle is possibly due to deteroriation of the Idle Speed Control Valve stepper motor. The motor controls a valve that allows air
around the throttle butterfly to maintain a steady engine speed at idle, if the motor does not work correctly then the idle speed cannot be maintained.
Note that if unstable idle occurs and
the car has been fitted with a vent to air blow off valve then this is the likely to be the problem.
There a few checks you can make before condemning the ISCV (the checks below apply to all
car models mentioned). The ISCV is situated underneath the throttle body (see figure 1) with an electrical connector on the right hand side.
Approximate cost of repair:
New ISCV for E4-7 = £210 + VAT & fitting
New ISCV for 1800GSR & E1-E3 = £164 + VAT & fitting
Second hand or reconditioned ISCV's are also available, contact [email protected] for details.
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