How do I solve the Apexi Power FC idle sticking problem on a Mazda RX-7 FD with the idle speed control installed?

Solving an idle sticking, high idle, or idle hunting issue on a Mazda RX-7 (FD) equipped with an Apexi Power FC requires dialing in mechanical throttle body adjustments, sensor calibrations, and PFC software parameters.

Difficulty: Moderate

Here is the step-by-step process to correct your idle:

1. Mechanical Throttle Body Checks

Too much airflow bypassing the throttle body is a primary cause of a high or sticking idle.

• Throttle Cable & Linkage: Ensure the throttle linkage has enough slack so the throttle body (TB) is 100% closed when off the pedal. A binding or overly tight throttle cable will cause the idle to stick or drop very slowly ¹.
• Fast Idle Cam: Verify that the hot wax rod unit (fast idle cam) is totally disengaged once the engine is fully warmed up ⁷.
• Air Adjusting Screw: Close the air adjusting (bleed) screw fully clockwise, then back it out about 1/2 turn less than it was previously. If this screw is completely closed and the idle is still high, the main throttle adjusting screw may be turned in too far ⁹.

2. Vacuum Leaks

A massive vacuum leak will introduce unmetered air, causing a severe lean condition and a persistent idle hunt or stall. Ensure all intake and vacuum routing is properly connected and sealed ⁶.

3. Throttle Position Sensor (TPS) Calibration

Check the TPS voltages in the Power FC sensor check screen. If the ECU doesn't recognize that the throttle is fully closed, it will not properly enter idle control mode. Mazda's standards are:

• VTA1: 0.10V to 0.70V (Closed) / 4.2V to 4.6V (Open)
• VTA2: 0.75V to 1.25V (Closed) / 4.8V to 5.0V (Open) Ensure these values are strictly within spec ⁷.

4. Power FC Settings Adjustment

• Fuel Cut Settings (F/C): If your fuel cut parameter is set lower than the desired idle speed, the engine will aggressively hunt. Make sure the F/C - AE setting is higher than the actual desired idle. To rule this out, you can temporarily set all fuel cut (FC) tables to around 2,000 RPM ⁶.
• PIM Volt / Fuel Drowning: If the engine is running too rich, it may drown in fuel in the 1,000 RPM range, struggling to hold an idle and eventually stalling. Using the PFC-Commander or Datalogit, you can lower the PIM Volt in the 1,000 RPM range (e.g., reducing it to 90% or even 70% depending on your setup) to lean it out, allowing the idle to stabilize. Once stable, you can adjust the physical idle screw down to your target RPM ².

5. Idle Self-Learn Procedure

Once your mechanical adjustments, TPS, and base fueling are corrected, execute the Power FC's idle self-learn procedure. This will only work if there are no vacuum leaks.

1. Let the car idle with no electrical load for 10 minutes.
2. Turn on heavy electrical loads (headlights, defroster) and let it idle for 10 minutes.
3. Turn on the A/C and let it idle for another 10 minutes ⁶.