idle high while wheels are moving, low idle when below 1mph

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2 months 2 weeks ago #201813 by jayw
1998 Honda Civic DX 5-speed Manual Hatchback

The Problem:
  • The IACV is opening up while the wheels are rolling, and closing when the wheels have stopped moving.
  • The idle range depends on the engine temperature when the car is started (cold start = higher overall idles, hot engine start = low idle range, causing the engine to shake while at a complete stop)

  • Evidence:
  • Rolling the car out of the driveway in neutral -> once above 1mph, the idle goes up
  • Idle high while moving -> rolling to a complete stop without any braking -> idle goes down
  • while moving, car accelerates in gear without pressing gas pedal (indicating IACV is opening up whether in gear or not)
  • clutch in while moving -> idle stays high

Things I have done to try to solve it:
  1. Bled the coolant
  2. replaced the thermostat
  3. replaced the IACV
  4. checked for vacuum leaks

Other things:
No check engine light.
When I unplug the IACV while the car is running, idle shoots up. Plug back in, idle goes back down. This causes the check engine light to come on. While the check engine light is on (from replugging the IACV) the idle performs normally (does not change depending on wheel speed). When I reconnect the battery to get rid of the check engine light, the problem returns.
Engine temps are normal.

Something is sending the wrong voltages to the IACV, depending on whether the wheels are moving or not. From what I have researched, wheel speed does not influence the voltages going to the IACV, if I'm wrong someone please correct me.

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