My 1999 Honda Civic EX has been throwing intermittent p0505 codes for the idle control system. When the problem comes up, I just barely depress the pedal, and I get surging (around 1500 rpm), but if I press it more than 15% there's no problems. I have checked for vacuum leaks, replaced the idle air control valve, cleaned the throttle body, flushed the coolant and replaced it. I cleaned the throttle body when it was surging really bad, and when I started it again there was not problems for over a week. Yesterday the surging came back, with the ECU throwing another p0505 code. I felt the IAC unit and it was burning hot to the touch, way hotter than the cylinder head. Is it possible that such heat could affect the resistance inside the solenoid and give incorrect signals back to the ECU? This problem ONLY happens when the engine is warm. If so, does anyone know if bypassing the coolant lines from the IAC valve would be a reasonable solution?

  • "coolant lines from the IAC" no such thing on a Honda, some cars have coolant lines that run to the egr, bad idea to bypass those. – Moab Nov 15 '19 at 15:47

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