Sep 042018

1998 Honda Civic

Intermittently stalls

Initial start up car idles 1500 to 1700 open loop all good for about a minute or less. Then idle slowly drops down to 500ish, sometimes lower for a few seconds then back up to 1500.  (Usually does that idle drop just once sometimes twice). Once its been idling for ten minutes or so(temp half way up on gauge) it will not drop lower unless I turn the ignition off and restart the car. Idle goes normal 870ish. Depends on how its feeling that particular moment. Also my LTFT is averaging -10 sometimes higher or a little lower. Seen the STFT get as high as -27 while live scanning. Could the car possibly not be transferring over to closed loop?

The car intermittently stalls. Sometimes during warm up, sometimes while doing 60 mph down the road and sometimes when I arrive to my location as soon as I stop and start doing the routine of parking. I’ve replaced IAC, TPS, TB gasket, IM gasket and plugs/wires. Also bled air from coolant.  I keep throwing a P0420 cause manifold/converter has small crack in it right down the middle into the O2 sensor hole. Any response back with new suggestions would be greatly appreciated.


Hello Matt,

The crack in the converter may be causing an issue if it is the upstream O2 Sensor(A/F Sensor). The idle is adjustable, I will add the procedure below.

Idle Control System

System Description

The idle speed of the engine is controlled by the Idle Air Control (IAC)Valve.
The valve changes the amount of air bypassing into the intake manifold in response to electric current controlled by the ECM/PCM. When the IAC Valve is activated, the valve opens to maintain the proper idle speed.

1. After the engine starts, the IAC valve opens for a certain time. The amount of air is increased to raise the idle speed about 150 – 300 rpm.
2. When the coolant temperature is low, the IAC valve is opened to obtain the proper fast idle speed. The amount of bypassed air is thus controlled in relation to the engine coolant temperature.


1. When the idle speed is out of specification and the scan tool does not indicate Diagnostic Trouble Code (DTC) P0505 or P1508, check the following items:

  •  Adjust the idle speed
  • Starter switch signal
  • Air conditioning signal
  • ALT FR signal
  • Brake switch signal
  • PSP switch signal
  • A/T gear position signal
  • Clutch switch signal
  • Hoses and connections
  • IAC valve and its mounting O-rings

2. When the above items are normal, substitute a known-good IAC valve and readjust the idle speed
If the idle speed still cannot be adjusted to specification (and the scan tool does not indicate DTC P0505 or P1508) after IAC valve replacement, substitute a known-good ECM/PCM and recheck. If symptom goes away, replace the original ECM/PCM.

How to Adjust Idle Speed

(best viewed with Google Chrome)

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