IAC Problems


I have been having an odd problem with high idle. What happens is that once I'm driving (engine above 2k rpms), as I am coming to a stop, I push in the clutch, shift the transmission into neutral, and let out on the clutch, and the rpms stay at approx 1750 (data from pocketlogger it varies ~100 rpm with 1750 being the most common). Then I will brake more, and as I slow down and am near a stop, the rpms will suddenly drop to normal (700 rpm). The car is coasting, and does not feel like the clutch is still engaged, however I did check the the clutch pedal engagement height (it was ok), and the throwout bearing arm is moving when the clutch is pressed. Also, I did a flush/bleed of the clutch fluid through the master and slave cylinders. Also, as an interesting note, if I engage the clutch and keep the rpms under 2000, then the high idle doesn't occur when I press in the clutch. Also, if I leave the transmission in gear and slow down with the brakes to low rpms (~1000), it doesn't occur either. More things that I've tried when this is occuring: revving the engine and/or pulling up on the gas pedal seem to have no effect on the rpms; depressing the clutch pedal a few times seems to have no effect either. I have also checked the routing of the accelerator cable, and can find nothing that appears to be catching. Note: if I rev the engine (above 2k rpms or higher) with the transmission in neutral with the car stopped, the high idle never occurs. Other potentially relevant info: I have already had the ECU capacitors replaced recently (a little over a year ago). I was unsure what the cause was, after checking all of the things listed above, but after reading many posts on 3si.org, I began to suspect the IAC. (This part has many names, and is often called the IAC=idle air control motor, or ISC=idle servo control motor.) I did a few quick logs with my pocketlogger and determined the ISC step count is going to 100 when I depress the clutch and put the transmission in neutral. For troubleshooting hints, the service manual states that if the stepper motor step increases to 100 to 120 steps or decreases to 0 step, faulty stepper motor or open circuit in the harness is suspected. I looked at the wiring, and nothing appeared broken or disconnected or otherwise compromised. Then I attempted to clean the IAC. Pics of the cleaning are below.

IAC Cleaning






















Update

After cleaning, the same problem still occurred. Next, I did more research, and found that people who've either had ABS problems, or intentionally disabled the ABS have had the exact same symptoms that I am describing. This also correlates with the info that my ABS light has been on for a while. So I disconnected the plugs on my ABS (under the splash guard in the front side passenger wheel well) and unplugged my pocketlogger, and the idle problem went away. However, when I plug in the pocketlogger, I immediately get the high idle problem, even with the ABS wiring unplugged. For now, I will live with the problem, and leave the pocketlogger unplugged unless I am tuning.



Update: New IAC motor

The idle problems continued, and I began to occassionally have problems with the idle dropping far enough for the engine to die when depressing the clutch. I finally decided to replace the IAC. I purchased one of the "new" version IACs, which is Mitsubishi part number MD628053. The appearance of them is notably different (new style is black, old style is white with a tan top), and the weight of them is notably different (new style is heavier). After replacing the IAC, I adjusted the idle speed, and it appears to have helped. Also, I replaced the base idle set screw (BISS) o-ring. I used a 3/8" OD x 1/4" ID x 1/16" thick o-ring and it was a perfect fit.









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