Mysterious Idle Surge

[SpawnedX]

Car:

1999 Mitsubishi Eclipse GSX
5 Speed

Problem Description:

Problem started as minor fluctuation 800-1200 range. Problem escalated to a 800 to 1700 idle fluctuation. Then car began to have fast idle, sometimes over 2000 at operating temperature after moderate driving time.

What I Have Done:

Tonight I pulled my ISC motor from the car, checked the resistance and confirmed the ISC was bad. I replaced it with a brand new OEM ISC from Mitsubishi. I replaced the BISS screw, o-ring and BISS screw cap with brand new ones from Mitsubishi. I then did the proper way of adjusting the ISC by going into the Solus and adjusting the BISS, target range is 750 +/- 50 RPMs. I was unable to get it better than 950, may be because these cheap plastic screws that only Mitsubishi would use strip out and I couldn’t safely tighten it down some more (have another screw inbound to try with a larger screw driver). So I tried adjusting the throttle stop on the back of the throttle body, to no avail, that got me to right around 1000 RPMs. Called it a night since I had to close up and leave.

The Problem Worsens:

Pulled the car out of the garage, the moment I got on the throttle and then released it, fluctuation came back immediately upwards of 1500 RPMs. After the highway drive home, car didn’t want to drop below 2000 RPMs, sometimes saw upwards of 2500, and say I was at 3500 RPMs, the car did not want to drop to 2500, it took quite awhile to idle that down.

I am lost. I have no idea what else to check. No codes are thrown.

Topic

[SpawnedX]

Ended up being a bad FIAV in the throttle body.

[dreamer2355]

Glad you have it fixed! How did you go about diagnosing it?

[SpawnedX]

By replacing the FIAV portion on the throttle body with a known good one.

[Author]

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