Jumpy idle

subaru
legacy

#1

I have a 1994 Legacy wagon with 2.2, 5-spd. and 180k miles. The idle has an inconsistent jumpiness to it. Sometimes it just starts idling at about 1300 rpm, and this can last for a short time, a whole day, or days at a time. There’s no consistency to it in terms of when it will happen. It happens in hot weather and cold, warm engine or cold, you name it. It might not do it for 3 months and then do it regularly for a couple weeks. Then back to nothing. I’ve had a shop look at it, but they said the idle was fine the whole time they had it and that it would need to be acting up WHEN they had it in order to diagnose it. Makes sense, but it’s frustrating. Anyone had this problem on their Subie or have any ideas what it could be? Thanks in advance for the help!


#2

Idle speed is controlled by the Idle Air Control (IAC) valve. If the valve is dirty or sticking (certainly possible at this age and mileage) the idle can fluctuate or be inconsistent. I’m surprised your mechanic didn’t check the valve and make sure it’s clean and working correctly.

A vacuum leak can also cause an uneven idle, but if you had a vacuum leak the erratic idle would be constant. I’d check the IAC system.


#3

I’ll look into that. Thanks for the help!


#4

I would first get the idle air valve and throttle body cleaned.