Have only had this happen a few times on my '97 Legacy 2.2L with 200,000 Km on it. It has only happened after the car has sat for 30-45 minutes after running and up to temperature. It has only happened a handful of times and does not do it everything. The idle will fluctuate from 700 to 1200 rpm. A quick flick of the throttle or putting the car into gear will bring idle to normal. Besides that the car runs great aside from "AWD" fuel economy. Recent work done in the past 4 months: timing belt, oem spark plug & wires, oem CTS, oem front o2, air filter, fuel filter, cleaned maf throttle body egr and iac. All work done by me. Only thing I have not checked or adjusted is the tps.

Edited February 14, 201412 yr by tayto

Welcome to the USMB.

 

The idle fluctuation may be related to the air fuel ratio being off a little bit.  The ECU does a lot of the air fuel calcs and look up table stuff from the reading it gets with the coolant temp sensor.  If you have the original in the car, I'd suggest replacing it as preventative measures.  They're not too much; around $25 USD.

What is idle speed when not in gear? Does idle change when put into gear?

CTS has been replaced with a OEM replacement. Idle rpm on the dash shows 500 when in gear. I hooked up my timing light and it showed closer to 700. I trust the timing light vs the gauge in the dash. Sometimes idle increases when shifting to neutral or park, sometimes it stays the same.

Edited February 16, 201412 yr by tayto

Just an FYI, most people incorrectly read the tach on the dash.  The small weighted hash mark just below 1,000 rpm is 750 rpm, while the darker hash just above 0 is 500 rpm.  You can see what I'm talking about in this picture of my tach.

 

Like most factory tachs mine reads incorrectly. This is why I checked it with my timing light which displays engine rpm. It was ~700 rpm when fully warmed up and in gear, right within factory spec.

Cleaning the idle air control valve and throttle body should help.

Make sure all the breather and PCV hoses are in good shape and connected properly, and check over all of the vacuum hoses for for any splits or cracks and loose ends.

I have cleaned the throttle body and iac. All vacuum hoses have been replaced. Pcv has been replaced along with all the hoses cleaned out, they were pretty crusty. I reused the iac and throttle body gaskets, they were not ripped and the thing runs to good at idle to have a vacuum leak IMO. I was also thinking about running some cleaner through the iac while running as described in the shop manual.

 

I do all city miles and drive about 20 minutes one way, I'm getting 18 mpg which I'm not happy about.

Edited February 17, 201412 yr by tayto