My old 89' 2.2 5MT Legacy has developed a bit of a strange, yet minor problem. It starts cold no problem 99% of the time, but once it's warmed up and I, say, turn it off to go into a store and buy something, come out and fire it up, it seems to hunt for idle quite bad for about ten seconds.

 

Idle jumps to around 1300 RPM, then down to about 450 - 500 RPM, then back up to 1100 RPM, then to 600, then 900, then usually settles itself down to a smooth, steady 750 or 800 RPM.

 

I've had a similar problem on a DOHC EJ20 Legacy a couple of years back, after a nasty timing belt accident and a replacement engine, and I'm pretty sure it had something to do with the IAC. But I'm a little dubious about pulling it off the engine when it's not really a huge drivability issue, more just an annoyance.

 

Is this a common sort of symptom for a carbon-clogged or failing IAC? Or am I missing something else?

Cleaning the IAC would be my first step, and it's really easy to do. Just pull the hose off and slowly pour in a half can or so of Seafoam, (you can do his by removing the hose from the crankcase vent and spinning it around so the open end points up) or spray in some intake system cleaner in it. You should do this with the engine running to avoid pooling of the cleaner in the intake and possibly hydro locking the engine on startup.

I'd suggest replacing the Coolant Temp Sensor. The ECU uses this to determine the temp of the engine in order to decide on fuel/air ratios. Usually, if the CTS breaks, the ECU defaults back to cold, which is why the car runs poor/won't start when warm.

Cleaning the IAC would be my first step, and it's really easy to do. Just pull the hose off and slowly pour in a half can or so of Seafoam, (you can do his by removing the hose from the crankcase vent and spinning it around so the open end points up) or spray in some intake system cleaner in it. You should do this with the engine running to avoid pooling of the cleaner in the intake and possibly hydro locking the engine on startup.

 

Not OP, but I've tried this before. I can never keep the car running when the intake is disconnected. Also, if the ECT is not OEM, how likely is it to work properly with the car?