As title really. Idle revs used to be very steady - something like 1500 when cold dropping to about 750 when warm.
For the past few weeks, the engine has seemed to hunt across a 300-400 rpm band. Now the bottom of that band seems to have dropped away, such that revs can drop to 100-200 before the engine "realises" and stops itself stalling. On one occasion it has actually stalled.
What can be causing this? I'm assuming it's one or more sensors failing.
Car is MY99, pretty much standard under the bonnet besides a Green panel filter.

 

engine coolant sensor perhaps...? The diagnostics will tell you this.. connect the green wire to the black one (under the dash)... i haven't done this yet on mine.. but have ordered a new sensor from subaru.. £40 + vat

 

to run the diagnostics and to perform an ecu reset, you dont connect the green to black. You connect black to black and green to green thats why they are colour coded.

Have you had any CEL come on the dashboard???? It could very well be down to your MAF which are reknowned for being a bit of a weak spot on that year of car. Do you have difficulties in starting the car either hot or cold? If it starts both hot and cold and just idles erraticaly then the temp sensor is probably okay. As there is something definately wrong I would advise you not to drive on full boost (if it is the MAF it can through your fuelling out which on boost is not good, saying that its not really good at anytime)

What I would do first though would be to reset the ecu, go for a little steady drive, then check for error codes, do a search for ecu resetting on here and you will find a site that gives you very clear step by step instructions and also the error code meanings

 

Nope, no CEL at all.
Hot and cold starts both no problem whatsoever.
I was thinking MAF too (MY99 with 78k miles), but wasn't sure how that affected the idle speed.

 

ahh that was it.. green to green, black to black.. phew.. good job i never tested it!

 

idle controll valve i would say,

its on the pasengerside of the manifold at the back, take it off and clean it with carb cleaner, should be ok then. hope u dont need a new one £280 each

 

very likely a MAF sensor on it's way out.