Had a look about on Google for this with varying different things.

2010 WRX S spec. Almost feels like it's misfiring when it's cold. It's fine above 1k rpm but at 800 it's rough. As soon as the car is up to temp it's fine.

No fault codes showing.

drives absolutely fine though.

only ever since cold weather rolled in

 

Personally I'd start with a leak down test.

Cold rough running could be something as cheap as a coolant sensor but it could be a sign of a mechanical issue, namely a compression one. Valve and/or valve seat seal would cause this type of fault.

Start with the basics and elliminate the mechanicals first.

 

Thanks, didn't think it would be something to do with compression as it is fine after 5-10 minutes once it's warmed up.

Will put it into the garage then to get it looked at

 

It also runs fine when driven when cold. It's just on cold idle in cold weather when the ambient temp is down.

 

Which is exactly how a valve issue behaves, so it running fine when warm doesn't illiminate that.

 

That's fine thanks. I am not sure how I would diagnose. Just didn't want to put it into the garage and pay a fortune for them to strip it apart if it's nothing.

found some reference to an inlet manifold gasket expanding when it's cold let's some air in but expands when warm

 

Would they have to strip the heads down just to diagnose?

Thanks

 

Nope. That's kind of why I said really. You add pressure to the cylinder when the valves are closed and any notable air escaping can be heard in the inlet (if the inlet valves aren't sealing) or the exhaust (if the exhaust valves aren't seating) and obviously in the crank case if excessive piston ring leakage.

It's just a basic mechanical test to eliminate a motor problem.

The inlet gaskets are a possible, like many other things. You could run the engine and spray carb cleaner around the inlet gaskets and see if the engine picks up.

It could be idle control valve (ISCV), coolant sensor, vacuum leak, mechanical issue etc.

Easiest things you could do on your own are probably ISCV clean and ECU reset. Not sure on the ISCV setup on the newer cars so check if you need a gasket as the newest impreza I've done that on is a 2002 and it needed a new gasket once opened.

Maybe have a search along the lines I've mentioned and see what fits.

Does the 2010 wrx s have any form of AVCS?