Subaru Legacy winter RPM issues

Question

I am having some weird problem with my 2013 subaru Legacy 2.5i.

When temperature drops below 25F outside I am starting car normally everything is fine. After about 5 minutes of driving when revs should stabilize on about 700rpm since it is warmed up and it should idle in normal low range car drops revs and almost stalling. On trafic lights i have to keep it on acceleration pedal because as I am pressing clutch rev drops when approaching traffic light and it almost stalls.

After driving for few more minutes everything goes back to normal. When it is colder outside it takes longer for car to get to idle without helping it with acceleration (further distance from home, more traffic lights) it always comes back to normal and not need any help if I am driving long enough.

I was thinking about bad coolant temperature sensor but why is is doing only below 25F.

Please let me know your thoughts.

I hope I did not write it too chaotic.

Thanks a lot.

MX-5_NERD · Answer

One issue that may cause this is a dirty throttle body. Cars as new as that use electronic throttle bodies to control the idle, and as oil builds up on the back of the throttle plate, it can cause disruptions in the air going into the cylinders. This effect can be accentuated by cold weather, and as the inlet air is brought up to temperature by the engine warming up, it can go away.

If you aren't getting a check engine light for this issue, this is a good place to start. Subaru throttle bodies are usually pretty accessible. You may be able to clean it by simply removing the intake, pushing the throttle plate open, and cleaning the back of the plate with a shop rag and some carb cleaner. To get the best clean, you can remove the entire throttle body assembly, though keep in mind, there may be a few coolant hoses running through it that need to be disconnected, and the gasket underneath may need to be replaced if you do this.

HelloCivicByeCavalier · Answer

Do you notice any flickering of your headlights? I would want to test electrical system, including spark plugs, they sell a cool sensor that hooks into plugs, so you can see w/o removing the whole plug, if it's getting spark electricity. Coolant sensor pops up on other forums, that's possible of course.. I'd be thinking cam shaft sensor also, Scotty had a video on cam sensor awhile back. -- also sounds silly, but make sure you've got good oil too, and it's not bad Gasoline...