Thought I'd post this in hopes of helping the next poor soul who has an XT with a misfire issue. I own a 2018 Forester XT (purchased new) that developed an intermittent P0300 starting around 28k miles that seemed to occur on cold starts. I noticed that the car misfired on cold starts, not initially but after running maybe 5-10 seconds and then once the engine RPM's started to drop it would clear right up. On longer trips, the CEL would clear so I figured it wasn't anything too major.
Shortly after this first happened, I purchased the Subaru Maintenance Kit and ran a bottle of the PEA carbon clean through a vacuum port on the intake and removed and cleaned the throttle body. I don't think this made much of an improvement, if any. Subaru also has a TSB to do this work (although they do a full application over 1hr and use their software/computer to run the engine at 2k RPM for that period). See this link for details: ('14-'18) - 2017 - Carbon build up?
After this was unsuccessful, I took it to my local dealer and they said there was an open recall for the fuel pump and they would address this as well as investigate my P0300. Of course when I got there, the CEL turned off. They ended up completing the fuel pump replacement and said that my fuel pressure was indeed low and the fuel pump was bad. Ok, great problem solved. Nope, 2 days later CEL back on and no change in cold start misfire. I do think the car ran a bit better after this though so perhaps I had two issues. See this link for details: Fuel Pump Recall for 2018 Forester XT
After a bit of research, I was pretty sure that carbon buildup was the culprit. Our car does a lot of short trips which certainly doesn't help and if the fuel pressure was indeed low, I thought the combination of these two things may have made the issue even worse. Since I own a WRX also, I decided it would be worthwhile to pick up the tools to do a walnut blasting. After completing the walnut blasting (see below pic for before/after) and replacing the plugs (don't think there was an issue but since it was halfway torn apart anyways thought it made sense) the misfire seems to be gone. It's now been about 3 weeks and the CEL and cold start misfire are now gone.
I think the CVT keeping engine RPM's low most of the time may increase the buildup on the valves, but it's hard to say. Seems like the WRX's get more miles on them before showing signs of issues and most of them are manuals (and likely driven a lot harder). I'd think a bit of spirited driving in Sport mode should help to some degree. It certainly wouldn't hurt!
Hope this info helps someone else out!
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Shortly after this first happened, I purchased the Subaru Maintenance Kit and ran a bottle of the PEA carbon clean through a vacuum port on the intake and removed and cleaned the throttle body. I don't think this made much of an improvement, if any. Subaru also has a TSB to do this work (although they do a full application over 1hr and use their software/computer to run the engine at 2k RPM for that period). See this link for details: ('14-'18) - 2017 - Carbon build up?
After this was unsuccessful, I took it to my local dealer and they said there was an open recall for the fuel pump and they would address this as well as investigate my P0300. Of course when I got there, the CEL turned off. They ended up completing the fuel pump replacement and said that my fuel pressure was indeed low and the fuel pump was bad. Ok, great problem solved. Nope, 2 days later CEL back on and no change in cold start misfire. I do think the car ran a bit better after this though so perhaps I had two issues. See this link for details: Fuel Pump Recall for 2018 Forester XT
After a bit of research, I was pretty sure that carbon buildup was the culprit. Our car does a lot of short trips which certainly doesn't help and if the fuel pressure was indeed low, I thought the combination of these two things may have made the issue even worse. Since I own a WRX also, I decided it would be worthwhile to pick up the tools to do a walnut blasting. After completing the walnut blasting (see below pic for before/after) and replacing the plugs (don't think there was an issue but since it was halfway torn apart anyways thought it made sense) the misfire seems to be gone. It's now been about 3 weeks and the CEL and cold start misfire are now gone.
I think the CVT keeping engine RPM's low most of the time may increase the buildup on the valves, but it's hard to say. Seems like the WRX's get more miles on them before showing signs of issues and most of them are manuals (and likely driven a lot harder). I'd think a bit of spirited driving in Sport mode should help to some degree. It certainly wouldn't hurt!
Hope this info helps someone else out!
