Octane

[Batz]

Good day! A question has been haunting me. I have a 2007 2 litre Forester XC. Sometimes, I run it on premium fuel, sometimes not. I find that when I run it on regular unleaded, the performance goes to hell after a couple of weeks. It stumbles around 2000 - 3000 rpm, sometimes quite badly. Sometimes it clears up if I fill up with premium. The handbook says 95 is ok. Perhaps the intermittent performance issues are co-incidental but I ask the question anyway. Are flat fours allergic to regular fuel?

[Jay762]

They are not but takes a fair few miles for the ECU to 'learn' the drop in octane - It may not just be the drop in octane though that is affecting the performance - I have noticed increased amounts of Ethanol being included in pump fuel now

[Batz]

I was thinking of having the plugs and leads checked. Should have done that when it was serviced! I'm a bit remote and having it in the garage for a day is a real pain. Last time I did it they plugged in the diagnostic and found nothing wrong. Came back perfect. 

[Jay762]

Depending on mileage since plugs changed (10k or so) good to check / change. Depends on the tool they use not all will throw up the fault codes also if you had better octane fuel in the tubes at the time it may not show an error?

[Batz]

I filled up with premium yesterday but no change so far. And it's getting very lazy starting in the mornings. I've booked it into Adams Brothers in Aylesbury for a diagnostic. They're Subaru specialists. 

[Batz]

Tried disconnecting the Battery etc. For two days it was brilliant. Today, crap again. Adams Brothers then...

[Jay762]

hopefully that will throw up an error code that can be used in doagnosis