As above, could a build up of rainwater overnight or through the day on the air filter (no blanking plate) trigger the knock sensor, retard ignition & reduce boost Etc when first started & run from cold?

 

Is the filter the standard Subaru paper one?

I've not heard of it directly triggering the knock sensor, and struggle to imagine how it would. However, water going into the intake could make the MAF sensor misread, and potentially be quite dangerous if signficant amounts got into your engine. Water injection is a bit finer and more controlled

 

The filter is a Hks Superflow, The car sometimes experiences rough running for a few hubred yards, Misfiring, spluttering Etc Turning the car off then on seems to cure it. It has now started sometimes illuminating the check engine light but not all the time & reducing boost to 9psi until a reset. This happens even when you don't drive hard & never use boost. Is the Maf sensor Naf.?
Thanx
Dean

 

As you said in the other thread you need to post the code. Count the flashes.