I recently had headgaskets and complete timing kit replaced on my Forester X. I was noticing that engine stumbled a little bit when first starting for a few seconds an it works smooth out and seemed to run smoothly on the road. Today it ran even a little rougher after starting and threw to simultaneous codes- P0302 & P0304- cylinder 2 & 4 misfire. Interesting that both misfiring cylinders are on same side of the engine. Looked for loose or crossed spark plug wires but everything looked OK, 

I reset the ECU and it ran a lot smoother on startup but a I could feel a bit of a miss when revving engine at lower RPM’s. It then threw another code but just P0304 this time.

Given the work recently done, what works be the most likely causes of this issue?  I don’t want to keep driving it if this is a potentially serious issue. I just had bought the car prior to having my mechanic do the headgaskets so I can’t say if it felt like it missed before, but this is the first CEL I’ve gotten. It has to back to my mechanic anyway because of a small oil leak and some A/C work.

 

Edited August 14, 20223 yr by Stevo F

check and reseat wires into coil and on plugs (also verify proper order usually marked on coil)

spray water at night on wires to see if any arcing visible (sparkles)

I used to have an issue when damp/rainy outside  05 OB (#3 cylinder) and thought I needed to replace coil but just reseated wires and all has been good since.

Edited August 14, 20223 yr by brus brother

maybe cam sprocket timing off by a tooth?

if there's oil leaking in one place, maybe there's also oil leaking on the spark plugs?

3 hours ago, brus brother said:

I used to have an issue when damp/rainy outside

i had a similar problem with a 95 Legacy... but in my case, it was a bad coil - there is a video of it somewhere...

ah, found it...

 

On 8/14/2022 at 10:16 PM, brus brother said:

check and reseat wires into coil and on plugs (also verify proper order usually marked on coil)

spray water at night on wires to see if any arcing visible (sparkles)

I used to have an issue when damp/rainy outside  05 OB (#3 cylinder) and thought I needed to replace coil but just reseated wires and all has been good since.

My uncle recently acquired this Forester with a similar issue. The previous owner had replaced the head gasket now it's throwing P0304 code. We'll just finish installing the suspension and jeep wheels on the Wrangler this week before we can take a closer look.

Edited September 12, 20223 yr by Grennick