cylinder #2 & #4 misfire

I have a 2006 Legacy GT (MT) with 75,000 miles

I've been having a problem the last month or two with a cel and misfire. I first noticed the problem when the check engine light came on. It did idle a bit rough when cold, but not enough that I really noticed it before the cel. It spit out the code for a cylinder #2 misfire. I tried a couple tanks of gas with fuel injector cleaner to see if that would clear it up, and if I reset the code it wouldn't come back on for maybe 300-400 miles.

The light kept coming back on, and the rough idling became more noticeable, so I brought it in to the dealer. I was overdue for new plugs, so I had those replaced, but that didn't seem to do much. They said it was giving codes for misfires in #2 and #4. They powerflushed the injectors. They checked all the injectors and coilpacks and they all are good. They did a compression test which came back OK. The leak test passed with flying colors.

It might be unrelated, but the oil has seemed to be getting dirtier quicker and losing maybe 0.5 qt every 3000 miles. They noticed the same thing and have a theory that it has to do with the variable valve timing which is controlled by the oil pressure via tiny oil channels that they think are sludged up since they share the same channels between the cylinders. So they did an oil change there and said to get it changed in another 1500-2000 miles and see if it cleans itself up. Otherwise the dealer mechanic was at a loss for the cause other than this "guess".

Does anyone have any ideas/suggestions/advice? The rough idling seems be getting worse, and I'm hesitant to bring it back to the dealer mechanic if all they're going to do is a bunch of tests that don't tell them anything.

Check your vacuum lines, especially those under the intercooler. I had this exact same problem, one of them popped off and started throwing a laundry list of codes. Misfires on all 4 cylinders, oxygen sensors reading lean, car running lean, etc. It was just a stupid hose that popped off. Take the intercooler off and trace all of them, both ends, making sure they are properly connected.

Before taking off the intercooler, I asked the dealer if they checked for vacuum leaks, and they said they did. I am only getting codes for the misfires in the two cylinders though.

Reviving your old thread... OP, did you eventually figure out what was causing your misfires? Was it due to leaking o-ring gaskets that sit between the plastic intake manifold and the TGV?