Rough Idle Skip

I have a 2005 Legacy GT with 90,000 miles. i am getting a P0304 skip in cylinder 4. The car runs fine when you get going on it, but it is a pretty rough idle. For the life of me i cant figure out what it is. i have NKG iridium plugs that have about 30,000 miles on them. I swapped the plug from cylinder 4 to 3, the injector from 4 to 1, and the ignition coil from 4 to 2, still getting the P304. I checked all of my hoses, MAF, and anything else i could think of but still no luck.

 

Anyone have any ideas?

We're starting to see more and more rough idles caused by burnt valves or bad valve clearances. That could be it.

My rough idle turned out to be bad comp in cyl 2 and 4. Before you buy any parts do a compression and leak down test to determine the condition of your motor.

 

Looking for 140psi, or even figures around all 4. Mine were 110 (1&3) and 80 (2 &4). Motor is toast.

Here's a short list of what you might need to do.

 

Have the heads rebuilt, replace any burnt valves, check the turbo for shaft play, remove both banjo filters, put a new clutch and SMFW & TSK3 kit in it, and you should be good to go.

 

If you need, You can get a new OEM shortblock for $1900.

 

Afterwards, get the car of the OEM tune. It's less then idea.

I'm hoping i wont have to go as far as have the valves done but who knows. I am taking it to a mechanic Tuesday to do a comp check and diagnostic, hopefully that goes well.

 

In the mean time ill have the hood open checking around if anyone else has another ideas.

I was in the same boat a couple of months ago. Followed the same troubleshooting steps you did. Turned out to be bad valve clearance on the driver side.

ya, i have a burnt valve, having the valves and heads machined, new gasket, water pump and timing belt. If you have to go this far you may as well do it all at once...