Rough Idling engine

[Fredy]

I have 2004 Forester XT (Turbo). Engine runs very rough on cold start-up gets better when up to operating temperature but still not as smooth as it once was. Car has 110,000 miles.
Quick history of last few months. Engine had occational miss CEL came on. Error indicated miss fire on no.4 cylinder. Checked with two different code readers. After checking all the possibilities and not finding any problem with No.4. Decided that maybe the code was wrong and went to old school approach and started pulling spark plugs looking for a cylinder that showed a problem. Found the poclin cracked on the no.3 cylinder plug. The plug also was white indicating a lean burn. SInce this cylinder is so difficult to serivce the fuel injector was changed along with the plug. The plugs only have 20,000 miles on them. Plugs were NGK OEM spec.
I know that injectors are suppose to be replaced in sets because there is a 6% flow varriation in injectors and injectors are flow tested to get matched sets. Would just changing one injector cause the rough idle problem?
Any one with any thoughts?

[Robbks]

i doubt VERY much there is a 6% flow variation in the injectors
if you replaced a set of injectors with another set that flowed 6% less fuel, it would run extremely lean on open loop fuelling and die a nasty death very quickly.

back on topic

the injector o-rings could be seated incorrectly and as such fuel drains into one cylinder while the car is sitting over time and cause a very rich cylinder and rough running
It's probably a good time to remove ALL injectors, replace ALL seals and have them cleaned and flow-tested (leak tested too)

another likely suspect it the Coolant Temp Sensor, it's onla a $30 part and is easy to replace
-remove alternator
-swap sensor
-re-install alternator
-drink beer!

also, check you entire intake for leaks, there may be a small air leak that closes up once the car is warm

boosted cars are tough on plugs, i change mine every 15,000km (10,000 miles) it's not a tough job with the right tools

[Fredy]

Thank you for the input. Since injectors on #2 & #4 cylinder were swaped a number of times chasing a problem that was on no.3 it is very likely a seal was damaged.