Subaru Repair: CEL rough running, corrossion, subaru dealers

Question
I have a 1998 standard shift outback.  The CEL light has come on two times previously and I have taken the car to Autozone so they can plug in the readout machine.  Both times they told me that the one of the pistons misfired and that it was nothing to worry about.  This morning the car was running rough and seemed to be idling low (500 rpms) at stoplights and shook slightly as if it was going to stall out.  The CEL blinked and then stayed on.  It did this a few times.  It rained the last couple of days, and I noticed that the car sometimes runs rough on rainy days until it warms up.  Should I take it in for a tuneup?  What could be the problem?  

Answer
A couple of things...Autozone provides a very nice, and free, service of checking codes. But, they need to give you the actual code that is found in your system. It is probably a P0300 indicating a multi cylinder missfire, or a P0301, P0302, P0303, or P0304 - each code indicating which cylinder -1,2,3,or 4 is the offending cylinder. That would be helpful to know if each time they clear the code, is it the same cylinder or different.

Depending on the miles, yes you should have a tune up. If the plugs have been in for more than 60,000 miles, replace them. Always, always, always, use the NGK plug that Subaru recomends. The Subaru dealers will have them in stock, or, you can ask Autozone to get them for you. Stay away form any other brand on Subaru's as we have seen some problems with non-NGK plugs. If the wires have not been replaced since the car was new, that would be good to try at the same time. And finally, when the wires are off at the coil, check the coil towers for corrossion. If coroded, you definitely need new wires, and you may need a new coil.

Less likely than a tune up would be a more serious internal engine problem - and unfortuneatly you have a year that Subaru has had some cylinder head problems. Heavy carbon build up causing valves to stick is one problem - which sometimes can be taken care of by a BG or Subaru top engine clean. Another problem is valve guides moving in the cylinder heads. The cylinder head or heads will need to be replaced if that is the problem.

Knowing exactly which codes are set, and the condition of the plugs, wires, and coil will all help to determine the next step.

And, before I have you worried about internal engine problems, let me tell you that it is possible, but not likeley. Since you said it seems worse when it rains, plugs and wires are the best, first place to start.

Good luck