98 Lumina LTZ Service Engine / Idle Problem

Recently had to replace the alternator. After that, voltage was jumping up and down from 14v to 3v (at least at idle). Took the alternator back to autozone, they tested it, said it was fine, but gave me a new one anyways. After putting that alternator on, problem seems to persist a bit, but driveable and no voltage warning light. At an idle, but worse when in gear, the idle seems rough and the rpm's seem to fluctuate a bit. Service Engine light came on about a day after driving like this. Seems to drive great and still has plenty of power. Any ideas? I am taking it over to Auto Zone tonight to get fault code pulled.

wHile you're there, have them check the battery.

make sure you charge the battery fully while your replacing any alternator. If you just throw it in, and the battery has been drained down from a dying old alterator, the new one has to go full field right away and could fry it. I've seen it happen, a lady kept bringing her van back to our shop, her alternator went out, and she did buy the expensive one with the lifetime warrenty, and kept frying that one, untill i got the van and hooked up the battery to the charger while a replaced the frieed alternator with another free one. It hasn't been back since.

Pretty new battery, got it late last year.
Had 13 Volts when i installed the alternator.
Would a battery issue trigger a service engine fault code?

Pretty new battery, got it late last year.
Had 13 Volts when i installed the alternator.
Would a battery issue trigger a service engine fault code?
And what is the code?

And what is the code?

I will have to get back to you on that... going to Auto Zone tonight

Went in to Auto Zone and got the readout:
"Cylinder 4 misfire"

Drove it around some more this weekend and it seemed to be driving ok. This morning the service engine light went off and it is driving great.

Any ideas? Plugs and wires are relatively new.

It might have been an anomaly since it has went away. A cylinder 4 misfire could be tripped by a partially clogged injector, or a bad plug or wire on that cylinder. If you used Bosch plugs (especially their +2 and +4 platinums) get rid of them (they are JUNK!!!) and go with AC-Delco or Autolite double platinums. Try running some in-tank fuel injector cleaner (Lucas and Chevron Techron are good stuff), as preventative maintenance. If the code doesn't come back, don't worry about it.