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Find and go to an autozone and have them pull the codes. They will then go inside and look them up on the computer, and then tell you what you need. This is free unless you purchase the necissary part there.
My Haynes manual shows code 341 as an Octane adjust service pin open. This is a small grey shorting plug in the wiring harness around the rear of the fuse/relay box (at least in 91 ) Removing is is supposed to retard timing by couple of degrees to relieve pinging on really cheap gasoline. Removing it will set this code during key on engine off self test, but should not cause the symptoms you described. Replacing the shorting plug will cure the code.
As far as the rough idle, there are a couple of things to try. 1. Clean the MAF sensor and 2. clean the idle air control valve. A good dousing with carb cleaner on/into both then clear the computer by removing battery cable for around 10 minutes.
I would bet this vehicle has the infamous pinging problem, and someone removed the shorting plug as a quick cure. You may also want to check the lower intake manifold bolts are not loose. This also is a common cause of pingind and idle issues.
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