I have an early post here describing my problems with my ECM. The 89 is slightly differnet than the 91, but I would bet on the ECM being the problem. Those codes were the ones mine was throwing before the ECM failed. I checked the sensors and found both of them ok. 89 ECM I believe also contains caps that leak and destroy the ECM. I would pull the ECM and look for CAP leaking. You can reset the ECM by disconnecting it the battery for 5 minutes or so. Once reset you should see the 5 5 codes with nothing wrong. You can hook an analog meter up, there is a post to show you how.
You can drive around with someone monitoring the meter to determine when the codes get set.
Good Luck
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