Definitely check for buildup of old leaves in the ducting as mentioned above. My car was loaded with them and they can start to burn as described in that thread.
If you have any of the "5x" codes then I'd ignore the other codes until those are fixed, because they are messing with the sanity of the ECM.
If you have code 53 (system over voltage) then get a multimeter and check the voltage of the battery circuit while the car is running. This code is set if the ECM detects over 17.1V, which is excessive. I don't know how much input voltage the ECM components are specced to tolerate before they'd be damaged.
If your meter agrees that the car is running on over 17V, the other error codes might not be reliable. Fix the voltage first. Replace the whole alternator or test/troubleshoot it and at least replace the voltage regulator in it.
If you have code 51, this means the EPROM is failing the checksum, which means it is corrupt.
If so then you need to get into the ECM. Remove the shift knob, screws from underneath the ashtrays, get the shift console out. Then pull the top of the large center armrest piece towards the front of the car and it should tilt forward and come out, unless there's screws I've forgotten. ECM is behind where the map storage door thing is.
Open the rectangular door on the ECM. The EPROM is the wider (24 pin I think) chip, in a black plastic carrier on the driver's side of the ECM. Note the orientation of the EPROM - you have to plug it back in the same way. There's always a notch or a dimple on one end of the chip itself (not sure if it's visible inside the plastic carrier). Pull the plastic carrier to remove the chip and carrier as one piece. Look at what markings are on it. There should be a code on it as mentioned above.
If my info is correct than the factory chip for an 88 Federal 5spd V6 should be AKYN3442, and factory chip for California cars should be AKYP3447.
Assuming it's the correct chip then you don't need to remove it from the carrier. Doing so may lead to damaged pins, it's not easy to get out. I swapped to an older chip carrier from a 1984 ECM on my car because it was much easier to deal with for me.
Plug it back in, making sure the orientation is correct. If you plug it in backwards it will fry the chip. Try again, hopefully reseating it will have fixed the code 51. I've had this happen a couple times on my car and had to reseat the chip, but that might be because I've messed with it a lot.
When you first turn on the key, the "check engine" light should turn on, then blink off for an instant, then turn back on and stay solid. That blink tells you the ECM is functioning (but not necessarily happy).
[This message has been edited by armos (edited 06-09-2023).]
