Is it stock? Or supposed to be? Code 51 is a prom error. Could be from reading it wrong, or damaged prom or ECM itself. But if it's running great, then it's probably just getting an incorrect checksum error. Which in and of itself wouldn't do anything performancewise, but it would set the code and you'd get a CEL. Clearing it won't matter as it would come back likely the next time you ran the car afterwards.
Does it have an aftermarket PROM? I've heard of Hypertechs doing this. Not always, but sometimes. Also, if it's not the correct PROM compatible with the ECM firmware, it could cause the code.
Your ECM box should be a 16047963 production, and 1226457 service number. And the PROM for an 85 442 should be an 7987FCH code prom with service replacement number of 16047986.
It would take some digging into to find out exactly why it's doing what it's doing, but if you're wanting to buy it, you can use the fact that's it's throwing a code that won't go away to leverage a discount.
Save a few hundred, then buy this for 75 bucks and replace all that crap at once.
https://rover.ebay.com/rover/1/711-...0001&campid=5335822911&icep_item=173367058240
If you can access the ECM...
It should have a big FCH on the label, and if it's a GM PROM, it should have an FCH on it somewhere.
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