My 95 4.3 had the battery disconnected / reconnected while the engine was running badly due to fouled spark plugs.

Loss of power, unstable coolant temp, occasional marginal overheating esp. with AC on, cylinder(s) cut out cold, no power in 3rd gear or lockup. Fuel smells, unburnt fuel out the exhaust, black exhaust tips.

Put clean plugs in after changing leaky valve guide seals, still ran awful.

No codes set....

Pulled the two PCM fuses for 10 minutes, reinstalled them, the PCM reset and re- learned on good spark plugs, it runs like new now.

Its run poorly for YEARS....

It had been power cycled on misfiring spark plugs, so it
' learned ' to run that way.

Ignore the GM manual, reset it on a COLD engine. I could hear it exercising the fuel flow cold to get max RPM.

Its now almost 40 psi oil pressure on hot idle, temp gauge doesnt move and no fuel smells, just from an PCM reset!

The root cause was bad valve guide seals and bad lookup data in the ECM.