I have a stock 1991 Z-28 Convertible with a 305 tpi, I have owned since new. I have not had any previous over heating problems before with this car. Last week I got in the car to drive home and it had a horrible misfire that was not there on the way to work. Ses light was on and flashing. Limped car home and began to inspect it. Replaced spark plugs, which only number five was found to be fouled. Also replaced O2 sensor since it had been some time and the coolant temp sensor. Car ran much better for several minutes but then stalled out and began to have the bad misfire again which felt like multiple cylinders involved. The car also was smoking real bad from exhaust with tan/grey smoke and was shooting black soot on the driveway. Ses light would blink on & off while car was running like this. I then noticed the engine was getting really hot. Turned it off and check the coolant after it cooled down. Coolant was low and dirty brown even though it had antifreeze in it. I checked the oil dipstick which appeared clean with no mud on it. Cooling fans not coming on while car is overheating but turned on when I checked the codes from the ecu. The ecu codes were 12, 22,23,34,43,45 & 53. I pressure tested radiator and it held pressure for several minutes and no leaking found under the car. Pressure slowly dropped after several minutes though. I found the ground strap at the back of the passenger side head loose. I tightened it and no change car still runs horrible and overheats. Fuses are all good. Please help could the head gasket be blown even though the car didn’t overheat prior to these problems. I also checked all vacuum lines and they are fine.

 

Flush radiator, make sure there's no oil in your water. Maybe plugged cats? I've had plugged cats absolutely ruin a car before. It's possible (though highly unlikely) that your timing chain skipped a tooth.

Just throwing out a couple guesses, somebody with better knowledge of code readouts will doubtless shine some light on the situation.