90 convert update

Radiator prices are going sky high. Even at the junkyards. The price for copper and brass and aluminum is really high so anywhere you go, you'll be shelling out the big bucks. I got a cheap aluminum radiator for my 88 at NAPA a few months ago for around 200. Good copper core ones can go upwards of 300 bucks now. I would always buy a NEW radiator, but that is personal opinion there. As for the check engine light, the easy way to tell what's going on would be to run the codes. If you're not sure how, a Chilton/Haynes manual will tell you how. Sounds like something isn't working once the car is warmed up. Could be a multitude of things, but since it drives fine, I would almost suspect an EGR fault. That would only cause it to ping a little and the computer to pull back the timing a little. Easy way to check if that system is working is to drive for a while and then when you get home, carefully touch the egr pipe near the valve. If it is flaming hot, the EGR system is most likely working. If it is only as hot as your intake, then it's probably not working. But, running the codes would reveal the trouble quickly.

I put a junkyard one in my car when I replaced the water pump and it lasted about six months. I then bought one at Autozone for $88. It is aluminum and plastic but comes with a lifetime warranty. If your car is stock like mine you shouldn't have any problems.

Could have had air bubbles floating past the coolant sensor which would make the sensor read falsely. The computer would pick up on it and throw a code. Long shot, but it's possible.

I used to have a sticky egr valve that would randomly throw a code when the car would get warm. Some days it would, some days it wouldn't. Cooling system components generally don't throw many codes unless a sensor is bad. Since you only replaced the radiator and no sensors, my guess would be that the CEL will be coming on again soon. or perhaps the radiator is magical? : P