Recently installed a new Koyorad Radiator onto my 91 FC Vert and after multiple refillings of coolant into the radiator, I'm noticing that the "Add coolant" light comes on more frequently. Just today I had it come on 3 times and after refilling, the light comes back on not even 30 minutes after driving (both spirited driving and just driving around town in traffic). When I pop the hood there does appear to be something getting some of the components towards the bottom of the engine bay wet and there's a horrible smell (not sure if that's just from the car misfiring normally). Any ideas of what it could be? I can get pictures of it later tonight or sometime tomorrow morning.

 

As you may know already, the add coolant light is triggered by the sensor on the top of the radiator. The electrical bullet connector, male/female, on that sensor needed to be cleaned/sanded on my 86. I bought a new sensor, because that one is pretty cheap, and I still got the add coolant light going off until I got in there and cleaned the old side of the connector.

That being said, if you are truly missing coolant, and your overflow water tank is getting filled up, that's not good so you need to check that. On mine, it meant a break in the exhaust channel, (engine block) was pressurizing the water system pushing all the water to the overfill tank.. . .. So no good there.. . .I got about 1 to 2 thousand more miles on the engine after that discovery

Is your overflow tank level going up or down?. .If it is down, you are leaking water. . . If it is up, you are pushing water.

You can put blue dye in the radiator water and use a uv lamp to check for leak origination near the areas that are wet.. . pretty common technique.