I was taking my car (94 del Sol, mid engine B18C1) for a drive one day, and mid drive, the car started overheating. I pulled over and took a look at the radiator area, and there was coolant sprayed everywhere. I limped it home, where I refilled the coolant to look for a leak.

Turns out, it wasn't just a regular coolant leak. The coolant doesn't leak until I drive the car, when it starts to fill up the coolant reservoir on its own, and eventually, it'll get too full and spew out from the cap. When the reservoir gets full, the radiator gushes out coolant when I crack open the cap there as well. Even when the engine overheats, the hoses and radiator stay cold. After it overflows and I let it sit for a while, the coolant level in the reservoir and radiator will return to normal (except for a bit of lost coolant from spewing out). The coolant does not appear to be oily or anything, and I have not noticed any smoke or noises out of the ordinary. The radiator is located in the stock location at the front of the car.

Any ideas as to what happened? Keep in mind that the car has been running fine for a month prior to this happening. Now I can't drive it for even 5 minutes without the temp gauge reaching "H". Thank you!

 

UPDATE:
I ran another test, this time letting the car run with a funnel attached to the radiator cap for a bit over half an hour, just at idle. The top radiator hose did become hot, but still not hot enough for me to burn my hand on it. The lower hose is cold, and the radiator itself was somewhat cold as well. Initially, the coolant started flowing into the funnel (which explains why the overflow reservoir spewed out coolant). It filled up a LOT, with some bubbling, so I removed quite a bit of coolant using a large syringe. The coolant still fluctuated a lot in the funnel (more than the reservoir can handle).
The radiator fan did turn on as expected, and I let the car sit for a bit longer. The temperature gauge kept rising, and I shut off the engine when it neared "H". After I shut off the engine, the coolant flowed back into the radiator from my funnel. When I drove the car back into my garage with the radiator cap on, all the coolant emptied out from the overflow reservoir back into the radiator, so the reservoir's empty again.

Still overheating, and coolant levels are fluctuating a LOT.

 

Thermostat sticking ? Do you still have a coolant bypass hose ?

 

The thermostat could be a possible culprit. Right now, I'm thinking it's either that or the water pump is failing, since the coolant seems to be having trouble circulating properly. I'm not sure about the coolant bypass hose - what does that look like and where would it be?

 

It is the short "L" shaped hose between the thermostat housing and the passenger side corner of the intake manifold... passing through to the cylinder head.