My rear exhaust bolt has been hissing and I recently took it out cleaned it up and used Peramtex ultra copper on it. The next day when I started the car there was a stream of coolant squirting through the overflow tube in a timed sequence. The car was only on for 20 seconds or so before turning it off. Everything was cool to the touch but when I twisted the cap off it exploded coolant fluid all over the garage. There was some serious pressure! The coolant was room temperature so I think it is not a thermal issue but it is being pressurized by a cylinder and that explains the timed sequence of squirting through the overflow tube. There is no coolant in the oil/ vice versa. No smoke or water coming out the exhaust. Compression test is normal on all cylinders. The only thing I can think of is a breach between the exhaust port and coolant passage that is sending exhaust pressure into the coolant. I plan on replacing the rear exhaust bolts with studs and new gaskets but before I reassemble and try again would like any input on if this might be a cracked head. Any thoughts?