[Jonny Roden]

Sounds like to me that somebody at one point in time has ran block sealer through your cooling system and it has plugged your water jackets unfortunately the only thing that you can do now is tear the head off and clean out your water jackets. I speak from experience I used block sealer in my samurai it worked good for a long time but when it did start to leak again I decided to try to run the block sealer through it again. I found myself having major cooling problems like you are describing, It was running super hot and rough also when the cap was off it would shoot the coolant out. When I tore the head off to replace the head gasket my water jackets were almost completely plugged up from the block sealer. If you are going to fix this problem your going to have to pull the head off to clean your water jackets at this point I would highly recommend replacing your water pump and your timing belt as well at the same time because they are fairly inexpensive for the parts. But these take a lot of time cause you have to tear the motor down to get to them so that's why I recommend you change these out while your at it to save yourself some time and money in the future. Keep in mind if your not very familiar with working on cars, you must be very precise tearing the head off and putting it back on there is a torque patterns and torque specs that you must be followed when removing and replacing the head. If you do not follow these you might warp the head or end up not getting a good seal and have a blown head gasket.