Blown Head Gasket or Cracked Block

cool, poor man's pressure tester:
1 bike tire tube
1 hose clamp big enough to go around the filler neck
1 pressurized air source
1 tire pressure gage

~cut the tube at the center opposite the valve
~seal one end shut...fold the end of the tube like a flag and clamp it
~attach the other end to the filler neck with a hose clamp
~pressurize to 20 psi and check for leaks
~if no leaks are evident, recheck the pressure every 10 minutes and see if you are losing pressure