With the muffs on the drive, remove the hose that brings water to the thermostat housing, start the engine. Does water come out the hose? If so try to hold it back with your hand while someone revs the engine. If you can stop it easily then the pump is installed wrong. If you cannot hold it back then without the engine running connect your garden hose to one of the manifold inlet nipples to see if they are not blocked. If the manifolds are clear disassemble the thermostat housing and look for corrosion.