disconect the two pipes going into the heater matrix at the blulkhead just above the exhaust port for #4 cylinder
connect a garden hose with a water supply to one outlet on the bulkhead and another piece of garden hose to the other outlet,
put the heater control to full heat (no need to have the ignition on for this) and turn on the tap slowly and see if you get a good flow through the matrix if not make sure that the heat control cable has not come off the valve that controls water going into the matrix (it buried down there somewere) if all that is ok then check the stat is in place,or which ever you want to do first