After market heater cores?

Flush the engine so you don't have any sand in the cooling system!

Never heard of that one. There should be no sand in the engine. Maybe someone used some stop leak before you owned the vehicle and it plugged it up. But FOR is right, flush the cooling system to get rid of it.

Thanks TDolby, flushing has dramatically improved it.

Sometimes a block is not cleaned properly and casting sand is left in it. Union workers.............