Did the so-called "rebuilder" flush the water jacket thoroughly? That usually involves pulling all the core plugs out of the block and either pressure-washing the cooling system repeatedly, and/or doing a lot of probing around in the water jacket with a steel rod to dislodge the crud. Once it's reassembled and in the vehicle, the process is much harder. I've used a mixture of muriatic acid (technical-grade hydrochloric) and water at times to chemically clean a block, followed by a flush with a full box of baking soda dissolved in water to neutralize the acid. Run the engine for about 20-30 minutes with a cup of the acid used to clean masonry and a full fill of water, followed by the baking soda flush. Repeat as necessary until the flush water comers out clean.
Jerry
