When I was running the maintenance shop for a sand, gravel, and concrete company, we used the same muriatic acid that was used for cleaning masonry work for flushing cooling systems. It's available at any home improvement store or swimming pool supplier. Pour 4 to 6 ounces of acid into the radiator along with a fresh fill of clear water, and run the engine at a fast idle for 10 minutes or so. If it heats up, shut it off. Running water from a garden hose over the outside of the radiator will help keep it from boiling over while you're flushing. Drain the flushing solution, and run clear water through the radiator, then pour a couple of boxes of baking soda dissolved in water into the cooling system, top off with water, and run for 5 minutes. This neutralizes the acid left in the system. Drain and flush with clear water. The acid flush will clean up any rust deposits or lime scale. It might also expose some pinhole leaks in the radiator core that were being plugged by deposits. If so, take the radiator out and get it repaired or recored.
Now- - - -here's another possibility. You could be getting exhaust gas into the cooling system from a cracked cylinder head. Here's how to check for that possibility. Take the fan belt and the top thermostat housing off, and fill the engine with water until it's level with the thermostat hole in the head. Start the engine and goose the throttle a few times. Don't be timid about it- - -let it accelerate for at least a couple of seconds. If any bubbles show up in the water, you're getting compression into the coolant. Take the head off and get a machine shop to pressure test it.
Good luck!
Jerry
