A step-by step guide for flushing an older vehicle's coolant system

We are often asked for a process to clean an old radiator and engine block. This process has been developed and refined over time by enthusiasts and we present it for your use. Always remember to put acid in the water an use all appropriate cautions with caustic chemicals and heat.

1. Drain coolant from the system.

2. Back flush radiator and engine until the water runs clear and is free or particulates
3. Add a Dawn Dish Soap water solution.
4. Run the car for 30 minutes and let it soak for 24 hours.
5. Drain the dirty soap solution.
6. Backflush the engine and the radiator.
7. Fill system with oxalic acid solution – 3 parts water 1 part oxalic acid 
8. Run the engine 15 minutes until temp > 200° and/or solution > 170°.
9. Let the engine cool down < 100°.
10. Drain the acid solution (should be dark green color)
11. Fill system with a neutralizing solution – 3 parts water 1 box bicarbonate (Arm and Hammer).
12. Run engine for five minutes.
13. Let engine cool to < 100°.
14. Backflush radiator and engine with water until clear and free of particulates.
15. Fill system with a diluted acid solution – 3 parts water 1 part green phosphoric acid .
16. Run engine until head temperature > 200° and/or solution > 175°
17. Let engine cool to < 100°.
18. Drained solution (solution will appear cloudy).
19. Fill system with a neutralizing solution – 3 parts water 1 box bicarbonate (Arm and Hammer) and run engine running for five minutes.
20. Let engine cool to < 100°.
21. Backflush the engine and radiator until water is clear free of particulates.
22. Fill radiator and engine with 50/50 coolant.
23. Cross your fingers and drive.