Why is my engine flaking?

December 1, 2012 | By Richard Prince


I have a restored 1967 Mustang with a cooling system problem.

The engine was completely rebuilt and after several hundred miles it started overheating. The new radiator was clogged up with what I soon found was flaking from the engine block.

The small pieces could be picked up with a magnet. I have flushed the cooling system and replaced the radiator again, but still have the problem although it’s not as severe.

What causes the flaking and can it be stopped completely?


The problem you’re experiencing is unusual but not unheard of.

After decades of use, rust, dirt, bits of metal and other debris accumulate in various low spots in the cooling passages inside an engine block. At the same time, some level of corrosion attacks the cast iron block inside the coolant passages. When your engine was rebuilt it probably was “hot-tanked,” which is to say it probably was put inside a machine that cleaned it with a very hot chemical solution. This will normally loosen up both the sediment in the low spots and the rusty areas inside the block.

It’s a good idea to use high-pressure steam or something similar through flexible tubes that allow you to manually reach deep inside the coolant passages. It’s also important to thoroughly flush the block after it’s hot-tanked or steam cleaned to make sure that all loose debris is washed out.

This is all good advice for someone who is going to rebuild an old engine but it’s not much help to you after your engine has already been rebuilt.

At this point, I suggest disconnecting the radiator and heater core, plugging the appropriate holes, and creating fittings that will allow you to run high pressure hot water through your engine’s coolant passages for as long as it takes to get completely clean water out.