Once rust gets a grip in a cooling system, it is only a matter of time before serious damage begins to occur. Even if steps are taken to clean the rust, and to prevent further rust, the damage has still been done. Maybe a breakdown will occur while on a vacation or climbing a hill of when you least expect it, but it will occur. Very easy to prevent. Not so easy to clean up. Expensive to repair.

Forklift Forklift
$10 for coolant would have saved him $350 for a recore. And that's not the end of it. The engine is just as rusty, or worse!

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