Manufacturers like GM and Toyota have for at least the past eight years been using coolants that last about 100,000 miles in their engines. In their maintenance guide they tell you to change it at about 5 years, or 100,000 miles. After that, their maintenance guides tell you to change it again in about 2 years, or 30,000 miles.

The question I had was: if you use the same coolant as the manufacturer, and completely drained the old coolant, why would need to do it again in 30,000 miles. Why not 100,000? Although I was unable to find an answer to this question on the web, the 30,000 interval may be based on the assumption that the do-it-yourselfers will only drain a portion of the coolant. If so, draining it again after 30,000 miles makes sense. They want you to do it every 30,000 to keep replenishing it. On the other hand, if you do a complete flush, or have the dealer do it, it seems reasonable that you would not have to change it again for 100,000 miles. But, nowhere did I see this on the web.

I just changed the coolant in my 2004 F150. It has a 5.4 liter engine, and according to the specs about 21 quarts of coolant. To drain the old coolant I decided to just disconnect the lower radiator hose. This resulted in 12 quarts of old coolant coming out. So, about 60% of the old coolant came out. I then added 12 quarts of the 50/50 mixture of the recommended Ford coolant. Doing it this way, I plan to repeat this at 30,000 mile intervals to keep replenishing the old coolant.