Odds are some of the maintenance was neglected on this car. Dex Cool and Trusty Ol' Green have there issues if neglected. Regular coolant flushing does the cooling system good, helps purge sludge and gunk that develop over normal use.

This is a topic of much debate over years, Dex vs Green. GM recommends and uses Dex in their modern cars from the factory. I know enough people (this is both online and also people I know local to me offline) who switched from Dex to green during a flush. Zero issues. If you spend the time to completely flush and purge as much to all of the Dex from the system and then use green, I don't believe it will be an issue.

Despite that statement, both my Grand Am and Monte Carlo are still running Dex (and I've done routine flushes on them).

 

So what's the recommended flush interval? I'd like to do one soon - soon as in the next few years. Mine has 94k and I have yet to do a flush myself in the 31k I've owned the car. I highly doubt that a flush was done by the dealership (Ford) that I bought the car from. That being said, I can't say if the coolant I have in mine now is the original or not.

 

I believe it use to be 5 years 50,000 miles when dex-cool first came out. I usually do a flush on about that schedule or when I change out the thermostat , When I flush system I usually try to back flush the heater core also, on gen 6 cars it's pretty easy to do just pop lines off heater core and would imagine gen 7 cars would be about same.

 

GM recommends:
So I may not always flush the system. But I will drain and/or flush it about every 2-5 years or roughly 30k +/- miles. I personally think 100k with any coolant is a bit much unless the car is never driven.