Silicate based coolant(green stuff) needs to be changed every 2 years, so flushed and changed.
Twice in 6 years means corrosion would have started.
You could change to a HOAT coolant, which is good for 5 years, and actually better in engines that use iron blocks and aluminum in the cooling system, i.e. intakes, rads, heads, etc....
First I would check for electrolysis(voltage) in your cooling system.
Set volt meter for 20vDC
Remove rad cap
Put negative meter probe on negative battery post
Put positive meter probe in coolant, don't touch the side or core of rad.
write it down, even 0.00
Now start engine, leaving rad cap off.
write it down
Have someone rev engine to 2,000RPMs and hold it there
Test voltage again
write it down.
Shut off engine
acceptable level is no higher than 0.3volts
at 0.4v you will start getting corrosion from electrolysis
Water pump fins are prime target for electrolysis, it will eat them away fairly fast.
If you are higher than 0.3v then check engine Ground to battery.
Ground alternator to that engine ground.
The main corrosion protection in coolant is the PH level, Acid=1--------14=Base, water is PH 7.
You can see all around you that rain(water at 7 PH) can cause metal to rust, when you add salts it gets an acidic PH so rusts faster.
Silicate Coolant concentrate has a PH of 10.5, when mixed 50/50 with water it's PH is 8.75 which helps prevent rusting, 60/40 mix has 9.5 PH which will last longer, but less cooling in hot weather.
Over time, 2 years, the coolants PH will drop, when it gets down to 8 you are looking at corrosion starting almost everywhere in the cooling system.
HOAT coolant is an organic acid coolant, so PH is only 8, it uses a different method to prevent corrosion, so it's use time is longer, it was also developed because of the issues from using different metal types in the cooling systems, silicate based coolant was developed in the 1920's, lol, and it still works just need to change it religiously on the newer engines.
If coolant gets acidic enough the engine becomes a battery and electrolysis voltage gets high.
You have to be careful if you use a chemical flush, these tend to be acidic so if ANY is left in the engine, rad or heater core it will change the PH of the new coolant, so 2 year coolant becomes a 6 month coolant.
Flush with tap water after chemical flush, a few times if in doubt.
I saw some Taurus/Sable 3.0l cooling system TSBs, they were related to the rusting out from an issue in intake manifold crossover in cooling system.
I don't recall any Ranger 3.0l issues along those lines, and not alot of posts regarding this issue either, like you would expect if it was a manufacturing defect.
Here is one of the Taurus TSBs: http://ww2.justanswer.com/uploads/fo...SB_01-11-6.pdf