Hi All,
New to the forums. Just wanting to flag to everyone my recent experience with my Mazda CX-9. It is the GT-AWD with the rear DVD.
Last year it developped a fist sized rust hole through the floor inside the trunk. Mazda was dumb-founded and replaced it on warranty, even though I had 101K and just about 5 years on it. I was fine with that. I tow a 17ft travel trailer weighing 2900lbs. I have a brake controller and the transmission cooler, and did not drive aggressively.

Two weeks ago, the head gasket went, and the coolant in the anti freeze likely scored the crank bearings. I did a lot of research and opted to have my garage replace the engine with a used one with a 6 month warranty. $4k later I have a running SUV again - for exactly 1 week. At this point the transmission failed. At 6 years and 140K it might be worth 15K$ used if it didn't have these problems, and an accident record due to the fact that I was rear ended while stopped at a light. Unfortunately when I replace the transmission I will have just sank 7500$ total (both engine and tranny) on a vehicle that wont get me more than $10K on a trade, and I will not be towing with it again. I don't trust it to drive my family around, and would not trust a new one either.

This will be cautionary tale to anyone who thinks that Mazda SUV's can tow. It is my own fault for trusting the 3500 tow capacity. I have no warranty due to its age, and Mazda is doing nothing for me.

 

You did the right thing by installing an external tranny cooler, however you did not state how many times you had the tranny fluid changed or the engine coolant changed as well as engine thermostat. Tranny fluid should have been changed every 20-25,000 miles due to trailer.

As a rule of thumb, every 20 degree increase in operating temperature above 175 degrees F. cuts the life of the fluid in half!
At 195 degrees F., for instance, fluid life is reduced to 50,000 miles. At 220 degrees, which is commonly encountered in many transmissions, the fluid is only good for about 25,000 miles. At 240 degrees F., the fluid won't go much over 10,000 miles. Add another 20 degrees, and life expectancy drops to 5,000 miles. Go to 295 or 300 degrees F., and 1,000 to 1,500 miles is about all you'll get before the transmission burns up.
Any number of things can push ATF temperatures beyond the system's ability to maintain safe limits: towing a trailer, mountain driving, driving at sustained high speeds during hot weather, stop-and-go driving in city traffic, "rocking" an automatic transmission from drive to reverse to free a tire from mud or snow, etc.
FROM: Transmission Fluid Flush Tips | Trucks Plus

Diesel engines are generally best for towing: make sure it's NOT 4 Cylinders!

 

Due to the fact that I knew we would be towing quite a bit last summer I changed the transmission oil at the beginning of the summer. We did maybe 1600Km that summer, and the rest of the year was simply my wife driving to the park and ride, and weekend groceries.
We did one trip at 100km this spring during which I felt a couple of lurching shifts while towing. At the time I was mildly concerned but we didn't feel any thing else for a few weeks. Then the motor died

The transmission guys tells me that the transmission cooler is actually a transmission warmer in that a hole from the transmission runs through the radiator to keep the oil from getting viscous on extra cold days. He sais also acts to keep the oil from getting too hot.

I don't know what to believe. Everyone I spoke to about this insists that my CX-9 GT AWD with the tow package had a transmission cooler.

When it failed, there was tonnes of oil, but something inside the trany had started to burn, so it seems like a loss of oil pressure occurred.

Now that I've spent 7500$ to get it running, I'm told that it's of no value retail for me to trade it and a wholesaler is only offering 4500$. I was offered 3000$ before spending the 3500$ on the transmission.....

 

I don't know what type of tranny cooler is on your vehicle. Normally with stock vehicles, the tranny does go thru bottom of rad which is useless as it is a tranny warmer. I hate to re-iterate, but when towing you must have an external tranny cooler, one that is separate from rad!
I thought you had an external tranny cooler, but my assumption may have been wrong. You must make sure it is an external tranny coooler and not one that goes through bottom of rad. A salesman may not know the difference.
This difference is explained in article that i suggested: Transmission Fluid Flush Tips | Trucks Plus

It is a separate rad installed in front of your coolant rad, like the small black one at bottom right corner on this one:

I suggest you get out and look for yourself to see if you can find something similar.

 

If the tranny guy actually looked at your vehicle, i would believe him.
Strongly suggest you get one installed now.