By John Cronin
Several weeks ago my daughter started having overheat problems with her Mazda 626. I learned years ago most serious engine problems are caused by overheating. While it is a good idea to change one’s oil and fluids regularly, when your engine’s cooling system acts up, it needs to be fixed and fixed correctly ASAP. The problem with my daughter’s car was the problem occurred randomly. Sometimes the car would run for days with no problem.
We took the car to a trusted local shop. They kept the car a couple days, tested it, checked it for leaks, etc. Aside from the antifreeze being low they could not find anything wrong. They replaced the antifreeze and didn’t charge us for their diagnostic time. A few days later the car overheated again. The radiator caps and thermostat were original equipment and about 10 years old. The shop recommended we replace them and keep watching the car. The next time the car overheated, less coolant was lost. So, while we hadn’t fixed the problem we were making progress.
The Mazda 626 has a Ford style coolant recovery system. Unlike many systems the Ford recovery tank is pressurized when the car is at normal operating temperatures. The same pressure on the coolant system is put on the recovery tank. I don’t know why Ford chose this design. In my experience as a Ford (and Mazda) owner when there is a cooling system problem, the pressurized recovery tank seems to me to exasperate the problem - loosing more coolant and injecting more air into the system than other designs.
We got our first break one cool evening. We had just finished test driving the car and examining it for leaks. While in the garage cooling off, I could hear a slight hissing sound. I traced it to the coolant recovery tank. There was a small hole. As the engine cooled the hole allowed air to be pulled into the system. The hole was so small very little liquid could escape. The tank would pass a pressure test, but fail a vacuum test. We replaced the coolant recovery tank. While this did not fix the overheating problem, it did fix the problem of air getting into the system.
I am not sure Ford’s pressurized recovery system is a good idea. In my experience, every time I’ve had a problem it has complicated the problem and the diagnosis. I’ve taken cars to dealer service departments only to find they often use a trial and error process to find the problem - charging me dearly for every hour spent.
The investigation continued… For my next test I connected my ODB2 Scan Tool and ran the car