99 Ford Taurus overheating, again

Hi, I have a 99 ford Taurus that is overheating, again. I must say this happens ONLY when it is HOT OUTSIDE about say 90 degrees, and I turn the AC on. If it is hot outside and I turn the AC on in about 20 minutes the car will start to overheat. The car never overheats when it is cool outside, even with the AC on. I have had this car in the shop for days trying to find the problem. Water pump is fine. Replaced the thermostat. Both fans work fine. Water pump puts out fine. Radiator is fine. We replace a hose and some other smaller hoses and thought that was it. IT does not leak. Flushed the radiator. Now it is starting to be hot again so I am using the AC, but it is overheating again. The mechanic can't seem to find the problem. He does not believe me. OF course, now it isn't that hot again so it won't overheat. I keep telling them it all has to do with the outside temperature and using the AC. Any other time it is just fine.
Does anyone have any suggestions??? I am at my wits end with this problem.
Thank you!!

Welcome to the forum!

Has the technician performed a pressure test on both the cooling system and the pressure cap? If the pressure cap is not properly pressurizing the system, it can cause the car to overheat.

Also, how do you know the water pump is fine? There have been a lot of folks on this forum reporting issues of missing impeller blades on their water pumps. The water pump looks fine until you remove it from the car.

If the pump was replaced, did the installer make sure the impeller blades were going the correct direction?

-Rod

Hi Rod,
Thank you for your reply. I do not know if he did a pressure test, or not. But, he told me the pump was working properly and putting out the right amount of fluid. About the cap, I don't know. I know it does not leak any fluid. This problem is related to outside temperature and the use of the AC. The car does not overheat at any other time. Even if it is hot outside, as long as I don't turn on the AC it is fine. If it gets hot with the AC on and it is hot outside, all I have to do is turn the AC off and the car will cool down. I was also told that the radiator was working properly, too. If the outside temperature is somewhat cool, below 85, you can use the AC without any problem. I just don't get it, and neither do the mechanics!
My solution is just don't use the AC, but it is the days are getting hotter and I paid the mechanic to fix it ( or so I thought) and I think it should be fixed!
The car works great otherwise, still has lots of power with 113,000 miles.
do you think it could be the belt? you know, the timing belt or serpentine, whatever it is? It was replace at 60,000 miles.

You may want to focus on the radiator cooling fan for the A/C. It is possible that, even though it is turning, it is not moving sufficient air. If the fan has been replaced recently, it could have been replaced with a universal fit version and wired backward. In this case, it would appear to be turning full speed, but moving the air in the wrong direction. Also, make sure the fan is tightly engaged on the fan motor. Maybe the fan blade is worn out and, even though it is spinning, when it has a load on it (traveling down the air) it is slipping on the motor shaft and not moving enough air.

-Rod

Yea, check The Pressure, my buddy has a 99 Ford Taurus and had a similar problem, he got it flushed, new hoses and new Pressure Cap, and raiator inspected, good luck