My daughter has a 95 Pontiac Firebird with the 3.4L V6 engine that has developed an overheating problem in the last 6 weeks. I changed the thermostat to the stock 180-degree thermostat and flushed the cooling system. I used good quality coolant and flushing chemicals. When test-driving the car after making sure the cooling system was full, the heater was blowing hot air, and the coolant seemed to be circulating well, the temperature held at about 180 to 190 degrees. The next day, she took it to work and it overheated.
After the she got home and the engine cooled, I opened the cap and there seemed to be more pressure than usual in the system. I checked the coolant level and it was about a quart or half-gallon low. I filled it back up and retested it to see if the temperature would go above 180 and it didn?t. The next day on the way to work, my daughter said it overheated. I told her to go get a different radiator cap, thinking the stock cap was not relieving the pressure well enough. This did not fix the problem.
After several attempts at refilling the system every evening, she took the car to a repair shop near where she worked the next day, and they replaced the thermostat again. I don?t know why they couldn?t tell that the thermostat was not new, but they replaced it anyway and didn?t ask the question. They tested the coolant for exhaust gas and did not find any. They mentioned coolant leaking down around the timing gear cover but did not suggest replacing the water pump. The overheating problem persisted.
I replaced the water pump after noticing coolant leaking around the front of the engine. After adding 1 gallon of coolant and about 3 quarts of water and closing the coolant system up, I warmed up the car and thing seemed fine. I did not notice any coolant smell coming out of the exhaust and the oil did not seem milky. I drove the car for about a half hour and it did not overheat.
While taking the car to my workplace the next day, it overheated. No warm air was coming out of the vents even though the temp setting was set to hot. I pulled off the road and slowly opened the radiator cap. While doing so, coolant sprayed out of the overflow tank until I was able to relieve the pressure. After that, it did not overheat for the remainder of the trip to work. On the way home after the engine had cooled all day, it ran fine with no overheating. I took the car out for a short trip after that and the temp went up to 210 before settling down to 180. The heater put out warm air.
The next morning, the same thing happened ? overheating, running OK after relieving the pressure, no problems during a short trip over the noon hour.
What is happening?
The radiator is probably trying to operate without enough cooling fins. It works great until the engine has to do some work. Look at the radiator from behind and you might see some missing fins. 5% gone and the radiator is also gone. If this is the problem, you will see more than enough corroded fins to make you say AHA! Then there is also a chance that something is blocking the airflow from the front but that would have been too easy. If the radiator looks green or white, it is a hint of a problem. Make sure the leaking ws from the overflow and not from somewhere else. There is more to check, like the cooling fan to see if it even works. If the problem happens in town when you are stopped, that can be a problem.
I would replace the radiator. Flushing may not get out all the little pieces of crud, especially is the radiator is 13 years old. Pieces of crud can be dislodged so everything is fine, and the re-lodged to that coolant circulation is inhibited. Google radiators and you should be able to find a good deal. Several years ago I bought a heavy-duty radiator online for my Jeep for ~$160, and it was delivered free to my workplace the next day.
I have been chasing this EXACT problem for 4 years and 50,000 miles on my 94 firebird v6 automatic. In that time, I have replaced thermostat, radiator, coolant lines, fan, head gaskets, water pump. I have had this problem occur within 5 minutes of driving on a 3?F day. I keep a minimum of 2 gallons of water in the trunk everywhere I take it. I have made a 1200 mile trip without having the problem once, and then it will have the problem every day for a week. It is definitely a problem of lack of water flow, which is why hot air doesn’t come out of the vents. I can’t figure out which comes first though… does flow stop and then it overheats and water leave through the overflow…or does water leak out and then it overheats…
Pretty reliable car though…I do have 245,000 miles on it, despite having overheated it about 50 times…