Ordinarily I wouldn’t think to post this here because I figured it was a freako thing with my car, but other Chevy owners recently told me they have experienced the same thing.
For the first few years we owned our cars, they generally ran cool, never getting above the 1/4 mark on the temperature gauge no matter how hot it was outside or how much idling in traffic you did.
Around the 4-5 year mark, we all had our thermostat and/ or water pump die and since getting it fixxed at the dealer (all three of us went to different dealers) we ended up with cooling systems where, under normal driving conditions, the car runs at the 1/2 temp mark, and in traffic, it gets above the 3/4 mark and would likely overheat if you let it keep idling.
But here’s the thing…if you run your air conditioner, the arrow goes DOWN and it stays at the 1/2 mark! Now I don’t know about you, but every other car I’ve ever owned, the WORST thing you could do was run your air conditioner while idling in traffic because you’d overheat the car. Two of the three of us have taken our cars back to the dealer to complain about this, and the dealer swears they can’t duplicate the problem yet I duplicate it every other day! But, since the car doesn’t overheat and I’d prefer to run the air conditioner anyway, I guess it’s not worth complaing about. I just want someone to explain what the hell is going on that allows EXTRA engine strain of running my compressor to somehow cool my engine.
Oh, and if it matter, the cars we are taking about are the Chevy Cavalier and Berretta (early 1990s) and a 1996 Impala SS.
Just a couple of WAG’s based on my limited experience.
Thermostats are available that open at different temperatures than stock. Entirely possible the new one doesn’t match the old and doesn’t open until a higher temperature is reached.
My '90 BMW 325i had the same odd charateristic of running a tad cooler when the AC was on even in 110º+ weather. That series has dual cooling fans, one driven by the engine pulley with a viscous clutch and one electric that came on under extreme coolant temperatures and when the A/C was turned on.
Mine also ran down around the 1/4 mark when I firt had it but a bit hotter after having a water pump and thermostat changed.
From what I have seen, and this is by no means universal, newer cars seem to have moved away from the traditional fan belts and now use one or two electric fans to pull additional air through the radiators / Air conditioning cooling coils. Radiators are now designed to be more easily cooled by forward motion of the car and need less fan assistance to cool down. Also, the faster you go, the more work the engine does so their is more waste heat, but also the more cooling you get from the air moving through the radiator. The new design is more efficient in most ways.
In come the fans. If the car has one fan, it is usually a two speed fan, and if it has two fans, then one will be The electric ones are controlled by a microchip that has a thermostat, and turns the fan(s) on when you are doing a lot of slower city driving, or hill climbing, etc, and will come on even if you have just shut the car off, which can freak you out if you don’t expect it. If you turn on the AC, then either the second fan comes on or the single fan kicks up to it’s higher speed, pretty much automatically, which provides additional cooling. The AC fan is designed for the worst conditions a car is likely to experience, which is high heat/high humidity, no forward motion. If you are moving, and it isn’t a really hot/humid day, then the fan provides enough extra cooling capacity to drop the radiator’s temp.
Add another data point. My '95 Saturn (owned by GM, just like Chevy) SL does a similar thing. If it’s stopped during a warm day, it will overheat unless I turn on the AC, upon which it’ll drop over time to the 1/4 mark. It’s the original stock thermostat, and it started happening after about two and a half years. It has never overheated while the AC is on, even under a strong load, or stopped in bumper-to-bumper traffic for half an hour. Doing the same things without the AC on, I’d have to turn the heater up full blast to avoid overheating.
When you drive with the Ac on, the car does indeed do more work and get hotter. THe thermostat opens and STAYS open. Without the AC on, the thermostat may open and close repeatedly, since open may overcool, yet closed may overheat. It takes time for the circulation to cool the water temperature down. In the meantime the temperature keeps increasing.
TO see if this is the problem, let your car idle in the driveway until the thermostat opens. Make a note of where the guage is when it opens initially. The car will cool very fast at this point since the new water that is circulating is quite cool. It will gradually rise and open again, and again. After half an hour or so the car should reach equilibrium, and the temperature remain more or less constant. If this is the same temperature that it settles on when the ac is running then you’ve identified the problem. Actually it’s not a problem since the added workload of the ac just brings you to this equilibrium point quicker. While you are driving down the road, w/o AC, the action of the wind cooling your car slows down the process giving you the appearance of running hotter.
SOme thermostats aren’t as snappy as they ought to be. THey may be set to open at 190 degrees, but may only open partially and not get completely wide open until 210-220 or so. THat’s an alternate explanation.
Still a third explanation might be a classic mechanic’s mistake. When you get your car from the factory, the thermostat gasket is installed by a machine. WHen it’s replaced it’s done by hand and the mechanic uses this goo called formagasket to give you a tight seal. Unfortunately, he may get alittle of this on your thermostat and cause it to stick.
A fourth might be impeded circulation in your radiator due to corrosion. When was the last time your fluid was changed?
It might also be the fans as well as others have opined
As a general rule if your car settles into the normal operating range and doesn’t show any tendency to keep climbing into the red, you don’t have a problem, so don’t worry about it.
The fans on many cars of that vintage use electric fans, and yours fall into that group. Since the fans draw a considerable amount of current, they use a relay to turn them on. It is very likely that the relay that turns your fan on at a certain coolant temperature has failed. No big deal, just a simple plug-in device that should cost no more than $10-$15 at most dealers’ parts counters. Installing it should not take more that 10 minutes if you know where it is and are comfortable with the idea…alternatively, most repair shops will charge you an hour of labor for it.
The reason your car runs cooler when the AC is on is that the AC uses another relay to turn the same fan on since it also pulls air through the condenser coils mounted just in front of the radiator.
My Z24 does this, too. It used to run at 1/4 (180), but when it changed from winter to spring to summer, it’s started running up near 1/2 (220) but when I turn on the air, it usually drops down to about 195-205…