1998 Mercury Sable • 120,000 miles

The car puts out moderate but not hot heat only when you are driving, the faster you drive the hotter the heat but no heat when idling or going slower (35mph). Does not over heat. Have flushed radiator and heater core and replaced thermostat
Brandi fabiano
March 4, 2012.

When the car is at idle it runs cooler because the number of combustions compared to number while driving is significantly different. If you held the idle at 4000 RPM at idle it would get hot like you were driving. However the lack of a load on the motor would destroy it. You would think the airflow from the car moving would cool it off but the flow is actually not as great from movement as you would think. This is because it is not good for fuel economy to let a lot of air flow made from the car moving into the engine bay or even under the car. The fan is more than adequate and you might see that car makers have shyed away from large open front grilles that are open and let air into the engine bay. It creates drag on the car as it gets trapped in theengine bay and creates a lot of drag. It used to be for performance to let cool air into intake but it has been found that the air drag far eceeds any Horsepwoer gains. With that said the fan is essentially the source of air for cooling the engine and there are a lot of hot explosions going on in the cylinders which make the temperature slgihtly higher. Even 5 degrees hotter than at idle will make the heater feel very different. Todays A/C systems are only required to make the car 20 degrees Farenheight cooler than outside and when it is 100F it can be hot in car. Heaters are more efficient and the heater actually aids to cool the engine. A heater uses a part called a heater core that looks just like a small radiator which has coolant running through it which is reulated by a valve. The valve opens more to allow more coolant flow and therefore the temperature of the heater is higher. So, one things to check is that the heater valve is opening all the way. It is located on the intake hose of the heater core near the firewall. Ther are two hoses that run to and from the engine and to the firewall near the center. They are close together, one right on top of the other. You will see the valve on one of the lines at the firewal. Most are cable actuated. Check to make sure the cable is sevurely fastened to the bracket which holds it in place. It is actually clamping the cable case so that the inner cable can move without the outer cable sheath moving so the cable can move the valve arm as far as possible. Sometimes they get loose and any movement will keep them from opening the valve all the way thusly not allowing as much coolant as possible to run into the heater core. Also check that the valve is moving all the way. Turn the heat off and note the postion of the valve arm and then turn the heat all the way up and it should be 90 degrees from the prior position which is full open.
You might want to see if you can get a higher temperature thermostat for the car. The stock thermostat starts to open at 193-200 Degrees Farenheight and is fully open at 224 Degrees Farenheight. Thermostats will have the fully open temperature stated when they are sold. So, you might try to find a special cold temperature thermostat that is fully open at 230 Degrees Farenheight. Look around the internet and try to stick with a reutable comapany like STANT thermostats. One thing you will note that freight trucks do is to have a radiator blanket over the front that can be opened or closed more or less to adjust for temperature. This is generally not only for running on highway temperature, but they spend the night in their cabs sleeping with heat coming from the engine idleing and the regular heater system supplying heat. So they can make the radiator opening smaller to make it a little warmer for the haeter. The cooling fan does not come on at the same temperature as the thermostat, it comes on when the temperature gets a little hotter. So, covering the radiator can help keep it warmer at idle yet not overheat the motor at driving or hgihway speed. Again aerodynamics is not a concern so airflow will be different from your car. If space allows, you might try a piec of cardboard to cover some fo the radiator and let it idle to see if it warms it up some and then drive it to make sure it does not overheat. If you don't have a temperature gauge, this is somwhat risky, but the overheat light on the dash will come on and you must immediately pull over and turn the car off and be able to remove the cardboard. If you try this the car actually gets hotter when the is turned off. So, it is possible the raiator cap might release pressure and hot steam will come at at high pressure. So wait a few minutes before you open hood to remove cardboard.
You might also try just replacing the thermostat and if you have not changed the ani-freeze in the last 2 years change it as well. In most temperatures the ratio of water to anti freeze is 50/50. But, you will see on anti-freeze containers that the manufacturer suggests a ratio with much more anti-freeze than water. This protects the motor to colder temperatures. Anti-freeze does nto cool off a motor as well as water and having new fluid may help as anti-freeze becomes more like water over time in its anti-freeze and heat disipation qualities. The greater amount of anti-freeze will also help to warm it up some.
Another thing you can do which is probably the best option is to change the fam temperature switch. You can get higher temp switches or you can get adjustable relay fan switches all in one that will overide the whole system. They are available at most auto parts stores and are not hard to install. They have a sensor that can be pushed into the fins of the radiator, DO NOT DO THIS. Simply attached the sensor with calbe ties or other means near the upper raiator hose at the radiator. Pusing it into the fins can cause damage. You can just turn the turn on fan temperture the a lower setting to accomadate for not following the maufacturer direction. This is just my opinion not to shove the sensor in between the fins when the near sme can be acheived via aforementioned. The realy will have an adjustable rheostat that you can play with as far as when the fans come on. You could sit with the car at idle and turn the fans on just after heater gets how. They are genrally not a huge range of adjustment and are within most engine specifications as to not allow a motor to overheat. So, this may be the way to go.
I reccomend changing the anti-freeze first and maybe even use a higher than reccomended ratio of anti-freeze too heat up the system just a little which may be enough. A new thermostat can be replaced at this time since the coolant is out.
Sometimes car manufacturers use different temperature thermostats depending on where the car is going to be sold. So, you mgiht try asking a dealership in Canada what the thermostat temperature is. It could very well be hotter and since it is made by manufacturer designed for your car is always the best.
So, you don't necassarliy have an issue. If you did the temperature of the coolant would dropp rather drastically at idle being that it is so cold. If the car was purchased in more souithernly state the thermostat may very well be cooler. Finally changin the anti freeze and raito will help and is safe to do. Antifreeze is actually very good at lubricating and I had a car tht I ran in the state of Maryland with pure anti-freeze and no water and it never overheated. So, it is required for maintenance and has low risk factor.
I also had an Altima in Maryland tht would do the same thing your car is doing in temperatures above what your car sees. So I don't think you have an issue. You may have a slow to close thermostat that when replaced may help and the other items I mentioned. Use the cardboard over the radiator as a last resort as it is risky to an extent. You only need a few degrees to get the heater to feel warmer. Doing the maintenance cheacks and services may add up to what you need and are necassary anyway.
Good luck and I hope this information helps you to get a little warmer.