External temperature reading keeps rising when driving. Inside the car temperature falls no matter if weather is cold. Vehicle: Buick park avenue 1991.

First of all the guage inside the vehicle is not very accurate. The temp reading will rise and fall about 20 or so degrees when the engine idles as opposed to when it is moving, the reason is airflow through the radiator and the water pump speed.

Probably a lean fuel issue . Engine coolant temperature sensor reading warmer than it actually is causing a too lean mixture , restricted injectors, restricted fuel filter , weak pump or bad regulator on return systems causing too low of fuel pressure , Mass Air Flow contamination .

that is the opening temperature for the thermostat
it opens at 160 degrees which is good for hot weather operation or another with 180 degrees which is good for cold weather operations
The differences is 160 opens sooner to start coolant circulation to keep operating temperature low where the 180 keeps shut and allows the operating temperature to rise up as soon as it open the cold air will quickly cool the coolant.

When you said "it wouldn't start" did it turn over (starter cranking the engine) or did it act like the battery was dead and wouldn't crank, even though all the dash lights lit up?

If it simply didn't crank over but then by you shifting it out of park, then back into park, may indicate a park/neutral switch problem.

It may have been something as simple as the shifter, when the vehicle was last parked, didn't fully engage the park/neutral switch into the park position. BUT by you moving the shifter out and back into park it engaged the switch.

Cold weather can do some strange things sometimes when it comes to mechanical devices. IF it acts up again, simply try the "trick" that worked the last time.

It may just be temperature related and may not give any more problems once the weather warms up.

Hope this helps.

You may have developed a vacuum leak that the idle air controller is trying to compensate for. Or, it could also be caused by a dirty mass air flow sensor. You need to post the engine type and the other conditions the symptoms occur. It's hard to diagnose a problem on such limited information. Conditions such as engine temperature, foot on brake, in or out of gear are extremely important factors to take into consideration when solving engine performance problems.

Could be the neutral safely switch is still not adjusted properly. With your foot on the brake and the transmission in neutral, move the shifter a little bit in either direction while holding the key in the start position. If the starter engages, the switch needs to be adjusted.

Thermostat may not be working properly, hot coolant has to circulate via the heater core in order to get heat. Normally operation temperation should be 180 to 200 degrees.

If the vehicle is an manual gearbox, hold the clutch in until the engine has taken properly. If it is a manual rather start i neutral with the park brake on than in Park. Once the engine has started, let it idle for a minute or so before driving off slowly. Until the temperature gauge has at least reached the 1/4 mark, try keep engine revs below about 2500. All the while keep an eye on the temperature gauge, if it rises very sharply and the fan does not come on, there is a possibility that the radiator is frozen and the engine will boil and burst a hose. If the temperature does rise suddenly, stop and carefully feel if the radiator is still cold. If it is, shut off the engine and wait till the radiator has thawed. This will take a while but can be hastened by starting the engine for a few minutes to warm the engine compartment to help thaw the radiator. When water is circulating freely, you should see a sudden drop in temperature. Do not rev the engine too much, the incoming water will be ice cold and could crack something. This is not something that occurs often, especially if you try park the car where air cannot readily blow through the radiator overnight.

If you are not getting a signal to your gauge, your car can still run. You just won't know if your car is overheating until it is too late. A bad thermostat won't keep your gauge from working. The temperature sensor that sends the signal to the gauge will keep it from working.

The thermostat helps your car in extreme weather. It helps your engine warm up in extreme cold so your heater will work sooner. And it helps you car cool itself off in extreme hot weather. As your engine water temp rises, the thermostat will open allowing the water in the radiator (that has been cooling) to flow into the engine block. The water that was in the block will go into the radiator to begin cooling.

In fair weather, you don't need a thermostat.

If the fans are electric, then replace the fan control module.
If the fan is belt driven, replace the fan clutch.

There is a chance that the thermostat is failing, but if the temperature fluctyation occurs during driving, as opposed to stopped and idling, then it's probably the fans.