You must think I'm psychic. I have SOME people fooled into thinking I'm good, but I'm not that good. I should say, "hold your computer closer to the car so I can see what's wrong". (That won't work either).
Seriously, if I were peeking under the hood, I'd start by checking the coolant level in the reservoir, then in the radiator. If the reservoir is full, that suggests the engine overheated and the coolant expanded into the reservoir, and that overflowed onto the engine. Coolant hitting the hot exhaust parts would make a huge cloud of steam and there would be a mess all over the engine. Some of that coolant will be drawn back into the engine from the reservoir when it cools down and contracts, so there might be enough to reach the temperature sensor and make the gauge work. A good suspect for all of this is an electric radiator fan that didn't turn on when the engine got up to the right temperature. The clue for that is the radiator fan is never needed at highway speed because natural airflow is more than sufficient, but once you come down to city speeds, and especially when stopped at a red light, the fan will turn on typically within a minute or two.
A hose could be leaking too, but when the coolant cools down and contracts, there will be no vacuum to suck coolant back in from the reservoir. The reservoir will remain at whatever level it is at and never change, but the radiator will be low or empty. To identify that, your mechanic will fill the radiator with water, wash the entire area, then watch for where that water is leaking out. He can easily pressurize the system if he has to. Once that is repaired, the concentration of water will be too high and the threat of freezing in cold weather exists. If it's time for a maintenance flush and fill, now is the time to do that. If that is not on the schedule, he will drain some of the coolant and discard it, then add straight antifreeze. The goal is to get the concentration back to approximately 50 percent antifreeze and 50 percent water. It's okay to go with more water, by percentage, if you live in a southern state, but if it's possible to get down to minus 35 degrees, you must have 50 percent antifreeze, no less.
Antifreeze doesn't carry as much heat to the radiator as water does, but besides lowering the freeze point, it has additives in it that wear out in about two years. That's why we never run with straight water.
The least desirable cause for this is a leaking cylinder head gasket. Most commonly that will cause coolant to be lost and go out the tail pipe as white smoke, or combustion gases will get forced into the cooling system and will push the coolant into the reservoir where it will likely overflow onto the engine. That gas can pool under the thermostat which will cause it to not open. Thermostats have to be hit with hot liquid to open. Hot air won't do it. A thermostat that stays closed will prevent hot coolant from flowing to the radiator to give up its heat. On some car models the hot coolant doesn't circulate at all, so it doesn't flow through the heater core in the dash. You'll get cold air from the heater even though the engine is real hot. On some car models when the thermostat is remaining closed, the radiator will feel cold but the coolant will still flow through the heater core and you'll get real hot air from the heater.
A dead radiator fan is probably the most likely suspect right now since you didn't run the engine long enough to get hot after the tow. Most leaking head gaskets start out with more subtle symptoms, namely white smoke from the tail pipe, or an unexplained partial loss of coolant and the need to fill the reservoir once per week or month. That can go on for many months before a definite diagnosis is possible.
Your mechanic can perform a chemical test at the radiator to check for a leaking head gasket. That involves drawing air from the radiator, while the engine is running, through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present, that liquid will turn bright yellow. If this problem only occurs at random times, the test may not be conclusive, but typically the leakage is always there, then the test takes just a minute or two to confirm the bad news.
Be aware too that while a leaking head gasket usually causes overheating, (due to the thermostat remaining closed), an overheated engine, due to some other cause, can cause a cylinder head to warp and therefore leak past the gasket. That wasn't really much of a problem 30 years ago, but today almost all engines are made from cast iron and have heads made of aluminum. Those parts expand at different rates leading to eventual leaking head gaskets. Every manufacturer has some problems related to that and some engines are more tolerant of being overheated than others.
Tuesday, April 5th, 2016 AT 9:11 PM