Cooling system service at your location in DFW

A radiator works because of the gap between coolant and air

On a hot summer afternoon, the radiator is handing engine heat to air that is already warm.

A contributor to the Mobile Air Climate Systems Association (MACS) blog makes this point: a radiator sheds heat because of the temperature gap between coolant and air. His example is 225-degree coolant meeting 100-degree air, where the large difference lets the radiator release heat efficiently.

A 15 psi cap, a 16 psi cap and a gauge that reads in the 210s

The same author gives the arithmetic. The author puts the boiling point of a 50-50 mix of antifreeze and water at roughly 220 degrees Fahrenheit, and each pound of cap pressure raises that by about 3 degrees. A 15 psi cap lands near 265. With a 16 psi cap the author says coolant will not boil over until about 268, and some vehicles now have factory caps up to 22 psi, which push the boiling point past 285.

The author says customers, and even some technicians, start to panic when the temperature gauge creeps into the 210 to 220°F range, but with a 16 psi cap the engine can efficiently run at 220 to 235°F as long as the coolant reaches that temperature and then stabilizes. That is one author's view, and it assumes the cap holds its rating and the system is not boiling over. APEX Tech Nation, by contrast, describes a typical operating band of 195 to 220 degrees Fahrenheit. A gauge climbing into the red, steam from the hood or a warning light means pull over (see below). The cap does not make an engine run cooler; it raises the boiling point.

A cap that cannot hold its rating undoes this. APEX Tech Nation's cooling-diagnosis article gives the example of a 16 psi cap that holds only 10, which lowers the boiling point by about 18 degrees, and says replacing the cap is standard practice on a cooling service.

The mix, the water and the reservoir marks in one owner's manual

One Kia owner's manual (the 2023 MQ4 English edition posted on Kia's owner's-manual site) shows how specific a coolant requirement can be. Because the engine has aluminum parts, it calls for an ethylene-glycol, phosphate-based coolant, bars alcohol or methanol coolant, and says to add only deionized or soft water, never hard water. It allows 35 to 60 percent antifreeze, and its table lists 35 percent for an ambient temperature of 5 degrees Fahrenheit and 50 percent for minus 31. The level belongs between the MAX and MIN marks when the engine is cool. Other makes, models and markets differ, so check your own manual.

Ethylene glycol is a key ingredient in most antifreeze, and it tastes sweet. The ASPCA calls it highly toxic to the nervous system and kidneys, and the Merck Veterinary Manual puts the lethal dose at 3 to 4 teaspoons for dogs and 1 to 2 for cats. Merck says to clean up leaks right away and cover the spot with cat litter, sawdust or another absorbent.

Hot at a stoplight but fine at 65, or the other way around

APEX Tech Nation separates two patterns. On a vehicle with an electric fan, overheating at idle that cools off at highway speed points first to the fan: at speed, air is pushed through the radiator, but at idle the fan supplies the only airflow, and a failed motor, blown relay or unplugged connector produces exactly that. The reverse, hot at highway speed and normal at idle, points toward a flow restriction or coolant volume problem, such as a clogged radiator, a pump with a worn or split impeller, or very low coolant.

A repair shop's thermostat guide says a thermostat stuck closed makes the temperature creep up while idling and drop a little once the car is moving, the same pattern as a fan that is not switching on, and that a stuck-closed thermostat can overheat the engine within minutes. One stuck open gives a low gauge, weak heat and sometimes code P0128. APEX says a gauge that swings up and down often means air in the system.

A pump that spins without pumping, and the heater core behind the dash

APEX says a water pump that turns is not necessarily one that pumps: on some pumps the impeller can separate from the shaft while the pulley and bearing spin freely. Its field check puts an infrared thermometer on the upper and lower radiator hoses at operating temperature; a gap of about 15°F to 30°F is typical depending on ambient temperature and engine load, and under 5°F suggests little or no flow. A pressure-test guide says the test is best for finding external leaks such as cracked radiator tanks, split hoses, loose clamps, leaking thermostat housings and seepage at the water pump weep hole.

The heater core sits behind the dashboard. According to a fluid-handling and motion-control supplier's guide, a feed pipe carries hot coolant from the engine to it and a return pipe carries the coolant back. A heater-core article from the fluid-handling supplier RG Group lists a sweet, fruity smell from the vents, coolant dripping under the dashboard on the passenger side, and windows fogged all over (not just a hazy edge on the windshield) as signs of a leak.

When the needle climbs: the order of steps and what never to do

A national service chain's overheating guide gives this order: A/C off and heater to maximum, pull over safely, shut the engine off, and let it cool at least 15 to 30 minutes before opening the hood. That time is for opening the hood, not the radiator cap; see Kia's warning below. Coolant goes in only after the engine has cooled completely, in the type and ratio the manufacturer specifies, and the cause still needs to be found.

Kia's manual says not to remove the radiator cap or drain plug while the engine and radiator are hot, because scalding coolant and steam can blow out under pressure. It also warns that its electric fan may run even when the engine is off, so keep hands clear of the blades. The chain's guide adds no cold water on a hot engine, since the sudden change could crack parts, and warns that driving on can warp cylinder heads and crack the block.