Keep It Cool

The primary function of the cooling system is to remove the heat of combustion from the engine in order to maintain a safe and consistent operating temperature. When problems occur with this system, the ensuing trouble can take many forms. These include overheating, no heat during winter driving, increased emissions and abnormal engine wear.

Although automobiles have undergone many changes over the last 35 years, the cooling system hasn’t changed a lot. It has, though, become more reliable by way of enhancements to a proven technology. Nonetheless, problems do occur. Let’s review diagnosing cooling system and water pump problems so you can quickly find the root cause and fix the problem.

Cooling System Basics

The primary purpose of the cooling system is to maintain engine temperature in a range that will provide satisfactory performance and emission levels under all expected driving conditions. This is done by circulating coolant throughout the engine to absorb excess heat. This hot coolant also flows through the heater to warm the vehicle in cold weather. This excess heat is then expelled when the coolant flows through the radiator.

When the engine is cold, the thermostat (which is nothing more than a temperature-sensitive valve) is closed, preventing coolant flow through the radiator. Once the engine reaches operating temperature, the thermostat opens, allowing coolant flow through the radiator and the entire cooling system. Figure 1 shows a typical coolant circulation pattern.

Checking Coolant Flow and Thermostat Operation

In order to determine if coolant is flowing through the system (and the thermostat is opening when the engine temperature reaches normal operarting temperature), a simple test can be performed. When the engine is cold, remove the pressurized radiator cap. Then, remove a small amount of coolant. Start the engine and allow it to idle until the engine reaches operating temperature. The thermostat should open and you should see coolant flow when you look down the filler neck of the radiator.

An alternate test (and less preferred) is to start the engine when it is cold. Allow it idle until the normal operating temperature is reached. Feel the upper radiator hose. If it is hot, coolant is flowing.

Just because coolant is flowing doesn’t mean that the thermostat is operating properly. Thermostats sticking closed are a rare occurrence. On the other hand, thermostats sticking open can be a problem. If the thermostat is stuck wide open, the coolant can never reach operating temperature. During normal operating conditions, the thermostat closes a bit when the coolant temperature drops, then opens all the way when the temperature increases (this is an ongoing cycle). So, how do you know if the thermostat is bad?

If the coolant temperature is below operating temperature as indicated on the temperature gauge on the instrument panel (usually there is a colored zone indicating where the needle should be during normal operation), there’s a problem. Also, if it’s winter and you just can’t get any heat, there’s a problem. Finally, computer controls will illuminate the Check Engine light if the thermostat is bad.

Overheating and Coolant Leak Problems

Now that we’ve looked at below normal coolant temperatures, what about the other extreme, overheating? When does this occur? In most cases, overheating occurs when the coolant level drops below a safe level. The coolant recovery container (see Figure 2) has coolant level marks (usually Max and Min) that allow you to monitor the coolant level. When the vehicle overheats, you’ll usually see a low coolant level in this container.

How does the coolant level drop? Obviously there’s a leak somewhere in the system. The leak can be from the water pump (we’ll discuss that later), or somewhere else in the system, such as a hose, the radiator or internally within the engine.

Pressure testing the system is a good way to locate a leak. Attach a cooling system tester (see Figure 3) to the radiator and apply 15 psi of pressure. If the pressure drops more than 2 psi in 2 minutes, inspect all attachments points for an external leak. Re-apply pressure back to 15 psi and shake all the hoses to simulate driving conditions (some leaks occur due to engine movement).

If no external leaks are found after the pressure drops, disconnect the tester. Start the engine and run it until the normal operating temperature is reached. Reconnect the tester. If the needle fluctuates, that is an indication of a combustion leak. If the needle does not fluctuate, rev the engine a few times. If coolant and/or steam is emitted from the tailpipe, the coolant leak is the result of a faulty head gasket, cracked engine block or cracked cylinder head (such damage can result from extreme overheating).

Leaks can also be detected by using an ultraviolet light method. This can also be used in conjunction with the pressure method. Add one ounce of leak detection additive (available from your Mopar parts dealer) to the cooling system. Set the heater control to the Heat position, or the maximum temperature setting. Start the engine and let it run until the normal operating temperature is reached.

Aim a commercial ultraviolet light at the various components of the cooling system. Because the additive is highly visible under a black light, any leaking coolant will glow a bright green color. This test works well because sometimes leaks are very difficult to see under normal light.

Water Pump Problems

Quite often, when cooling system problems are diagnosed, water pumps are usually included in the discussion. The reason is simple. Leaking water pumps are probably the most common cooling system problem. And usually the leak is due to the pump exceeding its service life.

The telltale sign that a water pump is leaking is a wet stream of coolant running down on the pump body and the components below the pump. This leak occurs at the weep hole, as illustrated in Figure 4. A small stain below the weep hole is considered normal. Not surprisingly, a water pump leak can cause a drop in coolant level that will result in an overheating condition.

Cooling Fan Problems

Probably the most noticeable change in the cooling system since the dawn of the environmental age in the early 1970s is the electrical cooling fan. Prior to that time, cooling fans were run directly off an engine belt. The fan operation is controlled by a solid state relay. The relay provides voltage to the fan motors that is proportional to the pulse width received from the Powertrain Control Module (PCM). The duty cycle ranges from 30 percent (low-speed operation) to 100 percent (high-speed operation). This allows infinitely variable fan speeds for reduced fan noise, optimal A/C performance and better engine cooling.

It should be noted that the electronics that control the cooling fans do not, as a general rule, fail; however, the fan motors do from time to time. The usual suspect is the motor bearing. When this occurs, under most circumstances, the Check Engine light will not illuminate. As long as the relay receives the pulse width from the PCM, the PCM assumes the fan motors are operating. So, if you’re experiencing a rise in coolant temperatures, and the coolant level is good, check the operation of the cooling fans.