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When most of us think of modern cooling systems, we tend to think overheating , leaks , and maybe even poor cabin heat . But another valid reason for maintaining and inspecting cooling systems is their direct impact on the engine's emissions level and fuel economy.

Let's start by reviewing the combustion process. The engine compresses the mixture to raise pressure. This, in turn, increases the temperature of the air / fuel mixture and aids in combustion. If the combustion chamber is too cool, the process needs extra fuel to insure combustion. Think of the old carbureted days of thermostatically controlled chokes that enrichened the mixture on cold starts. There is even a powertrain DTC assigned to this very issue … P0128, Engine is cold too long . Some manufacturers have unique code assignments for this issue. If the combustion chamber temperature gets too hot, Nox emissions will increase. While normally the job of the EGR system, it can only cool so much. If the engine itself is running hotter than normal, the benefit of added EGR is reduced.

Diagnosing the P0128 Diagnostic Trouble Code

The ECM (Engine Control Module) is charged with keeping an eye on any system that will have an impact on what comes out of the tailpipe. Typically, the ECM monitors the ECT (Engine Coolant Temperature) sensor after a cold start, looking for the engine coolant to reach normal temperature in a preset time frame. If the ECM does not see its threshold value in the allotted time, it assumes there is a problem in the cooling system, preventing normal operating temperature from being attained. This is a 2-trip code, meaning that if the ECM gets the same results after 2 consecutive tests, it will record the code and turn on the MIL (Malfunction Indicator Lamp).

The code criterion provides clues on what to test. First, the ECT sensor must be reading correctly. The ECM can only base its decisions on the information it receives. Second, the cooling system must be functioning properly … coolant levels must be correct, no air in the system, cooling fans cycling normally and the thermostat opening and closing as it should. Fortunately, all these factors can be tested at one time with minimal effort.

In my experience, the thermostat is usually the culprit. But to make sure, I set up a few monitors of my own during the test. First, I hook up the scan tool and display the ECT reading, positioning the scan tool under the hood where I can watch it. Second, I use a temperature probe attached to my DVOM (Digital Volt-Ohm Meter) and place the probe in the radiator fins as close to the outlet hose from the engine as I can get. (Make sure it is the outlet from the engine and not the return.) Last, I use an infrared temperature gun to measure temperature on the inlet side of the thermostat housing. Now I can start my test.

Start the engine and watch the ECT reading. It should climb smoothly. You can even graph the data if your scan tool has this feature to see if the signal drops out during warm-up. I use the infrared gun to measure the temperature on the inlet side of the thermostat housing to compare to the ECT reading. While not accurate, it is usually within a few degrees if the sensor is reading correctly. The color of the surface, and the material it's made of, will affect your temperature gun's reading. At the same time, I'm watching the temperature reading on my DVOM. If it, too, climbs almost immediately after start up, I know the thermostat is not closed. Of course, you can tell this by feel as well, but bear with me … I'm not done yet.

If the DVOM reading stays cold, I keep monitoring the engine temperature readings. As soon as I see the ECT reading near the specification for opening of the thermostat, I look to see if the outlet temperature begins to increase. If so, the thermostat is opening as it should. If not, the thermostat is stuck closed, or there is potential air in the system affecting its operation. If the thermostat opens prematurely, it may have been replaced with one of the incorrect rating, or have a damaged spring or seat.

If all is well to this point, the fault is not present at the time of testing. This could indicate a problem in the wiring from the ECT sensor to the ECM, and electrical checks may be in order. Remember, the ECM can only base its decisions on the information it receives, and if the ECM thinks the engine is cold it will still adjust its initial fuel calculations accordingly.
Of course, if the cooling fans come on immediately and stay on, the cooling fan control system needs to be diagnosed and corrected before proceeding.

While You're At It

Since I'm already connected, I continue to run the engine to normal operating temperature, making sure it stabilizes. Running at idle, with no airflow across the radiator, will allow the engine's cooling system to be stressed to perform at its best. I can test the operation of the electric cooling fans by measuring the temperature at which they come on, and by how much of a temperature drop I see as they cycle. In my experience, the fans should come on between 210-240 degrees F as measured at the radiator, and continue to run until temperature drops by 40-60 degrees F. Try out your measurements on known good cars to establish a diagnostic baseline of your own.

On vehicles with viscous fans, temperature should remain relatively stable. If engine temperatures persist in rising after running them for awhile, it's time to look at system coolant flow and air flow as contributors to the problem. Now we're in the area of ​​higher than normal engine temperatures and emissions … specifically the formation of Nox. Higher combustion chamber temperatures can also cause "spark knock", and this, in excess, can lead to engine damage. Troubleshoot these systems as you would normally.


Proper cooling system function is also an important contributor to fast idle or idle surges complaints on many vehicles that use Idle Air Control systems that rely on coolant temperature as a control. In these systems, typically, a wax pellet is built into the valve and is heated by engine coolant. If the lines are restricted, or there is air in the system, the valve never adjusts and the engine can race at a fast idle speed or more. In some vehicles, this situation is complicated by the ECM's need to control idle speed. The ECM will first try to adjust the IAC system, and failing that, start varying timing in an attempt to get the idle speed to where it should be. Look for IAC systems that use both an IAC stepper motor and a Fast Idle Air Valve when dealing with this type of complaint. A quick test to verify the fault is to remove the intake boot and close off the IAC port in the throttle body with your finger. If you feel high vacuum, or the engine idle tries to smooth out and stabilize, you're on the right track.

Today's engine cooling systems are more complex than ever, with multiple flow paths and some with multiple thermostats and water pumps. While their main job is protecting the engine from excessive heat, they also play an important role in emissions control and fuel consumption. So next time you speak to your customer about cooling system maintenance, educate them on the total job the cooling system is expected to do.

Automotive Cooling Systems and Emissions