People need to know how to diagnose a bad thermostat. It’s one of those often misunderstood car parts. When an engine has a cooling system problem people want to head directly for the engine thermostat. This raises a bunch of questions. For one, how long can we expect the thermostat the last? Second, is there a for sure fire way to diagnose a bad thermostat.
Not only will we answer these burning questions, but we’ll talk about other common possibilities for engine overheating problems. That’s right, if you have an overheating engine you’re just as likely to have a malfunction in other automotive components as with the thermostat. We’ll cover a few of the big ones like the engine cooling fans and the water pump responsible for circulating the antifreeze.
Diagnose a Bad Thermostat
Before we talk about how to diagnose a bad thermostat let’s talk about the two most common symptoms of the problem. When the thermostat gets stuck in the closed position the engine overheats. However, if the thermostat should get stuck in the open position the engine doesn’t get hot enough to produce heat for the passenger compartment. So let’s talk about how to identify the thermostat as the root cause of either of these problems.
The sure fire way to tell if you have a bad thermostat is to test it. I’m talking about testing it the old-fashioned way. You remove the thermostat and place it into a pot of boiling water. The thermostat should open quickly and completely. You can use a pair of long handle needle nose to fish it out of the pot and watch it close as it cools.
With that said, there’s a lot of people on the Internet talking about how you can feel the upper and lower radiator hoses to determine whether the thermostat is working or not. This is a half truth that can give you an idea if the thermostat might be a problem. Generally speaking, the upper radiator hose should be hot on an engine at full operating temperature. The lower radiator hose should be cooler. However, when this is not the case it doesn’t tell you that you have a bad thermostat, it tells you that a cooling system problem exists.
Diagnose Engine Cooling Fan Problems
Whether you have one big electric fan or two smaller ones these circuits remain relatively simple even on newer automobiles. The fans usually have a dedicated coolant temperature switch to tell them when to kick on. When the switch closes current flows to a cooling fan relay. This in turn closes the relay and allows battery voltage to flow to the cooling fan motors.
Your vehicle specific auto repair manual has excellent diagnostic charts to walk you through step-by-step diagnosis. However, we can discuss a few of the common issues found in electronic engine cooling fan systems. Sometimes the problem originates at the device that triggers fan operation. The coolant temperature switch is often mounted near the thermostat housing.
Although the diagnosis of the switch can vary with vehicle year, make and model these are often simple two or three wire switches. This allows us to jump the connector to turn on the fan. If the fan comes on when jumped, but won’t engage when the vehicle is hot you just found a bad cooling fan switch. If the switch is good, it’s time to move on to the cooling fan relay. Again we can jump the relay to send power directly to the fan.
Finally, we can check for power directly at the fan motor connector. Although we recommend an automotive meter for the test you can get away with using a test light. If you find power at the fan motors and they are not spinning, you have uncovered a failed cooling fan motor. Sometimes you can tap these with a hammer and they will start spinning. This indicates a brush or winding problem with the fan motor.
Bad Water Pump Causing Overheating
We all know that the water pump is the heart of the cooling system. It provides the movement or the circulation of the engine coolant necessary to stop the engine from overheating. Although this isn’t as important as how to diagnose a bad thermostat or cooling fan it remains a possibility in the engine overheating scenario.
The most common problem with a water pump remains a leak at the input shaft seal. This allows coolant to dribble out of the inspection hole. When enough antifreeze leaks out, the system can develop air pockets that restrict the flow of the pressurized coolant. Also in this situation there isn’t enough reserve to flow through the radiator.
With that said, there is an issue with some water pumps where they actually have impeller problems. The impeller is the internal fins that rotate to create the water pump pressure. Some Chevrolet Silverado models with the 5.3 L engine are known for failure of the impeller. In addition, some fancy German BMW automobiles have a problem where the impeller breaks loose from the input shaft. This is a hard to diagnose problem as the shaft spins, but the impeller does not. Many skill mechanics misdiagnose this condition as a bad thermostat.
Additional Problems that Cause Engine Overheat
There are a bunch of miscellaneous items that can cause an engine to overheat. With that said, they just don’t happen as often as finding a bad thermostat, cooling fan problems or even water pump issues.
This stuff can find its way into the passageways of the radiator and reduce the efficiency of the heat exchanging qualities. When discussing additional problems that cause an engine to overheat we need to mention that any cooling system leak can cause this issue. Whether it’s a worn through radiator hose or a more serious problem like an engine coolant leak from the intake manifold, when the antifreeze is low overheating becomes likely.