Why a Temperature Gauge Fluctuates

Why a Temperature Gauge Fluctuates

It can be a frustrating situation when your temperature gauge fluctuates on the dash. In this scenario, drivers note the gauge starts heading for the hot side and then quickly pulls back towards the middle where it belongs. This can create an uneasy feeling for motorists attempting to reach their destination as fears of overheating are validated by the radical movements of the indicator. This article will discuss why a temperature gauge fluctuates and some diagnostic steps that can help pinpoint the problem area.

How the Temperature Gauge Works

The gauge is an indication of engine temperature. Manufacturers put cold to the left and hot to the right. The normal range is in the center where the gauge should point when everything is operating as per design intent. It’s the job of the sending unit to relay the proper information to the instrument cluster. An automotive coolant temperature sender is a variable resistor often mounted near the thermostat housing.When the engine is cold the resistance in the sensor is high and the current flow to the gauge forces the needle to point to the cold side. As heat builds and the engine warms up the resistance in the sensor decreases, thereby increasing the current flowing to the temperature gauge. This forces the needle to start moving towards the hot side.

Testing the Sending Unit

When you have a fluctuating temperature gauge the first thing to find out is if the condition is a false indication. You must determine if there is a problem with the gauge and sending unit or if there is an engine problem that is causing a real fluctuation. An ohmmeter can be used to test a standard coolant sending unit.

Resistance is measured and compared against the values provided in a service manual.More importantly, the value should change in proportion to the actual temperature. As an example if resistance is checked before the engine is started and then rechecked after a few minutes of operation, the results should be different than the original recorded value. Many professional mechanics will leave the meter connected or back probed to the sending unit and verify a smooth change in resistance as the engine goes through its warm-up cycle. Any sudden changes in resistance value or drops to zero indicate a problem with the sending unit.

Coolant Thermostat Problems

After, it has been verified the antifreeze is full, the gauge is telling the truth and the engine is actually fluctuating in temperature, a good place to start diagnosis is the coolant thermostat. This device is responsible for the smooth circulation of the system. When closed, it restricts water flow to allow the engine to heat up quickly to its desired temperature. And when it reaches this goal it opens smoothly to allow full circulation.

Because of its inherent design, thermostat operation is a slow and steady process. If the valve itself is hanging up, or binding it is possible for it to stay closed longer than desired and then snap open. Problems like this can be verified by removing the thermostat and placing it into a pot of boiling water and physically observing the opening and closing. Testing before replacement is recommended, so you can verify that the problem has been located.

Engine Head Gasket Problems

The worst-case scenario and a likely cause of genuinely fluctuating engine temperatures is a cooling system that has become air bound. Air can obstruct the free flow of coolant. What happens is these air bubbles eventually work themselves up to the coolant reservoir in essence clearing the obstruction. This causes a cooling system that doesn’t flow and then suddenly operates correctly.

The tough question to answer is how air is getting into the cooling system. Most often the answer is compression leaking from the cylinder into the cooling system from a small leak in the head gasket. A cylinder head gasket is designed to separate and seal these two systems from each other. Since compression is high and cooling system pressure is low the leak is usually in the direction of the cooling system.

The leak can be small enough that a bubble develops and then dissipates until enough compression leaks in where the problem repeats itself. This can create a hard to find frustrating situation, leaving the driver to ask why my temperature gauge is fluctuating.To verify that compression is leaking into the engine cooling system exhaust gases can be tested for in a few different ways. Automotive companies have developed coolant test kits that will turn a certain color when exposed to exhaust gases. Auto repair shops with emission analyzers will sometimes place a probe over the coolant reservoir in an attempt to sniff exhaust gases. Finally, if the head gasket leak is severe enough bubbles or foam may be seen in the reservoir itself.

In conclusion, the first thing you have to figure out is if the gauge is telling the truth. This is done by testing the sending unit and the instrument panel. Dealership technicians have two tools at their disposal that make this a quick task. They can connect a scan tool and monitor the sensor resistance. Also they have testers for the instrument cluster to check gauge operation. Also note manufacturers can have separate cooling sensors to perform different tasks. It’s possible to have one for the gauge and computer and then a separate switch used to operate the engine cooling fans.

When everything tests okay with the gauge and sender the next step is to find out why the engine is experiencing a fluctuation in temperatures. If you’re lucky a sticking or binding thermostat is the cause of your engine overheating problems. If you’re unlucky a head gasket issue can also cause the coolant system to become air bound. Although this is the worst-case scenario, it is possible to verify this situation before making a decision on how to proceed with the repairs.

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