What is Automotive Fluid Analysis?

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  • Written By: Jeremy Laukkonen
  • Edited By: Allegra J. Lingo
  • Last Modified Date: 04 October 2019
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Automotive fluid analysis is the process of determining the condition of various oils and other liquids that are used by cars and trucks. Many automotive fluids degrade over time and should be replaced at regular intervals. Fluid analysis may consist of a visual inspection or some type of test that can indicate whether the fluid needs to be flushed or changed. Some automotive fluids that can be subjected to regular analysis are engine oil, transmission fluid, coolant, and brake fluid. These fluids may degrade over time, while others can be damaged by heat and slowly become less effective.

Most experienced automotive technicians can gauge the state of a fluid by using their basic senses of sight and smell. An experienced tech generally knows what fresh transmission fluid looks and smells like, and can tell if it will need to be changed soon. Antifreeze also tends appear differently as it begins to break down, and it is often possible to tell if an engine has gotten excessively hot by smelling the coolant. Other fluids, such as motor oil, can be more difficult to judge unless they have not been changed for a very long time. Brake fluid is another example that a technician typically must rely on timed intervals to judge when it should be flushed out, unless the rubber seals in the system have began to break down.


A number of chemical tests exist that can assist a skilled technician or allow a layman to perform their own fluid analysis. These tests usually consist of a strip that can be dipped in the fluid or a patch that can have fluid dripped onto it. The strips or patches may be made of a material that will react to impurities or other substances in the fluids, which can indicate that a change or flush is necessary.

Fluid analysis of engine oil, transmission fluid, power steering fluid, and other oils usually involves a blotter test. The technician may place a few drops of oil on a specialized blotter and then examine the way it spreads. Clean oil will tend to have a different color than dirty oil, and the presence of contaminants can create distinct stratifications. The technician may have a chart that shows oils in different conditions to compare the results to.

There are a few different methods to test antifreeze fluid. A specific gravity hydrometer will typically show how much protection the coolant offers against freezing, while a pH testing strip can reveal whether it has become acidic. Refractometers are sometimes used to test antifreeze as well, since they can show the freeze protection and whether there is oil contamination.

Brake fluid analysis typically involves testing for the presence of water. Since most brake fluid is hydrophilic, it will tend to attract water molecules throughout time. This can change the physical properties of the fluid, leading to brake fade and generally poor performance. Vehicles that are driven regularly and have brake jobs often enough may never experience this, though others can accumulate a significant amount of water in the brake fluid. A brake fluid test strip will typically change color if there is water present, indicating that the system should be flushed.


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