Causes and Solutions for Foaming in Oil

Ashley Mayer

Foaming is a common problem with oil-lubricated components. It can be difficult to troubleshoot, and for this reason, accurate testing to determine the root cause of the foaming is essential.


Foam is a collection of small bubbles of air that accumulate on or near the surface of the fluid. In severe cases, the foam can leak out of the machine through breathers, sight glasses and dipsticks. Foam is an efficient thermal insulator, so the temperature of the oil can become difficult to control. The presence of air bubbles in the fluid can lead to excessive oxidation, cavitation, the reduction of lubricating properties of the oil and hydraulic system failure.


The causes of foaming are many. The most common include:

  • Water contamination

  • Solids contamination

  • Depleted defoamant (possibly due to the use of excessively fine filtration and electrostatic separation technologies)

  • Mechanical issues (causing excessive aeration of the fluid)

  • Overfilling of the sump with splash- and bath-lubricated compartments

  • Cross contamination of the fluid with the wrong lubricant

  • Contamination of the fluid with grease

  • Too much defoamant additive, either by incorrect formulation or by incorrect reconstruction (sweetening) of the additive package


The first tests attempted should be water quantification and particle counting. The formation of air bubbles is often aided by such contaminants, as they provide a nucleation point for the air bubbles. It is important that degassing techniques are used to prepare the fluid prior to the particle count. If you have a foaming problem, the air bubbles in the fluid would otherwise trigger an abnormally high particle count, and that could lead to attempting an incorrect solution.

If the particle count does not reveal any significant contamination, try a patch test using very fine-rated (one micron or less) filter paper. Scrutinize the patch closely under high magnification. You might also want to run a pentane-insolubles test (ASTM D4055-E). This will quantify very fine contamination which can provide nucleation material.

Ask your laboratory to run tests for foam tendency and foam stability. These tests are described by ASTM D892 and are run together. Foam tendency describes the amount of foam generated immediately after the fluid is agitated and aerated, and foam stability quantifies the amount of foam remaining 10 minutes after the cessation of aeration.

The test allows for an “Option A”, which is in place to ensure the defoamant is well mixed and suspended in the lubricant prior to the commencement of testing. This option should be requested for gear oils due to the nature of the defoamant chemistry commonly used in these fluids. Comparison of the tendency and stability can indicate the presence of a mechanical issue, rather than an additive/contamination issue.

Cross contamination of the fluid by another lubricating fluid commonly contributes to foaming and other issues. Additive interferences prevent the defoamant from working properly. To check for this, analyze a sample of new oil from your storage for its elemental signature and compare it to a sample of used oil. The additive signature of the used oil should be similar to that of the new oil, but slight differences (due to additive depletion) are likely.

Pay close attention to elements commonly found in additives (calcium, magnesium, boron, molybdenum, phosphorus, sulfur, etc.) which are present in the used oil but not in the new oil. Also, keep an eye out for elements in the used oil which might indicate grease contamination, if this is possible.

Analyzing a sample of new oil from your storage facility and used oil together will let you know if the component got topped off with the wrong fluid, but it won’t trigger any alarms if cross contamination took place during lubricant formulation or in the storage facility. If you can’t see any signs of cross contamination here, see if you can secure a sample of new oil from a different blending batch.

The use of Fourier transform infrared (FTIR) spectroscopy should also be considered, once again using samples of new oil and the used oil. Advanced analysis of the spectra is necessary. FTIR is particularly useful if fluids with different base stocks have been mixed.

Possible Solutions

Hopefully your testing will have alerted you to some possible reasons for the excessive foaming of the oil.

In almost all cases, an oil change, or at least a partial drain and refill, will be required. If some type of contamination was the root cause, a flush will be required, too. This can become expensive for large-volume systems; so in certain cases, reconditioning of the lubricant may be considered. Be aware that this does not always work, and is likely to be a stay of execution rather than a pardon.

Make sure you address the root of the problem before conducting the drain and flush. For particle and water contamination, concentrate on managing contaminant ingress as far as possible before resorting to filtration. This is particularly important with gear oils where fine filtration can strip the additive from the fluid.

If cross contamination with another oil is the issue, address the solution with fluid identification (color-coding) and training. If grease contamination led to the foam formation, make sure the correct relube quantities and frequencies have been calculated and are being adhered to. Mechanical issues might be due to tank design, oil return-path geometry or suction-side piping air leaks.

Troubleshooting foaming can be a challenging process, but by a process of elimination, you should be able to identify and correct the root cause.

Read more on oil foaming issues:

How to Fix a Foaming Problem

Controlling Oil Aeration and Foam

Eliminate Conditions That Cause Foaming

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