Fluid Foaming in Automatic Transmissions: Causes, Consequences, and Possible Solutions
It is well-known that fluids (ATFs) in
automatic transmissions serve not only as a lubricant, but also perform a wide
range of other tasks. First of all, ATF is a working medium that controls the
transmission operation in the valve body unit and transmits torque in the
torque converter. ATF also helps to cool the transmission unit during active
During regular maintenance services, drivers of cars equipped with automatic transmissions sometimes can notice numerous bubbles on a dipstick when checking a transmission fluid level/condition. As a rule, this symptom indicates certain faults in the transmission operation. In most cases, transmission fluid foaming is caused by abnormal ATF levels. In this article we will consider the main causes of these problems, its negative consequences, and possible solutions.
Negative consequences of ATF foaming
It should be noted that if you see large and rare air bubbles on the dipstick during ATF check after intensive driving with high RPM rates, this does not mean that the fluid is foaming. But when there are many evenly spaced bubbles on the dipstick, it is a dangerous symptom that requires a detailed diagnostics.
The process of ATF foaming can be quite harmful for any automatic transmission. The level of transmission fluid and its density are crucial for proper operation of the automatic transmission unit. The appearance of foam, in turn, leads to negative changes in ATF density. As a result, the foamed fluid becomes compressible and cannot create the necessary pressure on friction packages. This leads to oil starvation, slippages of clutches, etc. With ATF foaming, wear products quickly contaminate the oil and hydraulic system, damage valve body channels, destroy valves, etc. Moreover, it should be remembered that automatic transmissions are extremely sensitive to overheating. The appearance of foam in ATF leads to significant reduction in heat removal capacity, thereby increasing the risk of the transmission unit overheating.
Causes of ATF foaming
In most cases, transmission fluid foaming is caused by insufficient ATF level or its overflow. If the fluid is above the norm, the foam is formed when moving elements inside the transmission unit “grab” the fluid in the pan and make ATF foamy. If the fluid level drops, the oil pump also intakes the fluid with air.Depressurization of the oil passage, leakages of ATF through seals and gaskets can lead to the fluid foaming, too. In some cases, ATF becomes foamy because of severe contamination or failure of the automatic transmission filter.
That’s how foamed ATF looks like
The foam can also appear as a result of mixing different transmission fluids, i.e. foaming is the result of a chemical reaction that causes the loss of fluid properties. The thing is that compositions of modern ATFs include a great number of chemical additives, which ensure proper operation of transmission units. As different transmission fluids include different sets of additives, they often turn out to be incompatible.
For this reason, it is prohibited to mix mineral and synthetic transmission fluids. Disregarding this rule can lead to fallout of precipitation that pollutes operating surfaces, clogs valves, channels, and other components. One more reason for ATF foaming can be the use of “old” ATF. Manufacturers recommend changing transmission fluid every 60,000 kilometers. Aggressive driving style, driving in hot weather with stop-and-start driving can reduce the service interval to 30-50, 000 kilometers without waiting until ATF starts foaming and blacken.
Possible solutions and recommendations
At the initial stage, the main symptoms of ATF foaming are kicks and jolts when driving and shifting from P and D or R. Moreover, automatics can also jerk when braking. In any case, if the transmission starts jolting, then it is time to check the transmission fluid. As has been mentioned before, the main reason for ATF foaming is the incorrect fluid level. Ii it is the case, then it is necessary to adjust the fluid level. (Note: the fluid level should be measured after a proper warm-up of the transmission unit.)
If there is a fluid overflow (when the fluid level is higher than the HOT mark), the excessive fluid must be removed. In case of the fluid overflow, the foaming will not appear immediately after the engine start, but after driving under loads and at high RPMs. The ATF removal can be done with a special fluid syringe and a thing tube.
Learn how to check ATF level accurately
In case of a low ATF level, it is necessary to
inspect the transmission unit for ATF leaks (pan, cooling pipes, half-axles installation
locations). If there are no visible leaks, then it will be enough to add the
missing amount of ATF. At the same time, it is necessary to use the same ATF
that has already been used in the transmission unit.
As has been mentioned above, there are several
reasons for appearance of foam in ATF. If the car owner notices bubbles on the dipstick,
then it is necessary to check the ATF level as well as the general condition of
the fluid (color, smell, transparency, viscosity). If there is visible ATF foaming,
then it is recommended to stop using the vehicle and tackle the problem without
any delays, since driving with foamed ATF can lead to the transmission failure
and consequent costly repair of the unit.