How Transmission Fluid Gets Into an Engine

by Tiana Pyer-Pereira
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Classic Car Engine Refurbished image by Janet Wall from

Transmission fluid is a thin, slippery liquid that lubricates and cools the moving parts in your transmission. Its movement is controlled by pumps, valves and an internal computer that governs the operations of your transmission. Transmission fluid can sometimes be found in a car engine, and is usually either added by mistake during an oil change or introduced purposely as part of an engine flush.


Transmission fluid serves three purposes: it lubricates the transmission, cools its parts while the car is operating, and transmits power from the engine to the transmission by providing fluid pressure. Transmission fluid performs similar functions to engine oil, but is designed for a different machine and is thinner, slipperier and less viscous.


Different transmissions require different fluids. Automatic cars take automatic transmission fluids. Manual transmissions can use a wider array of liquids, including motor oil, hypoid gear oil or automatic transmission fluid. Check your owner's manual to see what your car needs.


The main reason transmission fluid would be found in your engine is due to a mechanic's mistake. Your transmission is a closed system that consists of a number of seals, pumps and channels to guide the movement of fluid from one part to another. Transmissions connect at almost no point with your car engine. An internal computer regulates fluid amounts in various parts of the transmission, making accidental leakage of transmission fluid into the engine almost impossible. Internal seals are designed to leak liquid onto the ground, not internally to other parts of the car. If you suspect contamination, it may be that your mechanic accidentally switched out engine oil with transmission fluid. Some people put transmission fluid into their own engines on purpose in an attempt to flush out the engine's machinery and clean it out.


Since transmission fluid essentially performs the same task as engine oil, it's not the end of the world if you find it in your engine. However, since the fluid is designed to work with a different set of machinery under different strains, it's not smart to operate your car with only transmission fluid in your engine, especially not for an extended period of time.


Some people believe transmission fluid will help flush out their engine, clean it and help the machinery run more smoothly. In most flushes, a quart of oil is drained from the engine and replaced with a quart of transmission fluid. The engine is run for about half an hour. Then the fluid is drained and replaced with engine oil. Though most agree these flushes are relatively harmless, not everyone is convinced it's effective and some insist it's worse for your car in the long run.

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