Fuel Wash – What Is It – What Can Cause It – How To Eliminate It

Fuel Wash - What Is It - What Can Cause It - How To Eliminate It
Fuel Wash - What Is It - What Can Cause It - How To Eliminate It

There is a condition that contributes to many failures of engines generally referred to as “fuel wash”.

The only thing protecting your cylinder walls and bearings from fuel wash is, the thin layer of oil on them.

When you douse the inside of the engine with raw fuel, you wash-off that protective layer of oil.

As a result, any unburnt fuel in the cylinders can cause, fuel wash.  So, your left with a, wet mixture of raw fuel. Consequently, that literally “washes” the oil away.

Fuel Wash In Cylinder
Fuel Wash In Cylinder

Carburetors were partly to blame. Because, rich fuel mixtures, wash the lubricating oil off the cylinder walls, diluting the oil in the crankcase.

Fuel injection has all but eliminated the fuel wash problem, but it still happens. A common symptom of a potential issue with a fuel injector is, a fuel smell.

Two Simple Ways To Check For, Fuel Wash:

  1. Pull out your oil dipstick and smell the oil on it. If it smells like fuel, then raw fuel is getting past your rings and into the oil.
  2. Remove a spark plug or two and see what they look like. A plug fouling from fuel will have a fine, black sooty powder on it. Furthermore, it may smell like fuel and be wet.

What Can Cause, Fuel Wash:

Leaking Fuel Injectors

The number one cause would most likely be leaking fuel  injectors. This takes place when the engine is off. Due to pressure on the fuel rail; fuel will push itself down into the cylinder of any leaky injector and gradually drip into the oil pan. But in this case, you will observe that the engine would hard start and rough idle. Especially, when parked overnight, until the dumped fuel in the cylinder clears off.

Leaking Fuel Injectors
Leaking Fuel Injectors

So, fuel will flood out if a fuel injector is stuck open. Fuel will definitely get into the oil, when this is the case. Also, if the fuel pressure in your car is too high, that may cause; fuel to get into the engine oil.

Vacuum Leaks

Vacuum leaks in today’s computer controlled engines can cause, very serious engine damage.

Cracked Hoses Causing Vacuum Leaks
Cracked Hoses Causing Vacuum Leaks

When an engine that has computer controlled spark and fuel systems has a vacuum leak, the following events occur:

  • Because, the vacuum leak pulls in outside air, the air/fuel ratio is; altered. Causing, a lean condition.
  • The lean air/fuel condition is picked up by the exhaust oxygen sensors who, in turn; report this to the computer.
  • The computer then tells the fuel system “hey, we need more fuel”.
  • As a result, the fuel delivery is now; altered to inject more fuel.
  • The result of this is an over supply of fuel, can end in a, fuel wash condition.
  • The fuel diluted the oil on the cylinder walls until, metal to metal contact occurs.

This phenomenon can cause very serious damage to the:

Common causes are:

  • A fuel system malfunction
  • Over fueling when the engine fails to start

At this point metal to metal contact occurs and scuffing takes place. This condition is similar to and also sometimes referred to as, dry start.

Rebuilt or Reconditioned Engines

A rebuilt engine should that has been correctly assembled, will start almost immediately. But, if after a reasonable period of cranking the engine does not start, something is wrong!! At this point the compression, ignition, and fuel systems should be checked. Because, without spark, the fuel being drawn into the cylinders; will reach a saturation point and fuel wash will occur. Finally, it will severely dilute and thus weaken the oil film, and scuffing will start.

Engine Bearing Damage
Engine Bearing Damage

Although diesel fuel has increased lubricity over gasoline; fuel wash still must be avoided or, serious damage will result. Also, engine manufacturers’ manuals should be; consulted and closely followed. This includes correct fuel timing, fuel/air ratios, servicing of injectors, pumps, etc. Fuel filters and water removal systems, should be fully serviced at overhaul.


Finally, good engine maintenance, and good common sense, can all but eliminate this potentially serious problem.

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