What Is Normal Oil Consumption?

What Is Normal Oil Consumption?

Oil consumption is normal and necessary for internal combustion engines to run properly. At what level does oil consumption become abnormal, you may ask? This question is difficult to answer since different engine designs may lead to different levels of “normal” consumption.
Picture a car with an oil consumption problem. Most would imagine a car belching a smoke-screen for several blocks. Others might envision a vehicle which leaks a lake of oil on the driveway.
Neither of the above examples describe our typical call relating to oil consumption. The typical car we hear about consumes around 2 to 3 quarts of oil every 3000 miles. Is this normal consumption?
Possibly, but one needs to investigate further.
First, where did the oil go? Oil is lost by either an external or internal oil leak. What may appear to
be a small external loss could actually turn out to be excessive. Did you know that one drop leaking
every minute from an engine would add up to 7 gallons in a year? Also, one drop of oil leaking from
a vehicle every 20 feet would lead to 1 quart of oil lost every 100 miles, which equals 7.5 gallons
every 3000 miles! Since a small leak may often go undetected, it is very important to carefully and
thoroughly investigate external leaks before looking for other possibilities.
External leaks may be due to leaky valve cover gaskets, oil pan gaskets, front or rear main seals or
cracks in the crankcase or valve covers. To make leak detection easier, clean the engine before you
start searching. Using special dyes in the oil may also help identify evasive leaks. Keep in mind that
the evidence of a leak may not be found near the actual source. A clogged or inoperative Positive
Crankcase Ventilation (PCV) system may lead to oil consumption indirectly by increasing crankcase
pressures and forcing oil from the gaskets and seals. Crankcase breather elements, PCV valves, hoses
and related items should be inspected and replaced as needed, or as recommended by the
manufacturer, to prevent these problems. If there are no signs of external oil loss, the leak is probably
internal and will be more difficult and expensive to locate and repair.
Blue smoke exiting the tail pipe signals an internal oil consumption problem. Even if blue exhaust
smoke is not apparent, internal oil consumption may still exist. If an engine burns as much as one
drop of oil on every firing stroke it will use more than a quart of oil every two miles! The most likely
sources of internal oil consumption are worn valve guides and worn piston rings. If you think you
have an internal oil consumption problem, check with your local mechanic about performing a
leakdown test, a compression test or a ring seal (blow-by) test. A qualified mechanic will use these
tests to pinpoint the source of internal oil consumption. Other possibilities of internal and external
consumption are listed below.

Internal Oil Consumption
Improper oil levels Valve cover gaskets
Incorrect engine oil viscosity Oil pan gasket
Clogged PCV System Front and rear main seals
Worn valve stems and guides Cracks in the crankcase
Worn, broken or stuck piston rings Leaking oil drain plug
Improperly installed piston rings Porous crankcase casting
Worn ring grooves Cylinder Head Gasket
Cracked or broken piston lands
Incorrect pistons
Improperly honed cylinders
Distorted cylinders
Worn or damaged main bearings
Worn or damaged cam shaft bearings
Bent or misaligned connecting rods
Excessive oil pressure
Blown cylinder head gasket
Clogged oil passageways
Fuel dilution of the engine oil
Clogged crankcase breather element
Intake manifold leak

External Oil Consumption
Valve cover gaskets
Oil pan gasket
Front and rear main seals
Cracks in the crankcase
Leaking oil drain plug
Porous crankcase casting
Cylinder Head Gasket

An increase in oil consumption is most likely due to a combination of several of the above items.
Also, oil loss can be masked by other conditions such as fuel dilution. In this case, the fuel will
evaporate if the vehicle is driven under highway conditions for an extended period of time giving the
appearance of excessive oil consumption. In reality, the drop in oil level was a result of a change in
operating conditions and evaporation of the fuel in the oil.
Steps to Identify Abnormal Oil Consumption
1. Consider the year and model of vehicle, the type of engine, the type of service the
vehicle is driven in and how the engine was maintained.
2. Was there a drastic change in consumption? A part may have broken.
3. Were there recent changes in operating conditions?
4. Check for external leaks. Small leaks can lead to big losses.
5. Then, check for internal leaks.
Obviously, most engines begin to consume more oil as they wear. We recommend three basic steps
to minimize oil consumption for the life of your car. First, provide proper preventative maintenance.
Next, use a quality motor oil meeting your automobile’s manufacturer’s specifications. Finally,
change the oil and filter every 3000 miles or three months, whichever comes first. There is no way to
completely eliminate all oil consumption. In fact, that would be harmful for an engine. Problems
with oil consumption can be greatly reduced, however, by following the above advice.

stokes and poise

Stokes are the way kinematic viscosity is measured and basically measures how fast a steel ball bearing falls through a solution. Poise is a measure of the internal resistance of a liquid and basically tells us how fast it flows . The difference between Stokes and Poise is that Stokes is Poise divided by the specific gravity of the liquid.

fuel dilution

1) fuel dilution will naturally cause viscosity thinning due to the mixture of a low viscosity fluid (fuel) with a higher viscosity fluid.

2) Fuel in the oil will volatize. Some will be recycled through the combustion chamber and burn off. Some non-volatile components will be left behind as solids. These will tend to thicken the oil.

3) Fuel chemical interaction with the oil will cause increased oxidative thickening. For some oils, this thickening will tend to offset the thinning due to fuel dilution.

4) There are some highly active aromatic hydrocarbon molecules in fuel that do nasty things to some oil and VII molecules, snipping the long chain molecules apart. This seems to happen with some oils that use VIIs and causes what Terry Dyson calls chemical shear or aromatic damage to the oil.

5) Fuel is a solvent, and as such will soften the tribological additive layers on the bearings. This softening or washing causes an increase in additive depletion and increased wear of some bearing metals in some engines.

As a side note, if fuel does indeed cause aromatic damage to the VIIs, that would also subsequently cause a reduction in HT/HS, since HT/HS is primarily adjusted using VIIs in many oils.