Compression Check: Specifications

Interpretation of the Results

The indicated compression pressure are considered within specification if the lowest reading cylinder is within 75% of the highest reading.

CAUTION: If engine oil is sprayed into the combustion chamber, after carrying out the measurement run the engine at 2000 rpm for about 15 minutes, in order to burn the oil and prevent damage to the catalytic converter.

If the measurement on one or more cylinders is much lower than the specified value, spray some engine oil into the combustion chamber and repeat the compression measurement.
If the reading greatly improves, the piston rings are damaged.
If the reading stays the same, the cause is either damaged valve seats or valve stem seals.
If the measurements for two cylinders next to each other are both too low then it is very likely that the cylinder head gasket between them is burnt through. This can also be recognized by traces of engine oil in the coolant and/or coolant in the engine oil.

Excessive Engine Oil Consumption

The amount of oil an engine uses will vary with the way the vehicle is driven in addition to normal engine-to-engine variation. This is especially true during the first 16,100 km (10,000 miles) when a new engine is being broken in or until certain internal components become
conditioned. Vehicles used in heavy-duty operation may use more oil. The following are examples of heavy-duty operation:
- trailer towing applications
- severe loading applications
- sustained high speed operation
Engines need oil to lubricate the following internal components:
- cylinder block cylinder walls
- pistons and piston rings
- intake and exhaust valve stems
- intake and exhaust valve guides
- all internal engine components

When the pistons move downward, a thin film of oil is left on the cylinder walls. As the vehicle is operated, some oil is also drawn into the combustion chambers past the intake and exhaust valve stem seals and burned. The following is a partial list of conditions that can affect oil consumption rates:
- engine size
- operator driving habits
- ambient temperatures
- quality and viscosity of oil
Operation under varying conditions can frequently be misleading. A vehicle that has been run for several thousand miles on short trips or in below-freezing ambient temperatures may have consumed a "normal" amount of oil. However, when checking the engine oil level, it may measure up to the full mark on the oil level indicator due to dilution (condensation and fuel) in the engine crankcase. The vehicle then might be driven at high speeds on the highway where the condensation and fuel boil off. The next time the engine oil is checked it may appear that a liter of oil was used in about 160 km (100 miles). Per liter oil consumption rate is about 2,400 km (1,900 miles) per liter.
Make sure the selected engine oil meets Mazda specification and the recommended APE performance category "SG" and SAE viscosity grade as shown in the vehicle Owner Literature. It is also important that the engine oil is changed at the intervals specified for the typical operating conditions.