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Engine Compression Test Diagnose Engine Problems

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Engine compression test to diagnose engine problems is the best way to determining engine problems, engine vibrates if improper compression.

There are three important factors necessary to keep the engine running smoothly. One of them is that each cylinder has good compression. You will know how to measure the compression pressure in the engine. Improper compression may cause engine vibration due to the difference in output power among the cylinders.

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Engine Compression Test to Diagnose Engine Problems

The compression pressure values of gasoline engines are different from that of diesel engines. Refer to your car manual to know the standard compression value intended for your car model.

For gasoline engines, the compression gauge adapter is attached to a spark plug hole. For the diesel engines, the compression gauge adapter is installed to a glow plug hole. measure the engine compression pressure.

If you obtained a low compression pressure the following items are the reason for low engine compression.

1. Cylinder head gasket
2. Valve seat
3. Valve stem
4. Piston ring

Engine compression test

(1) Pour a small amount of engine oil into the cylinders through the spark plug holes then perform an engine compression test.

If the engine compression pressure is the same and did not change, the valve and valve seat contact is poor. Inspect and repair the valve seat, then perform the engine compression test again.

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If the engine compression pressure is improved, the piston ring(s) may be worn or damaged. Inspect and repair the piston ring, then perform the engine compression test again.

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The car gasoline engine is categorized by the internal combustion engine. It generates power by changing the chemical energy of the fuel into the heat that produces mechanical energy. The change of chemical energy into the heat is accomplished through combustion, thus good combustion is a key to engine performance.

The good air-fuel mixture, good compression pressure, and good ignition are the three essential elements for good combustion to occur as shown in the figure below. An engine in bad condition means that one of these three factors is not satisfied.

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The ECCS system is always in control to maintain a good air-fuel mixture and a good ignition spark. Good compression pressure is fully dependent on the mechanical factor.

Good Air-fuel Mixture

The air-fuel mixture is the material at which fuel and air are mixed to a certain ratio. The ratio of the air-fuel mixture is a significant factor that will dictate the vehicle performance such as engine power, fuel consumption, emission control, etc. A good air-fuel mixture is required for high combustion efficiency.

Air-Fuel Mixture Ratio

The air-fuel mixture ratio is calculated by dividing the weight of air contained in the air-fuel mixture by the weight of fuel, and this is called the air-fuel ratio (A/F=A by F).

This mixture ratio is expressed by the following formula:
Air-fuel ratio = Air amount (g) / Fuel (g)

Air Fuel Mixture Ratio

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When air is mixed with fuel, theoretically the air to fuel ratio in which the fuel is supposed to be burned completely is called a theoretical air-fuel ratio. The theoretical air-fuel ratio for fuel 14.7 and this is sometimes referred to as λ=1.0 (lambda).

When the A/F is smaller than the theoretical value, it is called rich since the proportion of fuel is large. When it is larger than A/F, it is called lean. Fuel consumption is better for the lean area, while the power increases for the rich area. The current A/F for a lean-burn engine is approximately as lean as 22. ECCS normally controls A/F to be close to the theoretical value.

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If the A/F ratio that is controlled by ECCS deviates from the theoretical value, the following symptoms may occur.

• Black smoke
• Misfiring of ignition
• Deterioration of fuel consumption
• Lack of power
• Deterioration of stability
• Insufficient purification of emission gasses

Air Fuel Ratio Chart

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Good Compression

Good compression means that there is no leakage of the air-fuel mixture from any gap between the cylinder, piston, and piston rings and also any gap between the cylinder, cylinder head, and head gasket. Valve timing and leakage from the valve seat are also important factors.
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Compression Ratio Formula

The compression ratio is calculated by the following formula:
V1: Volume of the combustion chamber
V2: Volume of piston displacement

Engine Compression Ratio Formula

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Compression pressure

Compressing the air-fuel mixture will increase the thermal efficiency. The reasons as follows.

• When air is compressed, its temperature will increase.
• Thus, the temperature and pressure of a gas will be high after combustion.
• The combustion time will be shortened.
• Thermal energy will easily be converted to work.
• As a result, the amount of heat that will be discarded as exhaust gas will decrease.

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Theoretically, the higher the compression ratio, the better the thermal efficiency. However, knocking will occur when a compression ratio exceeds a certain limit.

When the compression pressure is lower than the specified value, there is a possibility of compression leakage.
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When the compression pressure is higher than the specified value, there is a possibility that carbon, etc. is deposited inside the combustion chamber.
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When the compression pressure is out of standard, this will cause engine vibration due to the occurrence of irregular combustion at each cylinder. When there is a malfunction in the compression pressure, disassemble the engine to repair it as needed.

Good Sparking

If a good air-fuel mixture and good compression are obtained, engine performance will be optimized by a good ignition spark so that good combustion will occur. The condition for good sparking consists of the 2 elements shown below.

• The most appropriate ignition timing
• Strong spark

Most Appropriate Ignition Timing
ECCS always controls the ignition timing so that maximum combustion efficiency can be achieved. The ignition spark is controlled to ignite shortly before the TDC.

If the timing is delayed due to the overstretching of a timing chain or for some other reason, the ignition timing may differ. Therefore, after the repair is complete, check that the ignition timing is appropriate using Diagnostic Tools and the timing light.

Strong Sparking

A strong spark (approximately 10,000 to 20,000V) is also controlled by ECCS at all times. In other words, ECCS secures the strong spark by controlling the dwell period* to be constant from low speed to high speed at all times.

*Dwell period: In the figure on the right, the dwell period is the time period necessary for the ignition coil to be energized.

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Spark Plug

Although the spark plug is a small and simple part, and since it is used in an extremely harsh environment, its suitability is an important factor that directly influences the engine's performance. Examples of malfunctions caused by the usage of an inappropriate spark plug are listed below.

• Poor fuel consumption
• Poor startability
• Unstable idling

Recent engines use iridium-type spark plugs, explained down as shown.

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(1) By using iridium spark plugs with low wear and tear for the center electrode, a smaller diameter is achieved.
(2) Its melting point and hardness exceed that of the platinum type.
(3) By thinning a center electrode, the electrical potential will be concentrated and easily discharge sparks, so that a strong spark will be generated.

Service point of iridium spark plug

(1) Gap adjustment is not required.
(2) Cleaning should be performed in a short period of time with low pressure compressed air (0.59Mpa, 6kg/cm square, within 20 seconds).

Iridium plug installation

(1) Make sure that the seat side for plug installation contains no foreign materials, and then screw the plug manually until it stops.
(2) Tighten the plugs to the specified torque.

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Performance required for sparkplug

(1) Electric insulation (insulation performance against 10 to 30 Kv).
(2) Heat resistance (against rapid heating and cooling.
(3) Mechanical strength (against explosion pressure of 45 atmospheres or more).
(4) Sealing performance.
(5) Long life (wear and resistance of the electrode.

Wear and Tear of Electrode

Spark plugs are expendable. However, it is necessary to maintain them to ensure maximum performance. The figure on the right shows a worn spark plug and the gap has become wide after a certain mileage of use.

Spark Plug Wear and Tear

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Spark plug wear and tear

Although it varies by the type of engine or condition of use, the wear and tear of a center electrode normally range from 0.1 to 0.15 mm per distance of 10,000 km for a normal type spark plug.

Heat Range
A spark plug has its own heat range at which it works properly. The lower limit is referred to as the self-cleaning temperature and the upper limit is the pre-ignition* temperature. Therefore, a spark plug must be selected so that the temperature near the center electrode will be approximately 500 to 950°C. Because engine usage characteristics differ, such as for high-speed high power or mid-speed high torque, the spark plug heat range should be carefully chosen to correspond to the temperature of the combustion chamber.

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*Pre-ignition: A symptom that generates the combustion before spark ignition. The temperature that generates the pre-ignition symptom is approximately 950°C.

Spark plugs can be divided into the hot type and cold type as shown in the above figure. A cold type has the characteristic of high heat radiation efficiency with the short pole while the hot type has a long distance for the heat to be released to the cylinder head. The type of spark plug to be used is described in the Service Manual.

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Before the compressed air-fuel mixture gas is ignited by a spark, the gas is partially exploded for certain reasons. This phenomenon is called knocking, and the explosion speed of this abnormal combustion is very fast, and it may cause serious damage to the piston and cylinder head.

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Causes of Engine Knocking on a Car
Causes of engine knocking is an incorrect air-fuel mixture this will result in the burning of fuel or pre-ignition before the combustion cycle. Read more » Causes of Engine Knocking

The causes for the occurrence of knocking
• The normal combustion speed of the air-fuel mixture in the combustion chamber is 20 to 30 m/s.
• The combustion speed during the occurrence of knocking, or during abnormal combustion, will be extremely high, which is approximately 1,500 m/s.
• During this abnormal combustion, the impulse wave that is generated from it makes a knocking sound.
• For an automobile engine, a bore diameter of approximately 110 mm is the threshold limit for preventing the occurrence of knocking.

Preventive Measures for Knocking

• Retard the ignition timing.
• Remove the carbon deposit in the combustion chamber to decrease the compression ratio and to remove hot spots.

• Use premium high octane gasoline to raise the ignition point of the fuel.
• Change the spark plug heat range (to cold type). Using the above measures, knocking can be prevented. In most cases, high-performance engines are designed to use premium gasoline to obtain maximum performance as well as to prevent knocking.

Inspect Spark Plugs to diagnose engine

The spark plug is an important part because it can be a telltale of the combustion condition of an engine. Check the condition of each spark plug for normal combustion or abnormal combustion.

Properties of Gasoline

Gasoline is a mixture of hydrocarbons (HC). Actual gasoline is typically comprised of the additives mentioned below.
• Metal deactivator
• Antioxidant
• Antirust
• Coloring agent
• Detergent

Octane Number

The main factor concerning engine performance is the octane number. The octane number is the antiknock property of gasoline expressed by a number. The larger the figure for this property, the less the gasoline will generate self-ignition and knocking.

The standard that expresses octane numbers contains the two types written below.
• RON: (Research Octane Number) (indicates antiknock property at low speed)
• MON: (Motor Octane Number) (indicates antiknock property at high speed) RON is normally used.

Octane rating for RON

• The octane rating for regular gasoline is approximately 90 RON.
• The octane rating for premium gasoline is approximately 98 RON.

Backfire is the phenomenon that occurs when the air-fuel mixture is blown back to the intake manifold while the intake valve is opened, and then the air-fuel mixture is ignited. Backfire occurs because the combustion speed of the air-fuel mixture becomes very slow when the ignition timing is retarded due to an excessively lean air-fuel mixture ratio. Backfire is caused by the following conditions.

• Engine start during the cold condition
• Rapid acceleration during engine warm-up
• Carbon deposit on the intake valve
• Clogging of the air cleaner


Afterburn is the phenomenon that occurs when the unburned air-fuel mixture is discharged into the exhaust manifold and is burned in the exhaust pipe. Afterburn occurs because the unburned air-fuel mixture is discharged into the exhaust manifold due to the improper ignition timing attributed to the lack of the air-fuel mixture in a cylinder (lack of intake air), or because the air-fuel ratio is too rich. It is necessary to pay proper attention to afterburn since there is a risk that afterburn may damage the catalyst because of the high temperature of combustion. Afterburn is caused by the following conditions.

• Water in the fuel
• No fuel cut occurs during deceleration
• Spark plug malfunction

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