The compression indicator is one of the most important parameters in the operation of an internal combustion engine. With its help, you can determine the serviceability of parts of the cylinder-piston group, the degree of wear of the CPG or piston rings. This indicator depends on the compression ratio of a particular engine (the ratio of the working volume to the piston stroke length). The more boosted the engine is with a higher compression ratio, the higher its compression parameters.
Over time, the CPG parts wear out, gaps appear, and the machine loses its power. The total loss from such work can reach 15-25 percent. The engine runs intermittently, does not display its rated data, and exceeds average fuel consumption. Therefore, there is a need to check the compression in the cylinders, and this can be done with your own hands, without having unique knowledge.
Methods for checking compression in cylinders
You can measure pressure in the field with only a set of wrenches and a rag on hand. But such a measurement will only answer the question of whether it is present in the cylinders or not. It is impossible to measure compression correctly and set its exact value. Checking engine compression is a simple procedure that you can do yourself. However, this will require a special device - a compression meter.
How to check compression on gasoline and diesel engines
Checking compression in engine cylinders using various methods allows you to more accurately determine the problem area where you should concentrate your efforts. You can take measurements in your own garage, without resorting to the services of specialists. If you decide to measure this extremely important indicator yourself, then you need to arm yourself with the following arsenal:
- The measuring device itself is a compression meter.
- Charged car battery.
- Candle key.
- Defective starter.
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Experts often carry out measurements on a cold engine. If you do not have proper experience in carrying out such work, then you must first warm up the engine. After which the air filter is dismantled and the low-voltage wires are disconnected.
A compression gauge is a regular pressure gauge with a special adapter. The device is connected to the spark plug hole and thus a measurement takes place in each cylinder while simultaneously starting the engine at idle. The compression gauge is held for several seconds. As soon as the arrow stops growing, the device is disconnected. It is advisable to perform this procedure several times, and then display the average value. It is a completely normal situation when the data obtained differs from those certified by the manufacturer. After all, during the operation of the car, natural wear of the piston group parts occurs, which helps to reduce compression. A discrepancy within 10% is considered acceptable.
If some deviations from the norm of compression of a gasoline engine are acceptable, then for a diesel engine everything is much more serious. It is worth taking measurements of a diesel engine not only to determine the condition of the piston group, but also to obtain the temperature range at which stable operation of a “cold” diesel engine is possible. In order to measure the pressure in the cylinders of a diesel engine, it is necessary to turn off the power, leaving only the starter in working condition. When measuring, it is necessary to observe, perhaps, the most important condition - the crankshaft must make 200-250 revolutions per minute.
To measure this parameter in the cylinders of a diesel engine, the following conditions must be met:
- Turn off the fuel supply;
- Unscrew one nozzle;
- Make sure the battery and starter are working properly.
It is necessary to de-energize the fuel supply solenoid valve along the line. After which the compression gauge is connected to the nozzle hole. The device must have a measurement limit of at least 60 atmospheres.
Devices for measuring compression
To check the compression value in the cylinders we need a compression gauge. This is a device consisting of several parts. These include:
- special pressure gauge. It is better to take a part with a scale marked from 0 to 35 Bar. This parameter can be measured in atmospheres, kgf/cm2. or MPa, but the generally accepted unit of measurement is Bar, equal to 0.99 atmospheres;
- a flexible hose that screws directly into the pressure gauge. Equipped with a pressure relief valve. This will help measure compression on both gasoline and diesel internal combustion engines;
- one or more attachments that are screwed into the engine in place of the spark plugs;
When choosing a product on the market, it is advisable not to buy the cheapest meters. As a rule, their durability is short. But if you are not a car mechanic and do not plan to use the device often, purchasing a professional kit is also pointless. You should pay attention to models in the mid-price segment. The price of such devices is 750-1100 rubles.
| Device brand | Minimum cost, rub. |
| ZMZ-406 | 510 |
| Will measure | 625 |
| Avtodelo | 670 |
| Friend PRO | 750 |
| Friend | 1100 |
| FuelMerPro VAZ, GAZ TopAuto | 1290 |
| JTC | 3135 |
Engine compression: what is it and what is it used for?
Compression, in simple terms, is the pressure generated at the highest point of movement of the crankshaft during the compression stroke. Factors influencing changes in the compression ratio can tell a lot about the condition of the engine, its level of wear and possible problems that affect its normal functioning and the behavior of the car as a whole.
Measuring engine compression using instruments
Such factors include timing defects, wear of the piston group, coking of parts and many others. Deterioration of the technical parameters of the power unit, a sharp jump in fuel consumption - all these are the consequences of changing the compression value up or down, regardless of what specific reasons were the impetus for this.
Pressure is measured in atmospheres. Some pressure gauges use other units of measurement - bar, Megapascal.
Factors affecting internal combustion engine compression
The compression level directly depends on the volume of incoming air flow, which, in turn, depends on the problems and defects listed above. Separately, it is worth mentioning the throttle valve, which has a significant impact on compression in the cylinders: its position is regulated by air flow. Its intensity, in turn, depends on the air filter: drivers often forget to clean and change it when it gets dirty, which leads to a deterioration in its capacity.
A dirty air filter reduces engine compression
Compression values can vary greatly depending on the set gas distribution phases: if errors are made during their installation, the closing moment of the intake valve changes, which moves the cylinder in a certain direction.
The compression ratio can vary depending on the width of the clearances in the valve actuator. For example, the pressure may decrease if the gap is too small, causing the closing of the intake valves to be delayed. At the same time, valve overlap increases - the angle at which the valves open at the same time. This also has a negative impact on compression levels.
Due to the low temperature of the internal combustion engine, the air flow entering a cold engine does not have time to expand and warm up sufficiently, without subsequently creating the required pressure - accordingly, the compression value of the cylinders also drops.
One of the reasons for reduced compression is valve clearances
The presence of valve clearances leads to air leaks. Their intensity is influenced by certain factors that have a similar effect on the amount of compression:
- Motor temperature. As it increases, compression also increases, since the distance between the parts decreases;
- Engine oil supply. A tighter fit of the engine's rubbing parts to each other is ensured by an oil layer on their surface, which also makes it possible to reduce gaps and reduce air leaks through them;
- Fuel supply. The oil film formed on the inner surface of the cylinders when fuel drops enter is quickly washed away, which can stall the operation of the entire internal combustion engine, increases friction and reduces the density of contact of surfaces to each other;
- Depressurization of pressure gauge hoses, check valve, high force applied by springs;
- Increase in crankshaft speed.
What does engine compression affect?
The engine compression level affects various processes and engine parameters:
- Completeness of fuel combustion;
- Engine oil consumption;
- Ease of starting the engine. A decrease in pressure below normal makes it much more difficult to start the engine, especially in the cold season;
- Correct operation of the cylinders. Low compression can increase the risk of engine stalling;
- Power. The lower the compression, the lower the power.
Preparing the engine for measurements
The operating instructions for the VAZ 2106, 2107, 2108 models do not contain specific recommendations after what mileage it is necessary to measure compression. However, it is worth making it a rule to carry out this procedure along with replacing the candles. Standard spark plugs last 15-20 thousand km. When replacing parts, it would be useful to measure the pressure in the cylinders - this will help to identify malfunctions in time. There are signs when this procedure should be carried out earlier:
- the engine does not start well when cold;
- the motor does not produce the required power and traction characteristics;
- one or more cylinders do not work (troit);
- increased fuel consumption;
- The oil guzzling has begun, and bluish smoke is pouring out of the exhaust pipe.
Before measuring compression, you need to prepare the engine. It should be heated to an operating temperature of 70-90 degrees. If it is a two-stroke boat unit - up to 60-80 degrees. We stock up on a standard set of keys, as well as a special spark plug attachment. Now everything is ready for measurements.
Compressometer
Measuring compression indicators is best done with a compression meter operating on the principle of a pressure gauge. If the latter measures tire pressure and is designed for light loads, then the compression meter is a more powerful unit. Compare the pressure inside the engine and the tire, and then the difference between the two devices will become visible. However, the principle of their functioning is largely similar.
Conventional compression gauge
Today, there are three types of compression meters.
- For gasoline units.
- For diesels.
- A universal option used for diagnosing power units of any car.
In addition to the fact that compression meters come in different types, they also differ in models.
A compression meter is a diagnostic unit that is part of the mandatory tools for maintenance. Experienced motorists use it to analyze the technical condition of the engine without using expensive service stations. A good compression meter allows you to carry out diagnostics yourself, without the help of an assistant, which is also considered a big plus. As soon as I noticed problems with the engine, I took it and checked it.
A prerequisite for a high-quality meter is the presence of a tip on it that allows you to screw the measuring device into the spark plug hole. Without such a threaded output, the measurement accuracy is reduced significantly.
It is also recommended to purchase a meter that is equipped with a compression breather in the tip itself, which is screwed into the spark plug hole. If the valve is located directly below the scale, the readings will be less accurate.
A conventional compression meter is most often used to determine the compression level in garage conditions. However, this measurement can be called uninformative, although it is still possible to clarify the overall picture.
For a more complete diagnosis, it is recommended to carry out the test using special equipment, preferably at a service station. For example, such an installation as the AGC is used to monitor the condition of the CPG. The measurement option is no different from the compression meter, but the AGC helps to measure the vacuum values, not the compression values in the cylinders.
AGC is also used to detect leaks that are associated with the combustion process. The installation helps determine the effectiveness of valve fit, assess the condition of the pistons, and detect coking or stuck rings.
Instructions for checking engine compression
The following provides step-by-step instructions for determining compression on a gasoline carburetor or injection engine.
- Warm up the engine.
- Remove the spark plugs from the spark plugs.
- For models with a carburetor, you need to reset the switch terminal.
- Disconnect the fuel pump hose to prevent the supply of fuel to the combustion chambers. You can take measurements without disconnecting the fuel line, but then gasoline will get on the cylinder bore and can remove the oil film, which will subsequently lead to scuffing.
- We unscrew all the spark plugs from their seats to provide the crankshaft with minimal rotational resistance.
- Screw the hose of the measuring device into the threaded connection.
- The assistant sits behind the wheel, presses the gas pedal all the way and turns the starter. The pressure gauge needle will begin to oscillate and stop at a certain position. To correctly determine compression, you need to limit yourself to the first four strokes. That is, the engine must complete one working cycle. With the engine running, the exhaust valve will then open and the pressure will drop. Therefore, even a faulty unit can “inflate” good values.
- We check all four cylinders and analyze the results.
The procedure for determining compression on diesel units is fundamentally similar to the instructions, but there are several points:
- Diesel engines have glow plugs. Compression must be checked through these technological holes.
- Power is supplied not by a carburetor or injector, but by a high-pressure fuel pump (HPF). If the engine is equipped with an archaic mechanical injection pump, then you can use a shut-off valve and then remove the fuel line. To turn off an electrical device, simply remove the corresponding fuse from the unit.
It is also possible that it is simply impossible to measure compression on a hot engine. This happens when:
- the car simply won’t start;
- The motor has been removed from the vehicle.
In this case, you have to measure the compression when cold. If there is no battery, a starter is connected. The procedure is absolutely no different from the one described above. However, the readings of the compression device will a priori be lower due to the presence of thermal gaps in the motor. It is worth subtracting 25-30% from the optimal value.
If the unit does not have a starter, then the compression can be measured using a special pneumatic tester. The device forces air into the cylinder and creates maximum pressure there. And the second dial shows the amount of leakage.
If you don’t have specialized instruments at hand, you can check for compression at home. A thick rag is inserted into the spark plug well and the starter is turned. A working engine with proper compression will simply push it out due to the created pressure. Some craftsmen even create their own homemade compression meters, but their serviceability is questionable.
The video below will help you understand all the nuances of the procedure in more detail.
Deviation of compression indicators from the norm
Compression indicators are very important for both gasoline and diesel engines. Deviation of measurements from the norm can lead to unpleasant consequences requiring repair work.
Values above normal
Problems associated with engine performance are often accompanied by certain symptoms, one of which is increased compression. There are a large number of reasons for jumping pressure indicators, one of which is incorrectly set valve timing. The direction of incorrect adjustment of the timing drive affects the compression - it increases or decreases.
Incorrect adjustment of the timing belt and valve timing can cause excessive compression
The appearance of carbon deposits in the combustion chamber or on the valves can also cause an increase in compression ratios. It appears mainly due to the use of auto chemicals, low-quality fuel or motor oil, and engine overheating - the higher the temperature at which it operates, the more soot accumulates in it. As a result, this can lead to a reduction in the volume of the fuel mixture and an increase in compression.
The appearance of carbon deposits due to the use of low-quality liquids
The next factor is the “occlusion” of the valve stem seals. It is detected after a visual inspection of the engine after removing the spark plugs. Excess lubricant accumulates as a thin film on the cylinder walls and indicates the need to replace the caps and increase compression.
Oil film on the surface of the cylinder due to the “occlusion” of the valve stem seals
Values below normal
The most common and easily solved problem of low compression is a dirty air filter. The part begins to leak air poorly, which negatively affects the pressure. A lack of air in the fuel mixture can be caused by a failure or incorrect operation of the throttle valve or its sensor and cause similar consequences.
A dirty air filter is one of the reasons for reduced engine compression
Compression indicators may decrease due to low engine temperature, which is associated with incorrect operation of the cooling system. Too low a motor temperature does not allow the air to expand and fill the required volume, which causes low compression.
The deterioration of engine parts, its depressurization, the formation of large gaps between components and elements of the power unit, wear and burnout of valves, the piston and its rings, failure of the cooling system - all this can contribute to a decrease in compression indicators. These factors can be provoked by anything, the main thing is to identify all the symptoms in time and eliminate the malfunction.
Elimination of low compression without repair
A major engine overhaul to restore normal compression levels is not always necessary. You can resort to the use of special substances that will help return the pressure in the cylinders to normal.
Known to many car owners, engine additives that guarantee incredible results can help increase compression. Their action is to remove dirt and restore damaged parts of parts. Despite the thin layer formed, it is enough to cover deformed areas. Of course, this helps to increase compression to normal.
Special additives for car engines
Low quality fuel can contribute to the formation of carbon deposits on vehicle parts, which also negatively affects compression. This can be eliminated by pouring a small amount of engine oil into the spark plug holes and leaving the car like that for half an hour. A clean lubricant will clean the parts and wash away all the fumes. After carrying out such a procedure, compression indicators soar almost instantly.
Data analysis
If you do everything correctly, you will be able to take correct values. Different types of engines have their own standards, with which it is worth comparing the results obtained.
| Motor type | Minimum value, Bar | Optimal indicator, Bar |
| Old gasoline engines with a compression ratio of up to 8.5 | 11 | 13 |
| New engines, with a compression ratio of 8.5 | 12 | 14-15 |
| Diesel units | 22 | 30 |
Measurement procedure
It all starts with preparation, as the basis for the competent conduct of any operation. In this case, it includes checking the condition of the engine, since the correctness of the measurement depends on this. For example, if even slight deviations are noticeable in the operation of the motor, they can affect the compression indicator during measurement.
Preparation also includes selecting the necessary tools:
- fully functional battery;
- pipe wrench;
- actually, the compression meter itself.
In some cases, especially for beginners, it is recommended to use the help of an assistant.
The measurement process itself looks like this:
- the motor initially heats up to a temperature of 50-60°;
- the spark plugs are unscrewed from the machine (using a pipe wrench or spark plug wrench);
- The crankshaft sensor is disconnected from the connector;
- the meter lead is screwed into the spark plug hole;
- the assistant turns on the ignition and presses the accelerator pedal;
- after the arrow on the meter stops, you need to write down the data;
- repeat the same procedure on all cylinders.
Why is the sensor disconnected? It's simple. This is done to prevent the controls on the computer from going wrong. In addition, in this way you can turn off the fuel supply to the injectors.
Compression can be measured on both a hot and cold engine.
| Hot engine | 1 cylinder | 2 cylinder | 3 cylinder | 4 cylinder |
| Open throttle and injectors disabled (ODZ) | 12.5 | 12 | 12 | 12 |
| Closed throttle and injectors disabled (ZDZ) | 11 | 10.05 | 10.05 | 11 |
| Open throttle connected injectors (fuel enters cylinders) | 12 | 11 | 11 | 11 |
| Cold engine | ||||
| Open throttle and injectors disabled | 12.3 | 11.5 | 11.5 | 11 |
| Closed throttle and injectors disabled | 12 | 11 | 11 | 12 |
| Open throttle connected injectors (fuel enters cylinders) | 13 | 12 | 12 | 12 |
Troubleshooting
Now we know how much Bar should be in the cylinders. If the compression measurement shows lower values, then it is necessary to troubleshoot the parts of the cylinder-piston group.
- The most harmless is the occurrence of piston rings after inactivity. It is treated by simply pouring decoking agent into the spark plug channels with a syringe;
- piston burnout. Diagnosed by probing the piston head with a thin wire through the hole;
- deformation, burnout or stuck valve. The cylinder simply does not work at low speeds, and the pressure value is critically low;
- worn rings and valve stem seals. Diagnosed using a nutrometer. Signals include increased oil consumption or blue exhaust smoke;
- a crack in the bridge or a chip in the piston. The oil enters the combustion chamber, where it burns, producing bluish exhaust smoke. The part must be replaced.
Of course, when serious breakdowns are diagnosed, the car body should be sent for service and defective parts should be replaced, but timely diagnosis will help to avoid unnecessary expenses. And measuring compression in the cylinders is just such a procedure.
