How to measure the compression of gasoline and diesel engines

Cylinder piston group (CPG) and gas distribution mechanism (Timing) in the process of internal combustion engine prone to wear. The condition of these engine parts affects compression. Engine compression is one of the important parameters for the normal operation of the unit.

Which shows the compression measurement: major faults

Measuring engine compression with your own hands is the easiest and most affordable way, which allows you to detect various faults of the power plant. The main task in self-measurement of compression of a diesel or gasoline unit is to detect deviations from the compression pressure in the engine cylinders from the allowable.

We also recommend reading the article on gas distribution phase change systems. In this article you will learn about existing solutions, design features and advantages of using such systems for changing the phases of gas distribution on modern internal combustion engines.

The solution to measure compression allows you to identify the fault as one, and in several cylinders. Additionally, the measurement of engine compression may indicate problems with the valves and the tightness of the valves to the seats. Measurement of compression can be done in different ways. Let's consider, how to properly measure the compression of a diesel and gasoline engine.

Equipment for self-measurement: compressor and AGC

To measure the compression yourself, the most commonly used is a simple compressor. It is worth adding, measured in this way uninformative, but the general picture of the state of the CPG is able to clarify. Also diagnostics of the cylinder-piston group is carried out by means of the special equipment, which is at the service station.

The main recommendation when choosing a compressor is the presence of a threaded tip. This tip allows you to screw the compressor into the hole of the candle. If the compressor does not have such a tip, then the accuracy of measurements decreases. It is also desirable to have a compressor with a pressure relief valve in the head, which is screwed into the hole of the candle. Devices with a valve, located under the manometer, less accurate and have the ability to underestimate the final evidence.

If the owner of a gasoline car is enough to unscrew the spark plugs, then the measurement of compression on a diesel car requires the removal of injectors or glow plugs. This will require experience and additional tools. The compressor for diesel must have a special adapter. An adapter is required for this, to securely fasten the tip in the hole for the injectors or glow plugs.

You can also use a special device to assess the condition of CNG and other elements of the internal combustion engine, which is called the analyzer of tightness of cylinders (AGC). The method of measurement is similar to working with a compressor, but the device does not record the compression pressure in the cylinder, and vacuum parameters.

This device determines the presence of leakage through the combustion chamber, valve fit efficiency, the burn of the bottom of the piston is found out, the condition and degree of wear of a sleeve is estimated. AGC also allows you to assess the condition of the piston rings, detect coking or ringing without disassembling the engine.

Measurement of compression of the petrol and diesel engine

In the list of general requirements for that, to accurately measure compression, are:

1. The engine must be warmed up 80-90 degrees;
2. The fuel supply must be switched off;
3. All candles must be unscrewed without exception;
4. The starter must be perfectly serviceable;
5. The battery must be fully charged;

• In the case of a manual transmission, the top can be engaged (increased) transmission, and avoid the possibility of moving the car. To do this, use the parking brake and / or stops. It is also allowed to measure the compression in the position of the lever manual transmission "neutral". In cars with "automatic" the selector is established in a parking mode (R).
• The next step is to check the condition of the air filter. Contaminated filter is changed to a new one before compression measurements. The second method is to temporarily disconnect the air inlet pipe.
• Gasoline engines require the ignition to be switched off. To do this, disconnect the low voltage wiring from the circuit breaker, which is fed to the ignition coil. Engines, equipped with an ignition distributor, require removal from the coil of the main high-voltage wire. how to properly measure the compression of a diesel and gasoline engine.
• For effective shutdown of fuel supply in designs, which include a mechanical fuel pump, the fuel line needs to be disconnected. Then the pipe is muffled. how to properly measure the compression of a diesel and gasoline engine.
• After cleaning of places for installation of spark plugs is carried out and removal of tips of high-voltage wires from spark plugs is carried out.. Then the candles are unscrewed.
• With unscrewed spark plugs you need to make a few turns of the engine starter. The procedure is necessary for that, Then the candles are unscrewed.
• The last step is to connect the compressor to the spark plug of the cylinder. It is better to measure compression with a partner, which will squeeze the accelerator pedal to the stop. The accelerator pedal is squeezed out for that, that the compression measurement was carried out with the throttle open, and the inlet resistance was thus reduced. With the pedal squeezed, the motor is turned by the starter close 5 seconds. It is necessary to monitor that, when the highest value appears on the scale. The specified value is noted and passed to further measurements in other cylinders of the engine.

As for the features of measuring compression on a diesel engine, then:

• Disabling the fuel supply in the diesel is achieved by squeezing down the clipping lever. This lever is located on the pump. The second way is to turn off the solenoid valve, which will also block the fuel supply to the diesel engine. This valve is located on the fuel line, which is responsible for fuel supply.
• The compressor in the diesel engine is connected through the holes of the injectors or glow plugs. It depends, which elements are easier to get to, and in accordance with the recommendations for the specific type of diesel engine.
• A special compressor is required to measure compression on a diesel engine, which will have a suitable tip for secure attachment in the hole of the nozzle or glow plug. The threshold of the maximum measurement at such compressor for diesel should be higher in comparison with petrol analogue. The diesel compressor must be able to record the pressure close 60 atmospheres.
• When measuring diesel compression, the need to press the accelerator pedal is excluded. The point is, that most diesel engines do not have a throttle. Only diesels stand out, which have a special inlet valve. A valve is needed to create a vacuum. The vacuum will provide the functionality of the vacuum regulator and brake booster.

The fuel supply must be switched off in order to do so, so that the fuel does not wash away the so-called oil wedge. To stop the fuel supply, you can use one of these methods to turn off the fuel pump, Pump or nozzle. Candles must be unscrewed.

If you take measurements, unscrewing only the spark plug in one cylinder, then marked increase in resistance to crankshaft rotation. At the moment of scrolling of a starter this resistance leads to decrease in turns. If the battery fails or the battery is weak, you can use a starter, although this method is not recommended.

Compression and throttle

Compression can be measured with the throttle open or closed. The different approach reveals some nuances of wear of engine parts. The decision to measure compression with the throttle closed will mean, that not much air will get to cylinders.

The compression ratio in this case is low (close 0,6-0,8 MPa). This method allows you to achieve high sensitivity readings to the smallest leaks, since little air entered the cylinder. Even small leaks can reduce the value at times. Based on the evidence obtained, a number of definite conclusions can be drawn:

• the valve is not completely adjacent to the saddle;
• the valve may depend;
• camshaft cam wear;
• crack in the wall of the combustion chamber;
• the gasket of the block head is worn;

Valve problems can occur as a result of wear, and after the repair, especially in mechanisms with hydrocompensators. The shape of the camshaft cam profile may wear unevenly, especially given the beating.

Measuring compression with an open throttle means, that the cylinder will receive much more air compared to the measurement with the throttle closed. Increasing the volume of air will lead to that, that the pressure in the cylinder will increase markedly. It turns out, the presence of leaks will lead to more intense loss of compression in such conditions. But potential leaks will still be able to pass less air compared to large volumes when it is fed. The result will be that, that the compression will not fall sharply (close 0,8-0,9 MPa). Measurement of compression of the engine with an open throttle valve allows to reveal serious breakdowns:

• destruction or burnout of the piston;
• fracture or deformation of the piston rings;
• sticking (coking) rings in piston grooves;
• burnout of the valve or bend of the valve stem;
• wear (bully) cylinder mirror surface;

Additionally, in the measurement process it is necessary to pay attention to that, as the pressure in the cylinder increases. This approach allows you to more accurately locate one or another fault. If at the time of the first stroke the compressor shows low compression to 0,4 MPa, but already on the next bar there is an obvious rise, then this may indicate wear on the piston rings. When at the time of the first measure the pressure is at about 0,7 or 0,9 MPa, but no longer increases, then probable problems with the tightness of the valve.

Why add oil to the cylinders

Problem with rings, pistons, valves or gasket is diagnosed more accurately by pouring into the tested cylinder about 3-5 cubic meters. see oil. Immediate increase in compression after the addition of oil on the first stroke will indicate the piston rings. If the compression is low at first, then the oil was poured, but the compression could not be raised, then possible leaks through the cylinder head gasket.

Most often, compression falls in only one cylinder. This cylinder with the least compression should be filled close 5 "Cubes" of pure engine oil. Oil in the process of pouring should not get to the bottom of the piston, and on the walls of the cylinder liner. Then the measurement of compression is repeated. In the case of increasing pressure on the readings in other cylinders is likely, that the piston rings are coked, there was their occurrence or destruction.

Also, the lack of changes in the readings after pouring the oil may indicate that, that the valve is not completely adjacent to the seats. Valve burnout or valve seat destroyed, valve clearances may be incorrectly adjusted.

To diagnose damage to the cylinder head gasket, burnout of the piston or cracks in the piston body it is necessary to install the piston closer to the top dead center (TDC), corresponding to the compression stroke. Next, you need to supply a portion of air to the cylinder at a pressure of about 2 — 3 atmospheres.

If the block head gasket is damaged, then the sound of the exhaust air from the candle well will be heard, located nearby. Air, coming out through the carburetor, means no normal fit of the intake valve. Additionally, the plug of the oil filler neck is removed. If the air goes from there, probable burnout or crack in the piston. Air in the exhaust tract will indicate a loose fit or burnout of the exhaust valve.

For maximum accuracy, it is recommended to use a combined method of measuring compression (measured "cold" and "hot", with open and closed throttle). At the initial stage it is necessary to measure compression in cylinders of the petrol unit at an open gate. Then the spark plugs, voltage spark plugs or diesel injectors (depending on the type of internal combustion engine) put in place and the engine starts. The engine must be warmed up, and then measure the compression (with the throttle closed for petrol engines).

It is necessary to measure compression on the cold engine then, when the motor cannot be started. The cold engine does not start due to critical LPG wear, and also as a result of occurrence of piston rings. In cold cases, the pressure on the cold engine drops two or more times. Warming up the engine allows you to increase compression, then the unit is reasonably started "hot".

It is recommended to perform compression measurements on a cold diesel engine. The measurement in the diesel depends on the presence of oil in the cylinders. Before measuring the compression of the diesel engine must stand at least 2 hours.

This time is needed for that, so that the remaining oil is completely drained back into the crankcase. The drop in compression for a cold diesel engine is lower 18 atmosphere will mean, that starting such a motor is almost impossible. If the figure after warming up does not rise above the mark in 24 atmosphere, then the motor must be repaired.

We evaluate the results obtained

After all measurements, analyze the readings on the cylinders. If the scatter on the cylinders to the figure in 1 atmosphere, then the engine failure is not related to compression. The problem should be sought in injection systems, ignition, fuel supply, etc.. The scatter rate is slightly higher 1 atmospheres point to those, that the CHP wears out unevenly, but the condition is acceptable.

The approximate compression ratio should be equal to the compression ratio, which is multiplied by 1,3. The values ​​of the degree of compression for a particular engine are in the technical manual of the vehicle, instructions or repair manual.

For petrol internal combustion engines, which are designed for 76th or 80th gasoline, the compression on the hot engine must not be lower 9,5-10 atmospheres (taking into account the serviceable cylinder-piston group). A necessary indicator for modern motors (gasoline brand AI-92 and above) is 11-14 atmospheres with a permissible scatter in the cylinders to 1 atmosphere. The diesel engine means compression from 28 (for older generations of diesel engines) to 45 atmospheres (modern diesel) with a scatter on the cylinders to 3 atmospheres.

The compression ratio is calculated approximately, as it depends on the internal combustion engine temperature during measurements, viscosity of engine oil, battery status and more. It is necessary to take into account the error of the compressor readings, which in some cases is close 3 atmospheres.

It is necessary to take as a basis comparison of indications on cylinders. It is a mistake to believe, that only an underestimated compression ratio will clearly indicate a malfunction. It happens, that in the faulty cylinder excessive penetration of oil is observed. Excess oil increases compression, thereby masking the fault. In this case, you need to carefully analyze the condition of the spark plugs. If there are traces of oil scale at admissible compression, then the broken oil ring may be the cause of the oil, bushing wear (directs) valve, etc.