Engine compression (Fiesta 4)

If over time you get the feeling that your engine was more dynamic at the beginning of operation, the loss of power may be caused by mechanical wear. The most common causes are high wear between the pistons and cylinders, worn piston rings, leaky or burnt valves, a damaged cylinder head gasket or spark plug. At the end of the compression stroke, high compression pressure is created, which then increases like an avalanche during combustion of the fuel-air mixture. For pistons and piston rings, cylinder walls, valves, valve seats, valve mechanism gaskets, and also the cylinder head gasket, this means high thermal and mechanical load. As a rule, engines with reduced power already signal their technical condition in advance. Symptoms include erratic cold start behavior or uneven engine operation, increased oil and fuel consumption, white or blue exhaust plume, increased coolant temperature, poor exhaust gas performance and reduced engine power. To get a clear picture, it is necessary to check the compression every other visit to the service center. This also applies to diesel engines.

Compression control values

The compression values for your vehicle vary slightly depending on the compression ratio. The approximate values shown in Table 4.2 are valid for engines in perfect technical condition. When analyzing compression, more importance is attached not to the absolute highest values, but to the same values in all cylinders. Deviations of up to 2 bar are allowed. If the deviations exceed the permissible value, it is necessary to find out the reason for this deviation. As a first step, a specialist will advise you to measure the pressure loss.

Older engines create less compression

In older engines, compression is significantly reduced, and this is a normal phenomenon associated with natural engine wear. Therefore, there is no particular reason to worry, it depends less on the absolute highest pressure value, and more on the same values in all cylinders. Only when the measured values reach the maximum permissible values, it is necessary to think about the upcoming engine repair or replacement with another one. If the difference in values is more than 3 bar, practice indicates the following reasons:

• wear of pistons or piston rings;
• jamming of worn piston rings in the piston grooves caused by combustion product deposits;
• damage to the working surfaces of the cylinders as a result of slight seizure of the pistons or jamming of the piston rings;
• residues of combustion products and oil in the form of a coating on the valve mechanism or on the working surfaces of the valve seats;
• cracked valves;
• valve burnout due to insufficient valve clearance or thermal overload.

You can measure the compression yourself. First of all, you need an assistant who turns the crankshaft with a starter and a pressure measuring device. In well-equipped auto repair shops, a compression measuring device is part of the basic equipment. First, unscrew all the spark plugs (in a diesel engine - fuel injectors) from the cylinder head and make sure that the valve clearance is adjusted correctly. During the check, the assistant should fully press the clutch and accelerator pedals, you check the compression in each cylinder with the device. Since diesel engines constantly suck in the same amount of air, you do not need to press the accelerator pedal. Start measuring from the first cylinder and then sequentially measure in all the other cylinders. Count the number of crankshaft revolutions until maximum pressure is created, this number will be the criterion for the subsequent cylinders: the faster the compression is created, the "healthier" your engine. In a "healthy" engine, compression should be created after 6–8 crankshaft revolutions.

The basis for a reliable measurement is a working starter and a fully charged battery

Although this is a truism, we still remind you once again - the basis for measurement is a starter with good traction force and a fully charged battery. If the crankshaft rotates very slowly, the compression in the cylinder also occurs slowly - the compression measurement loses its meaning. If you have received large deviations in the measurement results, conduct a pressure reduction test. Handling a compression reduction device requires some experience, so we recommend that you leave the compression reduction test to a specialist.

The reason for the decrease in pressure can be determined in the following way.

• If the compression is low, inject a little motor oil into each cylinder through the spark plug hole (in a diesel engine, into the injection nozzle hole), then repeat the check. The oil improves the sealing between the mating surfaces of the pistons and the cylinder walls.
• If the compression has increased slightly, then the leak is through the valves or the cylinder head gasket. Valve leakage can be caused by burnt valve seats and/or working edges, as well as deformation of the valve stems or formation of cracks on them.
• If after adding oil the compression increases, then we can make a clear conclusion that the piston rings are worn out.

Compression measurement

SEQUENCE OF WORKS



1. Warm up the engine to operating temperature before measuring. This creates optimal clearances between all moving parts.

2. Turn off the ignition (terminal "15"), remove the spark plug caps and unscrew all spark plugs.

3. Apply the parking brake, set the gearshift lever to neutral, press the clutch and accelerator pedals.



Fig. 4.13. When installing the compression gauge, the rubber cone plug should tightly close the spark plug hole

4. Insert the rubber cone plug of the compression tester firmly into the spark plug hole (in a diesel engine, into the fuel injector hole). If necessary, use a suitable adapter (Fig. 4.13).

5. Turn on the starter and turn the crankshaft at least eight compression strokes, monitoring the compression gauge readings. It is important to keep in mind that the best filling of the cylinders occurs only when the accelerator pedal is fully depressed.



Fig. 4.14. After measuring the compression on all four cylinders, compare the curves recorded by the instrument recorder



6. Record the highest compression value. Repeat the measurement procedure for the remaining cylinders and compare the results with the technical data (Fig. 4.14).



PRACTICAL ADVICE

Reduced compression

A decrease in compression accompanied by air noise can be caused by the following reasons:

• noise in the suction pipe or sound insulation of the pipe indicates a leaky inlet valve;
• if air is coming out of an open radiator or coolant expansion tank, the cylinder head gasket is blown or there is a crack in the cylinder head;
• if air comes out of the open oil filler neck or the engine oil level indicator tube, it means that the cylinder walls, piston running surfaces or piston rings are worn out;
• if air comes out of the exhaust pipe, then the exhaust valve is not tight.

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