1. Warm up the engine to operating temperature.
2. Turn off the ignition (see chapter Ignition system) and high voltage wire, remove all spark plugs.
3. On a diesel, disconnect the preheat control wire and engine switch.
4. Also, remove the glow plug wires and unscrew the glow plugs.
5. Insert or screw the rubber cone of the compression tester into the hole of the spark plug or glow plug of 1 cylinder (located on the right in the direction of travel behind the camshaft timing belt sprocket).
6. Put the machine on the parking brake. The manual transmission lever must be in the neutral position, the automatic transmission switch in the position «parking».
7. On a car with a gasoline engine, ask an assistant to turn on the starter with the gas pedal fully depressed.
8. On a diesel, ask an assistant to simply turn on the starter.
9. Rotate the engine with the starter for approximately 5 seconds until the compression gauge reading stops increasing.
10. Write down the result of the measurement. If using a memory compression gauge, proceed to measure the compression of the next cylinder.
working volume (2) called the space from the top (1) down to the bottom (3) dead point. Between top dead center, which is limited on the right cylinder by the piston mirror, and the surface of the cylinder head (5) the combustion chamber is (4).
Pressure value
To accurately measure compression, you must first adjust the clearances in the valve drive, as described in chapter «Valve clearance adjustment». For the normal operation of the engine, not only the compression ratio is important, but also its equal value in all cylinders. During the measurement at the same starter speed, the following values are considered normal:
Engine | Short motor type designation | normal compression pressure | wear limit | Maximum difference between cylinders |
1.4 l 1.7 l 1.8 l 16V Diesel | C3J F3N F3P F7P F8Q | 11-13 bar 12-14 bar 12-14 bar 13-16 bar 28-34 bar | 6 bar 8 bar 8 bar 10 bar 24 bar | 3 bar 3 bar 3 bar 3 bar 5 bar |
Too low pressure
A low compression value does not necessarily indicate a malfunction. The reason for this may be the inconsistency of the measurement data of various control and measuring instruments. It is much worse if the difference between the 4 results obtained when measuring the pressure in the cylinders exceeds 3, and on a diesel 5 bar. The reason for this may be:
- wear of pistons and piston rings;
- sedimentation of piston rings due to the formation of soot;
- oval shape of the cylinders as a result of piston sticking;
- soot deposits on valves;
- falling valves;
- burnout of valves due to too small clearance in the valve drive.
Note. Recommendation: In most cases, the cause of low compression pressure is a loose valve seat, which leads to a decrease in engine power. To eliminate the malfunction, either the valves are lapped or the cylinder head is repaired.
Troubleshooting
To determine the cause of too low compression, use the following method. Into the spark plug hole (incandescent) pour a little thick oil and measure the pressure again.
- If the pressure is low, then the cause is a valve malfunction.
- If the pressure has become higher, then the cause of the malfunction is the piston rings and, probably, the cylinders. The filled oil seals the gaps between the piston and the cylinder walls for a while, which leads to an increase in compression.
Compression reduction test
More accurate data can be obtained by conducting a control check, which is done in some auto repair shops. The compression pressure gauge consists of 2 compartments, one of which is maintained at a constant pressure. The second compartment is connected by a hose through the holes of the spark plugs (diesel injection ports) with a combustion chamber, and through a jet with the first compartment and with a measuring scale. If the combustion chamber under test loses pressure, this is indicated on the scale. Larger looseness can be determined by listening:
- A whistling noise in the exhaust pipe indicates a bad exhaust valve.
- Compressed air escaping from the air filter indicates a problem with the intake valve.
- If the cylinder head gasket is faulty or the cylinder head is cracked, the charge air will escape from the nearest spark plug hole (incandescent) or from an open coolant expansion tank.
- In case of wear of the cylinder walls, piston or piston rings, gases enter the crankcase and exit through the nozzle or tube of the oil dipstick.