2. The engine must be warmed up to normal operating temperature, the battery must be fully charged, all spark plugs must be unscrewed (see Chapter Routine maintenance). The help of an assistant will also be required.
3. De-energize the ignition system by disconnecting the ignition coil high voltage wire from the distributor cap and grounding it to the cylinder block (use a jumper wire). On models without a distributor, disconnect the electrical connector at the ignition coil assembly at the front of the cylinder head.
4. To avoid damaging the catalytic converter, release the pressure in the injection system and disconnect the power supply system by removing the fuel pump relay (see Chapter Power supply system).
5. Connect the compression tester to the spark plug hole of the 1st cylinder (it is preferable to use a meter with a threaded tip).
6. Ask an assistant to depress the accelerator pedal all the way and turn the engine with the starter. After the first two crankshaft turns, the instrument reading should reach its maximum value. Record the result.
7. Repeat the measurement for the remaining cylinders and record the results.
8. The pressure in all cylinders should be practically the same within the range of 11 to 15 bar. A difference in readings exceeding 1.5 bar should be considered as a sign of a malfunction. It is also unacceptable for the compression pressure to drop below 10 bar.
9. Remember that the pressure in a serviceable engine should rise quickly enough. A low reading on the first stroke with a gradual increase on subsequent ones indicates wear of the piston rings.
10. If the pressure does not rise at all, you should check the tightness of the valves and the condition of the cylinder head gasket (the possibility of cracks forming in the head is also not excluded). Deposits build up on the lower surfaces of the valve heads can also cause loss of compression pressure.
11. If the measurement results are below the permissible value, you should try to determine the cause of the pressure loss. The most basic of the tests is pouring about one teaspoon of machine oil through the spark plug hole into the corresponding cylinder and then repeating the compression pressure measurement.
12. If the compression pressure is temporarily restored immediately after adding oil to the cylinder, the degree of wear of the pistons, cylinder walls and piston rings should be assessed. Otherwise, we can talk about a violation of the tightness of the valve closing (for example, as a result of burnout) or damage to the cylinder head gasket (or the heads themselves).
13. Equally low pressure in two adjacent cylinders most likely indicates damage to the cylinder head gasket bridge located between them. The presence of coolant in the engine oil will confirm this assumption.
14. If the pressure in one of the cylinders is approximately 20% lower than in the others, and the stability of the engine idle speed is slightly disturbed, the cause of this phenomenon may be wear of the working projection of the corresponding camshaft cam.
15. Excessively high compression pressure indicates severe carbon deposits in the combustion chambers. In this case, the head should be removed and decarbonized.
16. Finally, screw in the spark plugs, reinstall the fuel pump relay and operate the ignition system.