4.1.2. Measurement of pressure of a compression - the description and interpretation

At decrease in overall performance of the engine, admissions of ignition which cannot be charged to malfunctions of system of ignition or fuel system measurement of a compression can be a key to definition of a condition of the engine. At regular conducting such check many problems connected with malfunctions and wear of the engine can be defined much earlier, than other their symptoms will be shown.

Test requires performance of the following conditions: The engine has to be heated-up up to the normal working temperature, the battery has to be completely charged, all spark plugs have to be removed (The head Routine maintenance). Also the help of the assistant will be required.


1. On models with system of ignition with the distributor, disconnect system of ignition by a detachment of a high-voltage wire of the coil of ignition from a distributor cover. After a detachment the wire has to be grounded on the block of cylinders. For ensuring good conductivity use a wire from a set for start of the engine from an auxiliary source or a similar wire of big section. Also the low-voltage socket can be disconnected from the distributor.
2. On models with direct ignition disconnect system of ignition by a socket detachment from the ignition block. More detailed information see the Head Sistemy of start and a charge. On models with the catalytic converter the fuel pump by removal of the corresponding safety lock also has to be switched-off.
3. Establish компрессометр in a candle nest of the 1st cylinder - preferably to use the measuring instrument which is screwing in in a nest.
4. Let now the assistant completely will open a throttle and will turn the engine a starter; after 1 — 2 turns pressure of a compression has to rise to the maximum value and be stabilized. Write down most recorded result.
5. Repeat check on other cylinders, writing down pressure in each of them.

Pressure in all cylinders has to be identical; the difference more, than in 2 bars between two cylinders indicates malfunction. Pay attention that on normally functioning engine the compression has to rise very quickly; the low compression on the first piston stroke which is followed by its gradual rise on the subsequent courses indicates wear of piston rings. The low compression on the first piston stroke which is not increasing during the subsequent courses indicates possible leakages of valves or a transmission of laying of a head of cylinders (also existence of a crack in a head of cylinders can appear the reason). Also existence of deposits on a plate of the valve can be the reason of a low compression.

In spite of the fact that SAAB does not regulate exact value of pressure of a compression, nevertheless pressure in cylinders provides 10 bars on malfunction or wear of the engine below. At emergence of any doubts when conducting this check it is necessary to address to representative office of SAAB or in a workshop of car service.

At identification of low pressure in any of cylinders, it is necessary to make the following checks for definition of the reason. Pour in a teaspoon of pure motive oil in a candle nest of the corresponding cylinder and repeat check.

If injection of oil temporarily lifts compression pressure, then wear of rings or a mirror of the cylinder is the reason of loss of pressure. At the same time suspicions on leaks owing to a progar of valves or violation of laying of a head of cylinders disappear.

The lowered compression pressure in separate two next cylinders almost for certain indicates violation of laying of a head of cylinders between these cylinders; at the same time existence of a cooler in motive oil confirms this assumption.

If in one of cylinders pressure of a compression is about 20% lower, than in the others, at the same time the engine works a little unevenly at single turns, then the reason most likely is in wear of cams of a cam-shaft.

In conclusion of check establish to the place of a spark plug and connect wires of system of ignition and where it is required, the fuel pump.