Mercedes S-Class W-140

1991-1999 of release

Repair and operation of the car



Mercedes S-Class
+ Mercedes-Benz cars of the class S (W-140)
+ Operation manual
+ Routine maintenance
- Engine
   Check of a compression in engine cylinders
   Check of the engine by means of the vacuum gage
   Belt of the streaming drive of auxiliary units
   + 6-cylinder petrol engines (M104.944 and M104.990/994)
   + 8-cylinder petrol engines (M119.970/981 and M119.971/981)
   + 12-cylinder petrol engines (M120.980/982)
   + 6-cylinder diesel engines (OM603.971)
   + 6-cylinder diesel engines (OM606.961)
+ Cooling systems, heating
+ Power supply system and release
+ Engine electric equipment
+ Gear shifting box
+ Automatic transmission
+ Coupling
+ Brake system
+ Suspension bracket and steering
+ Body
+ Onboard electric equipment






Check of a compression in engine cylinders

The compression gives an idea of a condition of the engine. Results of check show whether as it should be valves and pistons (piston rings), or they are worn-out. Whether Besides, results of check allow to make the decision to replace the engine whether complete repair is required to it or to replace sealing laying of a head of cylinders enough. For check it is required компрессометр which for petrol engines is offered in specialized shops for reasonable price.

Basic provisions and fluctuations of an arrow of the vacuum gage at diagnostics of malfunctions of the engine


For diesel engines it is necessary компрессометр with a limit of measurements approximately to 40 bars.


The engine has to be heated-up up to the working temperature, the rechargeable battery - is completely charged. You will also need the help of the assistant.

Take a safety lock of the fuel pump from the assembly block, (address the Head Onboard electric equipment). Start the engine and wait until it decays. If the engine is not started, scroll a starter about 10 seconds.

For receiving access to spark plugs uncover a maslozalivny mouth and remove a plastic cover of the engine. Establish a cover into place.

Disconnect system of ignition, having disconnected the socket from the ignition coil. Turn out all spark plugs (see the Section Check and replacement of spark plugs, check of a condition of high-voltage wires (petrol models) of the Head Routine maintenance).

Establish компрессометр in an opening of a candle of the first cylinder. It is desirable to use kompressometry which tip is rolled in a candle opening.

Ask the assistant to hold a pedal of an accelerator completely pressed and to include a starter. After several cycles the compression in the cylinder will reach the maximum value and will not change any more. Write down the received result.

The most admissible difference of pressure in separate cylinders can make 3.0 bars (for diesel engines of 5.0 bars).

The compression grows in the serviceable engine very quickly. The low value shown after the first cycle, increasing with the subsequent cycles indicates wear of piston rings. The low value after the first cycle which is not increasing after the following signals about that, something valves are untight, or laying of a head of the block is punched (also the crack in a head can be the cause). Can lead existence of a deposit on plates to decrease in a compression of valves.

It must be kept in mind that because of the different speed of rotation of a starter on different models the results received as a result of measurements can differ. The results received at measurement of a compression have to be approximately identical to all cylinders.

If pressure is in any cylinder at the level of minimum admissible and even below, then for clarification of the reason perform the following operations. Pour in the cylinder through an opening for a candle in a teaspoon of motive oil and repeat measurement of a compression.

If oil addition temporarily improved a compression, most likely, wear of the piston, rings or the cylinder is the reason of its decrease. If increase in a compression did not happen, then it is possible to assume that the reason - in leakage of valves or the punched laying of a head of the block.

Low compression in two next cylinders almost for certain - a consequence of breakdown of laying of a head. Availability of cooling liquid in combustion chambers or in a case of the engine will confirm this assumption.

If the compression in one of cylinders is about 20% lower, than in the others, besides turns of idling are unstable, then the reason, perhaps, excessive wear of a cam of a cam-shaft.

After completion of check establish to the place of a spark plug (address the Head Routine maintenance), put on them tips of high-voltage wires, attach the socket to the ignition coil. Establish into place a plastic cover of the engine and a safety lock of the fuel pump.

Values of pressure of a compression for various engines are given in Specifications.

Check on leaks

During this check are defined exit speed from cylinders of the compressed air pumped in them and directly the place of leaks. This check is an alternative to check of a compression. Moreover, from many points of view it is much more effective as visually it is simpler to reveal a leak source, than to comprehend results of measurement of a compression.

The equipment necessary for check on leaks, does not belong to public therefore performance of this check should be entrusted specialists of the service center.




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