In the event of unstable engine operation, poor starting, high engine oil consumption, the compression in the engine cylinders must be checked.
Compression in the cylinders is the highest gas pressure in the cylinders when the piston compresses the air at the end of the compression stroke.
Low compression pressure or its absence indicates wear of the piston rings, burning of the piston or valve.
The compression in the engine cylinders is checked with a compression tester which is inserted into the spark plug hole in the cylinder head.
The tip of the compression tester is rubber; it must be pressed firmly into the spark plug hole when measuring compression.
To undo the spark plugs, a special 16 spark plug spanner is used.
Before checking the compression, the engine must be started and warmed up to operating temperature.
After this, the fuel pump is switched off and the fuel pressure is released.
Releasing the fuel pressure is described in the article – «Replacing the additional fuel filter Renault Duster».
The engine ignition coils are withdrawn and the spark plugs are undone with the spark plug spanner.
For spark plug replacement, the article – «Replacing the ignition coils and spark plugs Renault Duster» – can be consulted.
The compression tester is fitted into the spark plug hole of the cylinder being checked.
The assistant presses the accelerator pedal (throttle actuator) to the floor to fully open the throttle.
The starter is switched on and the engine crankshaft is turned until the pressure in the cylinder stops increasing. This corresponds to approximately four compression strokes.
To obtain correct compression tester readings, the crankshaft must be turned at a speed of 180–200 rpm or higher, but not more than 350 rpm.
The compression tester readings are recorded and the compression tester pressure is released by pressing the air release valve.
The pressure in the other cylinders is checked.
The normal pressure in the cylinders must be at least 1.28 MPa and must not differ between cylinders by more than 0.1 MPa.
The minimum permissible compression is 1.135 MPa.
To determine the cause of insufficient compression, 20 ml of clean engine oil can be poured through the spark plug hole and the compression measured again.
If the compression tester readings increase, this means significant wear or breakage of the piston rings.
If the compression values remain unchanged, this indicates poor seating of the valve heads on their seats or damage to the cylinder head gasket.
The cause of insufficient compression can also be determined by supplying compressed air to the cylinder in which the piston is previously set to TDC of the compression stroke.
To do this, remove the tip from the compression tester and connect a compressor hose to it.
Insert the tip into the spark plug hole and supply air to the cylinder at a pressure of 2–3 kgf/cm².
To prevent the engine crankshaft from turning, engage the highest gear and brake the vehicle with the parking brake.
The escape (leakage) of air through the throttle body indicates leakage of the intake valve, and through the silencer – leakage of the exhaust valve.
If the cylinder head gasket is damaged, air will escape through the expansion tank neck in the form of bubbles or into the adjacent cylinder, which is detected by a characteristic hissing sound.
Measuring the fuel pressure in the engine fuel system
An indicator of the serviceability of the engine fuel system is the fuel pressure in the fuel rail.
If the fuel pressure is insufficient, the following are possible:
- unstable engine operation;
- engine stalling at idle;
- reduced crankshaft speed at idle;
- insufficient vehicle acceleration (the engine does not develop full power);
- misfires in engine operation when the vehicle is moving.
To begin with, we recommend checking the reliability of the electrical contacts in the engine management harness connectors responsible for the fuel supply (fuel pump module, injectors, fuel relay and the corresponding fuses).
The fuel pressure in the fuel system is checked using a professional pressure gauge and a hose with an original union for connection to the valve located on the fuel rail.
Switch on the ignition and listen: within a few seconds you should hear the sound of the electric fuel pump operating.
If it cannot be heard, check the electrical supply circuit of the electric fuel pump.
If you have switched on the ignition three times without attempting to start the engine, the next time the electric fuel pump will not switch on, which is not a sign of a fault.
It will switch on simultaneously with the start of the engine by the starter.
Reduce the pressure in the fuel system (see «Replacing the additional fuel filter Renault Duster»).
The pressure pipe is disconnected from the fuel pump (article – Repairing the fuel module Renault Duster).
To check the fuel pressure, a pressure gauge (with a measuring limit of at least 5 kgf/cm²) is connected between the fuel pump and the pressure pipe.
The relay removed when reducing the pressure in the fuel system is fitted into the mounting block.
The engine is started.
With the engine running at idle, the pressure in the fuel pipe must be at least 3 kgf/cm².
The following causes of pressure reduction are possible:
– fault of the fuel pressure regulator;
– fault of the fuel pump;
– clogging of the fine fuel filter;
– clogging of the mesh filter in the fuel pump module.
The engine is stopped and the fuel pressure is released.
The pressure gauge is disconnected from the fuel system.