Attention! Difficulties when starting a cold engine can also be due to low compression. Therefore, in such cases, the compression should also be checked.
Test conditions: battery voltage at least 11.5 V.
Checking the power supply
This test can be carried out without dismantling the glow plugs.
1. Connect a test light in parallel to the spark plug supply rail and to «mass» (-).
2. Turn on the ignition and preheat. Leave the key in the ignition in the preheat position and watch the indicator light. The control lamp should light up and go out after a maximum of 8 seconds.
3. Switch on the preheat again. As soon as the preheat warning light on the instrument panel goes out, start the engine.
Attention! The preheating signal light on the instrument panel in case of perfect operation of the heating system at ambient air temperatures of +20°C burns for no more than 4-7 seconds. The glow plugs themselves remain energized much longer, performing the so-called warm-up follow-up so that the engine warms up faster. Therefore, the connected test lamp must be lit when checking on engines with swirl combustion chambers (F8Q) for a maximum of 30 seconds, and for engines with direct injection (F9Q) - maximum 20 seconds.
If the voltage supply does not meet the specified requirements, the control unit must be checked by a specialist workshop.
Checking glow plugs
4. Remove the power wires and power rail from the glow plugs.
5. Connect an ohmmeter to each glow plug in turn and measure their resistance (see illustration).
2.5 Connect an ohmmeter to each glow plug in turn and measure their resistance
Nominal resistance values of candles:
- vortex combustion engine (F8Q) \u003d 0.63.0.2 Ohm;
- direct injection engine (F9Q) = 1.1±0.1 ohm.
If the resistance of the candle is below the nominal, then it is faulty and must be replaced.
Glow plug tightening torques:
- vortex combustion engine (F8Q) = 20 Nm;
- direct injection engine (F9Q) = 15 Nm.
Attention! Do not tighten candles with more torque than specified. Otherwise, the annular gap between the glow plug and the thread is compressed, and the spark plug burns out before reaching its expiration date.
6. Connect the power wire to the bus.
Attention! If the test fails to detect a malfunction and the engine still does not start well, then visually check the functioning of the glow plugs. To do this, remove the nozzles and follow the candles through the holes for the nozzles.
Glow plugs with burnt pins
Burnt glow plug pins are often the result of injector malfunctions. Such damage cannot be attributed to defects or malfunction of the spark plug itself.
If such candles are found during the check (see arrow in illustration 2.0), it is not enough just to replace them. Injectors should be checked for injection pressure and tightness (work is done in the workshop).
2.0 Burnt glow plug pin