By Andy Crook

This can be a problem for technicians who can see substitute data displayed on a scan tool and wrongly assume the sensor is working correctly, or test circuits that have been disabled by the ECU and wrongly conclude that the supply or control circuit has failed.

Live data should provide some clues and it showed lower than expected rail pressure, which deviated excessively from the desired value when the pedal was planted to the floor. So under high load, the rail pressure was unable to keep up with demand. This suggested the hesitation is part of the problem not limp mode.

High pressure faults can have many causes - injectors, pressure control valves, flow control valves, high pressure pumps and low pressure supply can all result in lower than desired values.

Testing the low pressure supply is a good starting point but breaking into the supply is time-consuming and sometimes difficult without the correct adaptors. If the vehicle uses an electric pump to supply the low pressure, an amp clamp can quickly prove the condition of the pump without the need to connect gauges.

The Range Rover was fitted with an electric pump, using an oscilloscope the supply voltage and current draw was quickly analysed, see Figure 1. The speed of the pump and the current provided clues to the fuel flow and pressure. The pump speed in this case was 4,646rpm (frequency x 60).

WHEN testing electrical circuits I always split the circuit into supply, protection, load, control, and ground elements. This makes testing the circuit much easier, and it is important to understand how the circuits are protected, and controlled before testing.

The supply and ground circuits are common to both sides of the coil, this eliminates them as causes of the misfire. However there is no circuit protection or so it appears. The load is the primary windings, and the control is performed within the amplifier circuits switched on by the 5V square waveform from the ECU.

Recently, I tested used an oscilloscope to test the coil on a misfiring Polo. The test proved both sides of the coil were receiving a good clean consistent square waveform. This is enough to condemn the coil pack.

To confirm the diagnosis I used current clamp to analyse the primary current. Using the data available it would appear that the primary current would be in the region of 28 amps. However, most primary circuits are in the region of 6-10 amps. This is due to the current limiting function of the amplifier. A low 0.5? resistance coil is used to ensure full coil saturation even at high engine speeds.