Most of the car's that we feature do not have the most common faults- if they did then we would soon run out of cars to write about, but they are the most interesting cars to work on since they stop boredom from setting in -just when you think you know it all another jumps out of the woodwork to stop complacancy taking over. I'm sure that they are the most interesting to read about to - I love reading about the latest car model out even though ownership is totally out of reach, so in a rather different way though people like reading about weird faults even though they are not likely to experience one.

     The Honda Accord kept breaking down intermittently despite having had everything that could be renewed  changed - including the distributor, Ignition pick up coil, and ignition ECU. We at first suspected an intermittent carburettor blockage so we stripped and cleaned the carburettor to no avail.

The car still refused to idle below 1500 RPM. We then found that the ignition Ecu was causing the intermittent cutting out, and because the price of a new one was a fairly significant proportion of the cost of the car a second hand one was fitted. We then found the reasom why the Ignition Ecu had failed since a further fault emerged - the engine refused to idle below about 1500 RPM caused by the dwell on the ignition osciloscope trace decreasing too low below this engine speed. This was found to be caused by the HT coil being the wrong type for the car and had been damaging all Ecu's fitted to the car. Renewal of the Coil for the correct one ensured that the new Ecu would continue operating without getting damaged and the car also then idled normally.

     The Honda brought to us had developed a problem after the clutch  had been renewed - the engine would not idle but ran at over 3000 Rpm when warm. The car was a fuel injection model fitted with an idle speed control valve and the cause of the high idle was soon traced down to this unit. The unit itself though was checked out and found to be Ok, it was only doing what the Engine Management Control Unit was telling it to do. At this stage of the investigations a full engine management systems diagnosis was required so the Control Unit was found and a diagnostic adaptor inserted between it and the wiring loom. This showed an abnormal signal from the throttle potentiometer so we traced the wiring back to that unit. The three wire loom to it had been damaged at one point, probably in the course of fitting the clutch. The throttle potentiometer tells the Control Unit the amount of accelerator pedal movement, and because the wiring damage told it that the accelerator pedal was full throttle it sent a signal to the idle speed control valve to open fully. Repairing the wiring brought the idle back down to a sane level.

Emissions failure.  The most common cause of rich running faults is the lamna sensor or the ECU (engine control unit) both of these parts are relatively simple to check,  to check the ECU strip back the insulation onthe signal wire from the lamna sensor, this wire is usually black. Then hold the bare wire between you finger and thumb then touch your other hand on to the battery positive if the emissions come down then you have a faulty lamna sensor but if the emissions remain the same then the ECU is most likely at fault. For more information on ECU testing click on ECU