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Earlier in February, Wheels24 reader Kevin Erasmus wrote in about his 2005 Ford Fiesta revving too high and constantly kept stalling when he came to a stop every time he drove his car.
Our Car Doctor, Paul Kasker said "This could be a possible air leak, or most probably a faulty idle stabilizer which is a common occurrence."Since there are still so many vehicles older than fifteen or even twenty years old on our roads, Erasmus' query inspired another reader, John Pearson, to share some of his knowledge about this common problem.
Pearson shares an in-depth look at why older vehicles might have this high-revving problem, and also why the reasoning has an alternative answer in more modern vehicles:
"Let's consider the essential operation of a petrol-fuelled internal combustion engine. It requires at least five things to successfully produce rotational power that can be transmitted from the crankshaft; viz, air, fuel, compression, and an ignition spark.
"In a petrol engine, the flow of air into the engine, to be mixed with the exact proportion of fuel to produce power is controlled by a throttle valve, be it in an older carburettor or a modern fuel injection engine. Diesel engines are different in this respect; linked to the accelerator pedal for the driver to control the required engine speed and power.
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"As we press down the accelerator pedal, a linkage will open the throttle valve (or butterfly, as it is sometimes termed), allowing air to enter the engine, drawn in by the vacuum created behind the throttle valve from the piston's inlet stroke.
"On old carburettors, there was a little screw to adjust the idling speed, by merely moving the butterfly to slightly open, letting some air into the motor on the most straightforward designs, to other carburettors having an independent idling circuit, letting a small volume of air and fuel into the engine downstream of the closed throttle valve.
"On older cars, the spark to ignite the air/fuel mixture was controlled by a set of points, with a condenser to control arcing when the points opened and closed, producing an electrical flux through a coil that generated a high voltage to flash across the spark plug electrodes gap.
"On these older vehicles, the condition of the points, condenser and spark plug gap made a significant difference to the car's performance and fuel economy hence the importance of ensuring the points & plugs were correctly 'gapped'. In fact, the variation of the plug gap, within limits, was not that noticeable. However, if too big, the voltage could be insufficient to flash across the spark plug electrodes in the desired pattern. Any weakness in the ignition lead insulation could cause the spark to jump to earth from the lead rather than doing so in the spark plug, leading to a misfire and a severe drop in performance.
"If the gap was too small, the spark might have been insufficient to burn the compressed air/fuel mixture thoroughly. Thus, the condition and settings of these key ignition components were always the starting point in diagnosing engine faults. Still, they certainly would not have been the reason for a significant increase in idle speed such as Kevin reported on his modern-day Ford. Too big or too small the set gaps, they couldn't have varied the idle speed by much. Such a significant variation must be the uncontrolled ingress of air or uncontrolled activation of the throttle.
Image: Wheels24 / Duncan Alfreds
"Considering more modern, (as well as earlier) fuel injection vehicles with electronic ignition, such as Kevin's example: Typically, the throttle butterfly is not set with an adjustable screw for idling, but by a supplementary bypass valve of differing designs. This permits factors such as engine temperature or outside temperature to be considered to optimise the idle speed further and corresponding fuel proportion to optimise fuel usage and emissions. Various sensors monitor these factors, and a central control unit will signal the air bypass valve to adjust the idle speed to an optimum value.
"Considering this, I fully agree with Wheels24's car doctor's assessment that the idle control valve, or its myriad of control sensors, and mainly their connections are prime suspects. A further clue is that the engine dies after the revs race up, possibly indicating that the admission of air is not accompanied by an injection of fuel to sustaining the rise in engine revs.
"Although diagnosis must never assume anything, and the necessary condition of the ignition system, spark plugs, HT leads etc., must always be included at the start of the diagnosis, modern high-voltage ignition systems will still successfully fire spark plugs in poor condition with electrode gaps way beyond optimum specification."
