by puddlepirate » Sat May 24, 2008 9:15 am
In the early days of my somewhat varied employment history I trained as a motor vehicle technician eventually becoming a registered Austin/Morris technician with British Leyland. By 1980 the MkI Cortina was an old car.
The Cortina was a Ford product. The MkII was introduced in 1967/8 so by 1980 even the newest of the MkI's was at least 13yrs old. As Obs rightly says the MkI had very basic engine electrics - battery, ignition switch, coil, distributor and within the distributor, a set of contact breaker points and a capacitor (aka condenser). I suppose it is possible that if the car were driven through an area of high electrical disturbance, then this might in some way affect the primary and secondary windings of the coil and thus the high tension output from the coil. This could result in the engine missfiring or even stalling because it would impact on the quality of the spark at the spark plugs. However, such interference would almost certainly be sufficient to affect other electrical equipment in the area, e.g. TV's and radios. Such electrical anomalies would not affect diesel engined vehicles because they are compression ignition, not spark ignition engines. An electrical anomaly might have caused a problem with the charging circuit - generator, control box, battery - which might in turn have caused a problem with the ignition circuit but other faults would probably have been evident if that were the case, perhaps a burned out control box or 'boiling' battery, burned out lamps or wiring. However, none of these would cause the car to acclerate of its own accord.
The MkI also had a very basic fuel system - tank, pump, carburetter, accelerator cable, pedal. There was no solid state stuff controlling the engine electrics or fuel system, much less any form of engine management system. Acceleration was simply a matter of pressing the accelerator pedal which, via a cable, operated a butterfly valve in the choke of the carburetter allowing more fuel/air mix to enter the inlet manifold. As engine speed increased it was necessary to advance the ignition timing and this was achieved by a set of bob weights fitted to and below, the distributor baseplate - as engine speed increased so did the speed of the distributor spindle, thus allowing the weights to spin out, moving the base plate in relation to the cam at the top of the spindle which opened the contact breaker points, thus advancing the ignition. There was a vacuum diaphragm attached to the distributor with a vacuum pipe connection to the inlet manifold. This was to balance ignition timing with engine speed / throttle opening.
Thus a fault with the ignition system would not result in the car accelerating of its own accord. Only a variation in the volume of air/fuel mix entering the combustion chambers would do that. There were faults, such as air leaks around the inlet manifold, that could result in the engine 'hunting' at tickover (hunting is a condition where air is drawn into the inlet manifold when the throttle is closed and the vacuum in the inlet manifold is high, The idle speed increases momentarily but as no fuel is drawn in, the idle speed slows again. This stops when the accelerator is pressed (and inlet manifold vacuum reduces) but as soon as the throttle is closed the process repeats itself).
The accelerator cable* was a 'Bowden' twisted wire cable, rather like the brake cables on a bicycle, contained within an outer sleeve. If the inner cable frayed it could stick as the frayed strands would not slide easily within the outer covering. If the car were moving, this could prevent the engine from slowing down when the accelerator pedal was released. However, releasing the clutch and applying the brakes would stop the car.
* can't remember if some models, depending on carb fitted, didn't have a linkage rather than a cable.
Vibration could be caused by faulty engine or gearbox mountings - or if the engine was suffering a missfire due to some problem with the plugs, points, coil, HT leads or even cyclinder compression. Even a suspension fault might cause unusual vibration.
Given the age of the vehicle, I suspect the problem was with the vehicle itself, not external factors.
You can fool all of the people some of the time and some of the people all of the time but you cannot fool all of the people all of the time (Winston Churchill)...causa latet, vis est notissima