A friend of mine owns an elderly Volkswagen Jetta, but the fault I shall describe could apply to many vehicles. The car had been bought new and had been in the family for several years and occasionally professionally serviced. Until the owner carried out a cam belt change it was running well, but then developed a misfire and would idle unevenly.
Not unreasonably, he thought perhaps he had fitted the belt incorrectly, a tooth out or something similar, and finally he noticed that the carbon brush in the distributor cap was badly burnt, so this was renewed with a quality component, but lasted only three or four months before it burnt right down to the plastic again.
Three months later, although the car was still running perfectly, he noticed that the carbon brush in the cap was badly worn again and this was again renewed with a Bosch unit, the same as the previous one.
The next step was to dismantle and check and clean everything in the distributor.
All HT leads were renewed along with spark plugs. The coil was cleaned and he checked every connection he could find and tested everything that could have a bearing on the trouble with his multimeter.
However, his troubles were not over because a couple of months later, although the car was still running well, he checked and found that the carbon brush in the cap was badly burnt again. And so he brought the car to me.
I do not know if the coil had been changed or was the original, but it seemed to me that the trouble was due to the coil – either it was defective, or the wrong one had been fitted. It sometimes happens that a short to earth somewhere in the primary winding alters the ratio between the primary and secondary windings. This increases the secondary output. Mind you, this also increases performance, but soon causes the coil to fail.
The answer was simply to fit a new coil of the correct type.
We tend to take coils for granted sometimes. I recall working on an old Hillman many years ago in what was then Rhodesia. The car would start well and run well for several kilometres and then the engine would die as if running out of petrol. The trouble turned out to be due to an incorrect or defective coil. All the more reason to make sure that, if you must buy a used coil, that it is the correct one for your vehicle.
Emissions tests are somewhat confusing and an example of this was when a guy bemoaned the fact that his car had failed due to a lambda reading. The owner asked me if I would get a new sensor for him to cure the fault.
In fact, the lambda reading which caused the failure had nothing to do with the sensor itself. Therefore renewing it would have been a waste of time and money. Instead I examined the car’s exhaust system and found a couple of holes. It was then up to him whether to renew the exhaust or do a bodge with some exhaust putty.
Knowing this fellow, I suspect he went for the latter option. - Star Motoring