Some faults are so obvious and easy to fix that they are just as easily overlooked. A chap with a well-maintained Ford Escort complained that while idling or when moving slowly in rush-hour traffic the engine revs would gradually increase, sometimes alarmingly to more than 4000 before returning to a steady idle.

He had owned the car for only a few months and, apart from this fault, it ran well and burned no oil. There was occasional smoke at the exhaust, but he felt it was nothing to worry about.

I had a look at the engine and it looked clean enough. Revving it showed no exhaust smoke and he assured me that the oil and filters had been recently changed and that the correct oil had been used.

It was only when I went to check the oil level that I noticed the dipstick seemed to be too long for its tube. In fact, it was obviously not the right dipstick. When I pointed this out to the owner, he told me that he had done the oil change himself and had refilled with the recommended 4.5 litres and that the incorrect dipstick would not matter.

Well, that’s all right in theory, but it is impossible to get all the oil from an engine by draining it. In fact, up to a litre could remain in the system. This is why to do a complete change a flush is sometimes necessary to get the remains of the old stuff out.

The first move was to get the correct dipstick and check the level – which proved to be far too high. Why the wrong one had been used was anybody’s guess. However, with the oil drained to the correct level the auto-revving ceased and everybody was happy.

Overfilling the sump need not have any disastrous consequences, but it does depend on by how much excess oil has been added. Oil pressure reading will give no clue because the pump’s pickup will be at the bottom of the sump. Oil pressure drops when there are excessive bearing clearances, or interior wear in the oil pump. But the pumps are designed to have sufficient reserve capacity to allow for normal wear.

In the case of excessive overfilling, the webs on the crankshaft will be disturbing the oil at every revolution and this unwanted disturbance may cause oil contamination of the throttle housing and air mass meter.

Such problems can be nasty, so be certain you do not overfill.

The final problem for today is jerking while accelerating. The car’s owner had changed the stepper motor and water temperature sensor, along with the old favourites, plugs, HT leads etc. But all to no avail.

An engine that jerks while accelerating is normally caused by a weak mixture.

It could also be due to a couple of other things, the throttle position sensor and the lambda sensor.

Testing is simple; while the engine is hot (being careful of course) briefly disconnect the lambda sensor and take a short test drive. If there is no improvement, do the same with the throttle position sensor.

When neither move shows improvement, pray to whatever gods may be because it could be a software fault. -Pretoria News Motoring