Why engines sometimes cut out

WE OFTEN hear of car engines that cut out for no apparent reason, sometimes at motorway speeds and sometimes at low speeds.

The latter was the complaint of a VW Golf owner. He told me that in a bid to solve the problem he had fitted new spark plugs, HT leads and even a spare distributor of known quality, but the cutting out continued.

He was told by a friend that the problem was perhaps caused by a faulty idle-speed control valve, but the friend had obviously not looked at the car because on this model there was no such thing as it idles through the throttle body.

This means that the air necessary to keep the engine running has to go past the throttle plate.

Everything works well under normal circumstances, but when dirt from the oil breather builds up in this area, the entry of air into the engine is restricted and eventually the electronic control unit/throttle body is unable to cope and the engine stalls.

Once the throttle body was given a good clean, the old Golf had no more trouble.

A distantly related problem occurred on a 1993 Passat, but this time it was an erratic idle.

As the car was not worth a great deal of money, the owner was worried about the probable cost of professional attention.

He told me that his son had cleaned the throttle body, renewed the air filter and idlespeed control valve with carburettor cleaner but did not know if he had cleaned off both throttle valves.

Failure to do so would probably allow the engine to idle properly initially, but rise to 1 200 rpm or more as soon as the throttle is opened.

He said that otherwise performance was about the same although petrol consumption was up a bit.

He took the car away and had the throttle valves cleaned and phoned to tell me that everything was now okay.

Had this not been the case I would have suggested checking the throttle microswitches on the throttle body and the wiring to the idle-control valve.

A man told me of cold starting difficulty on his wife’s Renault with single-point injection. It would fire as the key was turned, but almost immediately after starting would cut out.

When it eventually did run, it was essential to keep it revving until fully warmed up. After that, there was no more trouble for the rest of the day.

As with many single-point injection systems, the single injector may easily become contaminated and when such troubles occur, this is the first place to start looking.

Some say that cleaning the injector with a carburettor spray is pretty effective and it can work at times. But, ideally the job should be entrusted to a place with an ultrasonic cleaning facility.

There are other possible reasons for the problems and these include a faulty MAP sensor.

Have a good look at its vacuum pipe. The stepper motor on the idle-control valve is another suspect as is the coolant sensor, but often the injector itself is the villain.