Recently, my 1998 Ford Fiesta Encore Zetec petrol with 32,000 miles suddenly refused to idle. It was discovered that a blanking rubber had fallen off the throttle body and it was admitting unmetered air. Even after this blanking rubber was replaced, the refusal to idle has persisted. Before that, everything was perfect: the engine could be started instantly hot or cold with no use of the throttle and it would immediately idle perfectly. Now the engine starts immediately but dies after a second, unless very light pressure is maintained on the throttle pedal. If that light pressure is maintained, the engine can be kept idling more or less normally.
Sometimes it will start to idle normally after a short period of ‘throttle assistance’, at other times it needs to warm up for longer – eg, after a few miles of driving – before it will idle on its own. Even when the engine finally idles by itself, the revs are typically around 600rpm and not the 850+ that the ECU is calling for. There is some hunting, but the ECU is typically only opening the idle speed control valve about 65% – surely it should be trying to open the valve further to get the revs up? If the engine is stopped when warm and then immediately restarted, but with no throttle, it will die. However, if it is immediately restarted with just a touch of throttle it will keep running and idling, even if the throttle is released straight away.
I’ve replaced the idle speed control valve and throttle position sensor. A scan with a laptop running Forscan is showing everything normal for all sensors, that the ECU is running on closed loop and that no faults are logged. Connectors to the sensors and underbonnet loom have been checked and are good, and the throttle body is clean and the gasket is in perfect condition.
As you have already replaced the idle speed control valve and throttle position sensor, I believe the problem is a small vacuum leak. This may be from the rubber gasket at the base of the throttle body or one of the vacuum pipes. There is a small vacuum pipe under the manifold which connects to a T-piece to the manifold and the PCV valve. This pipe system also regulates the fuel pressure control valve and EGR valve.
As you have had a problem with a perished rubber component, I suspect that one of the hidden rubber vacuum pipes is the source of your problem. I would carefully trace and check the route and fixings of the rubber pipes under and around the manifold. If no fault can be found with the vacuum pipe network then I would remove the throttle body and check the rubber base gasket.