No idle

Car Mechanics (UK) - - Help! -

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.

J Ry­der

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.

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