Ford Fiesta Zetec 1999 - High engine idle speed. VSS?

Honest John Back Room » Technical matters
What is life like with your car? Let us know and win £500 in John Lewis vouchers | No thanks
View Threaded
Tue 10 Sep 2013 15:48
Ford Fiesta Zetec 1999 - High engine idle speed. VSS? - alastairq

Engine idles between 1200-1500 revs when hot...but this speed drops to 800 revs , immediately the vehicle stops moving.

Have experimented on a steep hill, bringing car to a halt [idle drops to 800 once stopped].....I then let the car roll forwards, and almost immedaitely the idle speed starts to rise to around 1100-1200 revs, dropping again as soon as the car is brought to a halt!

Is this a faulty vehicle speed sensor, telling fibs to the ECU?

Reply | Report as offensive | Link
Tue 10 Sep 2013 16:13
Ford Fiesta Zetec 1999 - High engine idle speed. VSS? - unthrottled

I appeared to 'solve' this problem by cleaning the ICV and resetting the butterfly valve (loosen the torx screws holding the butterfly to the spindle, fully opening the butterfly valve then retightening).

I can only guess that the ECU sets a minimum ICV position irrespective of RPM whilst the vehicle is moving-perhaps for emissions purposes?

I don't know if it will work on your car!

Reply | Report as offensive | Link
Tue 10 Sep 2013 16:27
Ford Fiesta Zetec 1999 - High engine idle speed. VSS? - alastairq

Very many thanks...seems a cheaper option than a new VSS? {£40 off eBay]...

Reply | Report as offensive | Link
Thu 12 Sep 2013 03:45
Ford Fiesta Zetec 1999 - High engine idle speed. VSS? - Terminator-17

If the idle speed is doing exactly as you describe, then that is normal operation and your VSS and ISCV are definitely functional.

1200 revs while in motion; 780 stationary. If you meant that you had excess idle speed when stationary, then that might have a different cause.

Reply | Report as offensive | Link
Thu 12 Sep 2013 21:14
Ford Fiesta Zetec 1999 - High engine idle speed. VSS? - alastairq

thanks for that..yes, pretty much as you describe.....however, until several weeks ago, the engine idle speed whilst moving, would fall lower than 1200 revs...

Now [if I depress the clutch. for example] the revs only fall to around 1200-1500 revs.. so whilst moving, the engine idle speed seems un-naturally high.

I guess I'm not used to engines that don't have a sharp cut-off in revs when the throttle is slammed shut?

Am I correct in thinking that, if I come 'off' the gas [for example, in top gear]..., ie, put the engine on over-run, is most/all of the fuel returned to the tank? [I'm considering economy when asking this.]

If so, would that be the case given the higher engine idle speed whilst in motion?

Reply | Report as offensive | Link
Thu 12 Sep 2013 21:23
Ford Fiesta Zetec 1999 - High engine idle speed. VSS? - unthrottled

Newer cars shut off the fuel completely when in overrun. I'm not sure if your car has deceleration fuel cut-off or not. Mine is '96 and doesn't.

The amount of fuel injected roughly equate to idle consumption-so yes, most is returned to the tank.

The limiting factor is misfiring under very high vacuum conditions.

Like I said, I had a similar problem. It is annoying. Changing up is a pain because the revs take longer to fall back. Also, you lose some engine braking effect-and your consumption is a bit higher.

Reply | Report as offensive | Link
Thu 12 Sep 2013 21:26
Ford Fiesta Zetec 1999 - High engine idle speed. VSS? - Terminator-17

If the revs previously dropped to 780 while moving, that might suggest you had a non-functional VSS. (Disconnecting its plug has that effect.)

The 'enhanced idle when moving' stage these systems employ may be an attempt to control on-off throttle jerkiness - I can't see it having much efect on fuel consumption.

With a closed throttle, the injectors are shut off on the over-run above about 1500 revs - the fuel circuit maintains a constant pressure and flow during this phase, so there's little effect on the amount returning to the tank.

Reply | Report as offensive | Link
Sun 15 Sep 2013 20:14
Ford Fiesta Zetec 1999 - High engine idle speed. VSS? - Dimebag

Hi guys, just wondered if i could muscle in on this thread let me know if i need to start a new one. You seem like a knowledgeable bunch and i always seem to get an unfriendly response on other forums!

Sister in laws 2001 Facelift Mk4 1.25 zetec is using more fuel than it should, no fault codes logged but it has the dreaded hanging idle problem which is the only thing i can see wrong.

The idle spped tends to hang for a fair few seconds between gear changes, the car idles at just about 1000rpm stationary

I have searched all the basics such as throttle body clean, vacuum leaks, new icv, battery disconnect for 30 mins in an attempt to do a re learn etc but am a bit stumped.

It looks like the tps problem to me but when testing the sensor with a multimeter i seem to be getting the correct readings. Throttle closed 0.98 volts, wot 4.95 volts, it shows position at 19 % on live data at idle which i thought was high.

I noticed on the live data readings too that the front lambda is a little erratic, it sometimes cycles correctly at 0 to 0.8 but then goes all over the shop on a test run switching too quickly if anything.

Map was between 20-30 Kpa at idle but was also a little erratic jumping to 90 Kpa with only light throttle openings, i put this down to the icv being open too far maybe?

Sorry for the long winded post but any tips on where to go next would be appreciated.

Reply | Report as offensive | Link