Fiesta revving problem - Ford Forums

ttaunton · 09-10-2009, 06:53

R reg Zetec Fiesta. Suddenly started behaving strangely. On starting it would idle OK for 20 or 30 seconds then slowly increase the idling speed to about 4K. It was not in gear/feet off everything. This seems to be intermittent: do it once then OK for a few hours or days.
Also the revs do not drop when you depress the clutch, say at 40mph. Take your foot off the accelerator and the engine carries on going at that speed, then sometimes slowly decelerates, but never at the speed it should.
I have removed the throttle position sensor and measured it: it measures 632 ohms when shut and 4.15k when fully open. Measuring the voltage on the centre contact when the ignition is on, and I get 0.9v. Nothing appears to be sticking: the throttle snaps shut and the unit measures 632 ohms consistently. I have cleaned the throttle butterfly. No hoses are split.
I have unplugged the road speed sensor (triangular plug at rear of engine bay) and the revs drop when you are going along and you depress the clutch more as you would expect, but they they still seem to be more related to the speed you were going at than the position off the throttle.
It has just done the "climbing rev" thing again, as I just went out and started it. Googling this type of problem it seems that the TPS should be at about 0.4v not 0.9? This would point to high end resistance on the pot. Has anyone ever done an ohms measurement on one?

**rtg143** · 09-10-2009, 08:48

What year is it and whait engine size?

ttaunton · 09-10-2009, 09:39

It was first registered in August 1997. Fiesta Fusion 1242cc

**rtg143** · 09-10-2009, 10:11

Here's what we use for specs. A scope works best but you can watch the readings with a good voltmeter.

ttaunton · 09-11-2009, 16:21

Thanks for the info. Mine measures 0.9v with the throttle closed, which agrees with your figures, within realistic limits. I went out this morning and started it, and within a few seconds the revs started climbing towards 4k. I went and got my meter and the centre contact measured 0.89v. ( I did it by sticking a needle through the wire to contact the copper inside) That proves, really that it probably isn't the pot, although the best test would be to disconnect the plug from the pot while the fault is on and see if the revs drop. Could the fault lie with the idle speed control valve? Is resetting the engine management system going to do any good, or is that something you should avoid?

**rtg143** · 09-11-2009, 18:23

I'm bot familiar with that particular system, but if it has a IAC, it can cause high idle but that high seems extreme. Hopefuly someone that knows that system will jump in here. Can you get a Haynes or Chilton makual for it?

ttaunton · 09-12-2009, 02:11

Right....
Just been out there (lovely and sunny) took the clip off the TP sensor pot in readiness and clacked the throttle against its end stop a few times and started the car. It immediately started doing its thing..revs started climbing. Measured the pot wiper and it was 0.91 volts, which is OK. Quickly took the plug off the TPS and the revving stopped. Throttle revs restore more quickly without the pot, they drop away as you would expect when you take your foot off the gas. Still after a few minutes things seemed to settle down and it idled at about 1400 with the plug back on, but it was a cold engine. Where are the plugs and sockets carrying the TPS signal and where is the management computer? It may be a connection problem, anyone had a problem like this? Whats the down side if I disconnect ithe TPS? It seems to run OK without it.

monkizzle · 10-06-2009, 13:01

I am having the same problem at the moment a brief history

My car was running like crap for a while loss of power acceleration wasnt smooth. I replaced the coolant temperature sensor and this fixed this problem. However then this over revving come along. I have been doing some research and many people say that the power steering pressure sensor is to blame if it has a dodgy connection. However i have tested this on mine and it seems not to make a difference.

When you disconnect sensors the PCM(ECU) will refer to default values for speed / revs and mix fuel to air accordingly perhaps this is why it seems fine without the TPS connected. I still havent figured out how to cure this on my car yet so any help will be appreciated. This seems to be a common problem.

monkizzle · 10-06-2009, 13:02

o mine is a 1996 mk4 fiesta 1.25 zetec-S

Update:

Try unplugging vehicle speed sensor. Not the speed but the sensor wires.