Bizarre Revving Problem

[Gorath]

Hey =)

This is my first post on the forum, hope i've come to the right place and you guys can help me with an issue I am having with my mk4 Fiesta.

The issue is as follows:

When accelerating or driving down the road, if I drop the clutch (so the car is essentially free wheeling) the car automatically ups the revs to 3-4k.

However, if I slow the car down to stationary and drop the clutch the car will go to 2k revs then drop to idle at 1.5k.

When I initally got the car the idle rpm was 1k which I believe is what it is supposed to be. I recently had the fuel pump and water pump changed on the vehicle as well incase that adds any clarity to what could of caused the issue.

The strange thing with this issue is it is NOT reproducable. Sometimes the car comes to life and works normally, other times the car gets out of the wrong side of the bed and starts acting strangely (described above).

At the moment I have 1/2 days a week where it acts up and the rest of the week it is fine.

If anyone has any ideas on what the problem could be and if there is anything I could do to fix it I would be greatly appreciated!! If more information is needed I would gladly supply.

Thanks for taking the time to read,

G

[FOCA]

2 things worth checking are the idle control valve and that the crancase breather (including the PCV Positive Crancase Ventilation valve) is not gummed up or there is no air entering the system

Do you know if it has an OBD11 port (codes? im thinking possible faulty sensor, eg - temp) and is the tickover controlled by the ECU?

[HandsonJohn]

I brought my Fiesta 1.25 Zetec with a similar issue, but it only surfaces once the car has begun to warm up (Just as the temp gauge clears the blue zone and is at the start of the warm-normal zone). The idle was fine before then and once it warmed up, unless I was stationary, the revs would climb up from 1200-2500 rpm. I did find a collapsed and leaky vacuum pipe under the inlet manifold and changed that, but the problem remained. It was only when I was doing an electrical connections diagnostic, that I noticed that sometimes, the power steering pressure sensor switch would show an open circuit, when under normal tickover and above idle rpm, the switch should show a "closed" or on condition. I removed and cleaned the contacts on the switch and the connector to the electrical lomb and the problem vanished for good. On reading about the operation of the switch, if the engine management believes that the pressure in the power steering is too low, it increases engine rpm, to prevent a stall condition (eg if you are holding the steering on full lock or the air con kicks in at idle and the headlights and other high loads are present).

To test this out, disconnect your sensor, which you can see by looking under the car on the drivers side and it is facing forward, where the power steering pump is. Once disconnected, hold the steering on a full left or right hand lock and see if the car now gets close to, or does stall. if there seems no problem, bridge the two connections with a paperclip (***Do not let that paperclip touch any of the cars exposed metal with the ignition on***) and then stick some electrical or selotape over the temporary "Bridge" you have installed and take it for a drive.

If the car idles fine once it has warmed up, then you have one of two conditions

1. the power steering pressure sensor is faulty and you will need to test it, to confirm if you need a replacement (how to test it yourself is included below.

2. The electrical circuit that the pressure switch is part of has a poor connection somewhere.

To test the switch, you will need either a continuity tester, or a small wattage car light bulb (5 watts or less, but you can pinch it from the interior light if you do not have one handy) and two longish (1 meter or longer) wires)

If you have the continuity tester, check that you have a closed, or on condition with the engine at idle (remembering that the pump is spinning down near your fingers) and then an open or off condition with the engine off (Normal operation)

If you do not have the luxury of the continuity tester, you must run one wire from the battery earth to the switch and then back from the other terminal of the switch (it does not matter which ones you pick, it's a switch). Make sure that the 2 wires are not touching, or anywhere near any moving mechanical parts. Start the engine and allow to idle, now connect the wire coming back from the switch, to one of the bulbs 2 metal tabs or to the outside metal surround of the bulb fitting and then touch the other connection of the bulb to your battery positive. If the bulb lights up with the engine running, the switch is good, but if it fails to light up with the car on tickover or above idle, the switch is bad.

I did bridge the connection originally and found that the engine has no "Stall" prevention, so if you end up doing this to keep you happy, you will need to be aware that the car can stall if the idle is low and you turn the wheels on a hard over position.

I know this might seem an odd place to look for an idle issue, but that's how Fords these days get the "Anti-stall" to work!

Good luck on finding the fault