Low Idle as it warms up

MIKE GLADWIN

MEMBER
2011 FORD FIESTA
1.2L
4 CYL
2WD
MANUAL
89,000 MILES

From cold and on tick over voltage drops from 14.3v, to 12.2v, after about 4 minutes. A few milliseconds later the rpm drops from 1100 to circa 500 (recorded on a diagnostic tool). This lasts a few seconds then voltage returns to 14.3, RPM surges and then stabilises around 1000. It may happen a couple of times whilst getting to temperature, but once at operating temperature is fine.

I have changed the coolant sensor, bank 1 oxygen sensor, throttle control body and the MAF sensor. I have also checked for vacuum leaks. There are no error codes.

5 Replies

JACOBANDNICKOLAS

MECHANIC
108,262 POSTS

Hi,

Does the engine idle get low and then the voltage drop and opposite? If so, it sounds like the idle air control valve may be bad. Take a look through this information and let me know if it helps. Also, I need to confirm this is a 1.6L engine. I have nothing on a 1.2L

______________

2011 Ford Fiesta L4-1.6L
Idle Air Control (IAC) Valve
Vehicle Powertrain Management Fuel Delivery and Air Induction Idle Speed/Throttle Actuator - Electronic Description and Operation Components Idle Air Control (IAC) Valve
IDLE AIR CONTROL (IAC) VALVE
ENGINE CONTROL COMPONENTS

Idle Air Control (IAC) Valve

NOTE: The IAC valve assembly is not adjustable and cannot be cleaned.

The IAC valve assembly controls the engine idle speed and provides a dashpot function. The IAC valve assembly meters intake air around the throttle plate through a bypass within the IAC valve assembly and throttle body. The PCM determines the desired idle speed or bypass air and signals the IAC valve assembly through a specified duty cycle. The IAC valve responds by positioning the IAC valve to control the amount of bypassed air. The PCM monitors engine RPM and increases or decreases the IAC duty cycle in order to achieve the desired RPM.

The PCM uses the IAC valve assembly to control:
- no touch start
- cold engine fast idle for rapid warm-up
- idle (corrects for engine load)
- stumble or stalling on deceleration (provides a dashpot function)
- over-temperature idle boost

__________________

Note that it indicates it can't be cleaned or serviced. Here is a link that shows in general how one is replaced:

https://www.2carpros.com/articles/how-to-replace-an-idle-speed-control-motor-iac

Let me know.
Joe

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Tuesday, November 17th, 2020 AT 8:38 PM

MIKE GLADWIN

MEMBER
3 POSTS

Hi,

It is a 1.25 (UK spec) which is the same engine as the 1.6, just with a smaller bore. It has the later electronic throttle, which has the IAC incorporated into the throttle control body, which I have changed. The voltage drops a couple of milliseconds before the RPM, as the vehicle warms up, which suggests a short to me, as something is activated as the engine gets to temperature. I have run it with a spare coolant sensor attached, it obviously leaves the 'choke' on thus keeping the revs high and the voltage drop never happens. Again suggestive of a circuit being opened as the vehicle reaches operating temperature.

Attached is a screenshot of it happening from my diagnostic tool

Image (Click to make bigger)

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Wednesday, November 18th, 2020 AT 3:13 AM

JACOBANDNICKOLAS

MECHANIC
108,262 POSTS

When you get the voltage drop, where are you getting that reading?

Joe

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Wednesday, November 18th, 2020 AT 8:19 PM

MIKE GLADWIN

MEMBER
3 POSTS

Forscan diagnostic readout via the OBD2 port.

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Wednesday, November 18th, 2020 AT 11:49 PM

JACOBANDNICKOLAS

MECHANIC
108,262 POSTS

I'm questioning if it has something to do with the alternator itself. Does it seem like it is consistent timing when it happens and the voltage drop is the same all the time?

Joe