Technical idle speed up and down

[TechNot]

Hi,
I'm drivin' a '97 marea, 89k miles, 1.6 16v and my idle speed is irrratic? At start up from cold it usually revs up high 2000rpm or more(maybe normal but seems excessive??) then drops back a bit as it warms up. Then it starts reving up and down from about 1500 to 750rpm constantly ???
Anybody let me know where to start lookin' ?
Thanks

 

[jug]

does it drive ok? is the injection system fault light illuminated?

 

[TechNot]

Hi Jug,
There's no fault lights on (except airbag!!!) and other than sounding like i'm lookin' for a race at every set of lights its running fine?

 

[jug]

excellent. its a common issue on this engine (isnt everything )

since the rpm drops from cold idle to warm idle, we can be fairly sure the idle speed control valve and coolant temp sensor are working.

once warm, the idle is controlled by the ecu using the rmp sensor (crankshaft position sensor). this is what i would suspect to be the problem.

the 2 attached images are copied from the fiat manual. notice that on the second page there is a description of how to test the sensor. it should have a resistance of 575-750ohms. use a multimeter to test your sensor.

also notice that a feeler gauge can be used to measure the gap between the sensor and phonic wheel. this gap should be 0.5-1.5mm (1mm recommended).

it is also important to check that the sensor is positioned and aligned correctly.

if in doubt, replace the sensor. this is an easy enough diy job.

 

[TechNot]

Thanks a mill for the all the info(pictures and everything), I'll give it a go when it stops raining. Will let you know how i get on!

 

[TechNot]

Checked the gap at the tdc senor and adjusted to 0.6mm. Checked the resistance and got an open circuit, think i confirmed this with continuity test failling. Will buy new sensor.