General Idling problems

[carrotronics]

Hi, got a problem with idling on a Marea. It is jumping from around 950rpm to 2500rpm and I have no idea why! I changed the idle control unit (the black box with a small motor in it) but to no avail. There are no error codes from a diagnostic machine, checked all connections to sensors etc. The car will run as normal when I take the air intake off and place my finger on the flap inside the throttle control unit. The revs reduce a bit when the engine gets warm. Been running like this for sometime now and the rev counter works intermittently. Any Ideas would be much appreciated in what to check for or replace next. Thanks in advance.

 

[20VMarea.Weekend]

You do not say what engine you have in your car....

My 2.0Ltr 20V engine used to randomly rev up to 2000rpm and drop again and then back to idle without me touching anything...!!

Removed and cleaned the idle control valve which does get very dirty and sticks open which helped but I still had the engine reving up on its own at traffic lights.!!

I used to get an intermittent injector light up on the dash whilst driving indicating an ECU error which I traced to the coolant temp sensor... Changed this and all the problems went away... Idles flawlessly and no injector light... MPG also has improved as ECU now knows how hot or cold the engine is..!!!

May be worth a try, very easy to change and does not cost that much...

Do not get confused with the temp sender to the temp gauge on the dash as this is a seperate sensor in the near side of the engine block.. The actual ECU coolant sensor sits in the thermostat housing..!!

 

[Migaard]

Hi.

Had the same problem on my 1.6 16V Marea, but i got the injector light ON as well.
The ECU had logged problems on the butterfly valve stepper motor controlling the idle. I left it soked in WD40 over night and refitted it next day.
It solved the problem right away.

Here's some info about idle control in the 1.6 16V engine:

Start up:
Set mainly by coolant temp and battery level.

Warming up:
Set mainly by coolant temp.

Warm:
Set by rpm sensor.

Since you have replaced the stepper motor the fault must be somewhere else.
Is the butterfly valve clean? make sure it moves smothly and doesn't hang.
Otherwise i would go for a defecive temp or rpm sensor or a air leak in the intake.

Good luck
Michael