Thread: Ecu
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Old 31-05-2017, 09:19 AM   #8
Dr Terry
FF.Com.Au Hardcore
Join Date: Jan 2011
Posts: 590
Default Re: Ecu

Originally Posted by wanneroo View Post
My fairmont is still at around 900rpm on idle when warm and out of gear, I called the RAC who replaced the ECU they said to run it a few days for the new ECU to learn sounds a little weird maybe its correct ?
Sounds like a little bit of BS. It only takes a few minutes for an ECU to '"learn" the idle routine.

Firstly I would verify, with another tacho, that it is indeed idling at 900. Don't trust tacho in the dash.

An engine can idle too fast for 1 of 4 reasons:-

1. ECU or associated sensors faulty.
2. Idle motor not sealing at rest.
3. Base idle screw set too high.
4. Inlet manifold (or similar) air leak.

1. This is easy to eliminate, just unplug the idle motor at warm idle to see if (& by how much) the idle speed drops. If the idle drops, check the electronics. If it doesn't drop move to no. 2.

2. To eliminate a faulty idle motor, remove it & temporarily block off the 2 holes, where it's mounted. If the idle drops, replace the idle motor, if not go to no. 3. Also you can check the idle motor off the car, by just trying to blow thru it, it should NOT pass thru any air off the car. If it does, it is faulty. This applies only to this style of idle motor. Stepper motor types, as used by various other manufacturers are not diagnosed this way.

3. Check to see if the base idle screw has been tampered with. If it has, try to unscrew it to check its affect. If in doubt, remove throttle body & check to see that it is almost fully closed when the throttle is at its stop. Quite often they need nothing more than a good clean & base idle reset.

4. Once you've made sure all the above 3 are OK, check for manifold leaks. Sometimes this might be a leaky brake booster or other device not actually at the manifold itself, so check all hoses. Maybe use a carby cleaner spray to spray around the inaccessible gaskets (with the motor running) & see if the idle alters, it might even increase. If this happens try & localise & then repair the leak.

All of the above assumes than the car is factory stock & is in a good state of tune & that the thermostat is operating to full temperature. If the engine does not reach full temperature, the ECU will hold the idle up slightly until it does.

Dr Terry
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