1- see that the throttle body flap is closed at idle, ie properly adjusted. if someone played with the adjustment then the iac will close to compensate but it won't be enough. proper adjustment for the flap is to turn the screw back until it disconnects from the flap and then turn it in 1/2 turn after it contacts the flap again.
2- pinch the vacuum booster hose while it is running; you may have a torn booster membrane.
3- stuck pcv valve- pinch off it as well and see if normal idle returns.
4- the iac is not being controlled by the computer; maybe a harness or computer problem.
expose the throttle body and with your finger close off the iac intake port; if the idle falls drastically or the vehicle stalls, then you can be sure that the problem is with the iac or its control.
SOURCE: miss fire #3 cyld. compression 125psi idle speed 2500 rpm
Hi
A faulty IAC would cause a random misfire rather than a misfire in a specific cylinder, as would an intake air leak, both of which would cause a high idle amongst other things.
Run the engine until its good and hot, disconnect the injector harness plug so the injectors are disabled, remove all the spark plugs, disable the ignition by unplugging the wiring connector(s) from the coil pack(s), and then carry out another compression test on al the cylinders.
If the compressions are now all relatively the same then you very highly likely have a faulty injector in cylinder 3.
Very best regards
Geordie
SOURCE: 1989 GMC 2500 5.7L - engine surges and stalls at idle. Loses power at cruising speed.
Wow you have replaced a lot of stuff. Lets go back to the Fuel pump. When you replaced it did you replace the hose between the pump and the sending unit. These are common for splitting. Have you hooked up a fuel pressure gauge to check what pressure you have? It should be real close to 13 psi and should raise to at least 18 or 19 when you pinch off the return line.
SOURCE: high idle with new iac valve no difference 1509 trouble code
With the engine running take a can of aerosol spray WD-40 and spray it at the vacuum lines, the throttlebody and the intake manifold gaskets. At one point you should here the engine speed decrease. If you do, concentrate the spray around the area until you are able to pinpoint the vacuum leak. If you don't find a leak, try clamping the line to the brake booster with your fingers and see if that makes a difference in the idle speed, if so the diaphram is at fault and needs replacement. Thanks, Dana
SOURCE: 1998 subaru rs impreza 2.5
Hi
Usually a vacuum leak will make the idle bog down rather than speed up, even if the ECU is compensating. It will be rough, but as you say, it's fine through the revs.
This leads me to think the PCV (postive crankcase valve) is not connected or is leaking. This raises the idle, although the ECU will suspect the IAC to be the problem. Check the PCV valve first.
Check the MAF sensor for dirt too.
Another point to make is the throttle body might be clogged/dirty. Get the intake manifold off again and clean it with petrol/carb cleaner and a toothbrush, check the condition of the rubber seals, if they are gunked up, the air speed will increase, causing a high idle. Check the exhaust manifolds for any build up/rust.
Finally, when were the O2 sensors last changed? If recently, are they the right way around? If not recently, try those, but make this the last resort as they can cost a lot.
I hope I have covered every eventuality as I can see how annoying this is for you. As you already mentioned, reset the ECu after each attempt.
Good luck, fire any questions back if you need.
SOURCE: loss of power and a slight skip through driving
Hello! Although the Throttle Positioning Sensor (TPS) has been changed the symptoms are pointing at it...When the new one was installed was it mounted with the throttle valve held shut; Then rotated counterclockwise until the mounting holes lined up? Send a comment...Guru..saailer
I’m happy to help further over the phone at https://www.6ya.com/expert/david_6df67de3b14de867
OPERATION
The Idle Air Control (IAC) valve adjusts the engine idle speed. The valve is located on the side of the throttle body. The valve is controlled by a duty cycle signal from the PCM and allows air to bypass the throttle plate in order to maintain the proper idle speed.
Do not attempt to clean the IAC valve. Carburetor tune-up cleaners or any type of solvent cleaners will damage the internal components of the valve.
Fig. Fig. 1: The IAC can be monitored with an appropriate and Data-stream capable scan tool
REMOVAL & INSTALLATION
Fig. Fig. 2: IAC solenoid location-4.6L engine
82 views
Usually answered in minutes!
×