I have had this problem since i put this complete motor in, it want to idle around 1200. Checked for vacuum leaks found none check pcv valve and the hose. I put new TPS on set it below a volt. But new Idle control valve and nothing. So i checked plates in the throttle body and the are closed completly so i put my hand over the intake part where air goes in and it sucks hard on my hand and idle does go down to around 900. But shoulnt it die? I think there might be intake gasket leak but would it still run with just that? Could the TPS not be right? I would think if it was set wrong that it would still die if i blocked air from coming in. I pinched off vacuum lines going to brakebooster to see if leak there and negative.
Any ideas would be great im working on it now outside and 20 degrees and snow and ice are on the way for tonight is my second vehicle but only one with 4wd so would perfer it.
Thanks
89 Bronco 302
Kansas city

 

Did you check the mechanical idle setting?? Try loosing the idle screw on your throttle body and see if your idle goes down. It should.

 

As Chrisvte says, you need to set your base idle. Get the truck warmed up, then shut it off. Unplug your IAC, then restart the truck. Using a tach, adjust the base idle setscrew at top of throttle body until you are at 650-700 RPM. Shut off the engine, attach a voltmeter between the green wire on the TPS (you will have to stick a pin in the wire) and chassis ground. Now turn your key to the "RUN" (NOT start) position, and make sure your TPS is still between .8 - 1.0 volts. If TPS is in range you should be all set, if it is not, then you need to re-adjust the TPS so that it is less than 1.0 volts with the new idle screw setting.

If you completely cover the Throttle Body throats and the truck still idles, you may have a more serious problem. Set the base idle and try it again. If the problem persists, then you probably have a pretty significant vacuum or intake gasket leak. The engine has to be getting air from somewhere.

If it still runs with the throttle body covered, then put something over the TB throats (duct tape works) then take a non-flammable spray (window cleaner or something similar) squirt it around your intake. Hit the TB gasket, Upper intake, Lower intake...just keep working your way around. If idle suddenly changes significantly, you have found your leak.

Good Luck

 

Thanks for the info
THe screw is backed out and plates are shut all the way.
I block off intake air at the throttle body and i removed the egr valve and covered that hole also and still runs a little above idle.
I have sprayed around the intake gasket and idle changed a little.
It is hard for me to believe it would idle so well with just a intake gasket leaking. But maybe it does leak enough to keep it running.
The main tree up top where the vacuum lines connect on top of the phlenum i have tried blocking off ones that go to brake booster and another one not sure which one is it is a rubber hose and no effect. I unhooked map senosor and it died. Is there any other place that it could be geting air in? I think its the intake gasket just not sure any other ideas would be very welcome.

89 Bronco 302

 

My 89 302 efi was idling at about 1500-2000 rpms, so i changed the upper intake manifold gasket and that helped a little, then i noticed that the IAC was on backwards, i turned it around and now it idles between 700-1000 depending on whether it is cold or not