1989 Bronco Full Size Idle Problems

Hey Gang, I have an 89 full size, 5.8, Eddie Bauer. The motor was rebuilt about 50K miles ago, the transfer case was replaced this month and the transmission was serviced at the same time. The truck also has a new
fuel filter, and the emissions were all gutted and rebuilt in the last 30 days.
The problem is with the low idle, if the truck kicks down from fast to slow idle, it stalls about 50% of the time. This seems to have started after the transmission service and new transfer case.
Would resetting the computer restore the idle to default? If so, how do I do it? If the computer won't do the trick, is there an easy way to up the idle speed on an EFI engine? Is there a chance that a vac hose is unplugged someplace that is causing the stalls?
Thanks!
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On 11 Oct 2006 10:05:26 -0700, snipped-for-privacy@gmail.com wrote:

The ECM can be reset by disconnecting the battery for 10 minutes or so. Before you do that, make sure all the vacuum hoses and wiring connections are proper and not leaking. Make sure the throttle body is clean. There are solvents to do this. You may want to make sure you spray some of the solvent into the IAC bypass holes inside the throttle body to claen up the IAC a bit. If that doesn't do it, remove the IAC and soak the valve end in a good solvent to clean it. If it is really bad, it may have to be replaced. If you disconnect the battery to reset the ECM, start the engine without touching a thing and let it idle for 5-10 minutes. Without touching the throttle, move the transmission selection into each position for a few seconds allowing the ECM time to stabilize the idle speed and relearn it's idle parameters. After that, drive the vehicle normally for a few miles around town to allow the ECM to relearn to operating paramaters. It should be good to go after that. If the stalling continues after cleaning the IAC, you may have to replace it. The IAC can get gummy enough not to work correctly without setting a fault code as it would in the event of an electrical related problem. If anyone has screwed with the base idle setting, you may have to get a manual and reset that by the book. If the base idle is not correct, the IAC cannot control the engine idle as it should. The base idle speed is set below the normal idle speed controlled by the ECM via the IAC. If it is not correct, the ECM may start to do funny things with fuel control. The correct idle speed for your engine is determined only by the ECM.
All of that said, it is possible that the transmission is not fully releasing the torque converter at idle as it should. This is rare but possible with the AOD trans if the OD band is setup a bit too tight. That can be checked by removing the pan and valve body. If you notice a hard thunk on engagement, that "may" be related.
Lugnut
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Hey Lugnut, Thank you!! Great post, I'll head out to Walmart tonight and get some cleanser for the IAC and see if I can get that thing cleaned up. I really appreciate the help, this is great advice!
lugnut wrote:

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One more question... I've never serviced an Idle Air Valve, I know what it looks like, but can you tell me where to look for it?
Thanks again!
snipped-for-privacy@gmail.com wrote:

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snipped-for-privacy@gmail.com wrote:

It is attached to the Throttle body by two bolts, where it routes air around the throttle plate Find where your accel/Cruise Control linkage attaches.. that's the throttle body, disconnect the air tube to TB. You'll see one port in front of the throttle plate. Clean the area around throttle plate, as well.
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