Weird Idle Problems
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Weird Idle Problems
1990 Full Size (Is there another?) 302, Automatic 200k+ mi
Warm weather... YoYo Idle
Truck will start, and idle will immediately stick to 2000 rpm's for about 10 seconds, then crash out and idle off. If I hold the gas down when it happens I can usually keep it from stalling, but the rpms will still YoYo for about 3 minutes, and I have to babysit the idle before it settles down
Cold Weather... Pegged Idle (2000RPM's)
Truck will start and immediately go right to 2000+ rpms and stay there. If I tap the gas, it will cause the RPM's to surge as expected and then the idle will come back down to around 8-900 and idle fine and continue to do so until the truck gets cold again.
Is this just a sticky throttle body? Maybe a bad oxygen sensor? Only other problem the truck has is a clogged pcv valve, which I will replace here soon. I think I have seen every crazy issue a bronco can throw at you, but they never seem to amaze me lol
Needless to say, I appreciate any assistance you can throw my way
Warm weather... YoYo Idle
Truck will start, and idle will immediately stick to 2000 rpm's for about 10 seconds, then crash out and idle off. If I hold the gas down when it happens I can usually keep it from stalling, but the rpms will still YoYo for about 3 minutes, and I have to babysit the idle before it settles down
Cold Weather... Pegged Idle (2000RPM's)
Truck will start and immediately go right to 2000+ rpms and stay there. If I tap the gas, it will cause the RPM's to surge as expected and then the idle will come back down to around 8-900 and idle fine and continue to do so until the truck gets cold again.
Is this just a sticky throttle body? Maybe a bad oxygen sensor? Only other problem the truck has is a clogged pcv valve, which I will replace here soon. I think I have seen every crazy issue a bronco can throw at you, but they never seem to amaze me lol
Needless to say, I appreciate any assistance you can throw my way
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Joined: Oct 2004
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From: China Spring, TX (Waco)
Surging and/or idle problems usually are associated with a bad IAC (idle air control valve). Check all your electrical connections first, then consider cleaning the throttlebody and replacing the IAC.
cds
cds
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Joined: Sep 2002
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From: Gateway to the West
First off, if things are operating properly, the idle should start off high and then drop after a while. The engine coolant temp dictates when the computer brings the idle back down. (The computer handles what used to be the job of the "High Idle cam" on a carburator). 2K sounds a little high for the top-end of the idle rpms as does the 8-900 on the bottom (may have a small vacuum leak somewhere). Since the idle will "kick down" when you tap the pedal it indicates that things are probably working correctly but may be dirty. I'll second checking the IAC for crud. Since your description would seem to indicate that the IAC is still working, you can clean it by removing it from the throttle body and using throttle body cleaner on it. (Take care to keep the cleaner away from the electronic half of the assembly).
Take the time to pull codes just in case its something that the computer recognizes as a problem too. It will save you time on the "wild goose chase" method of diagnosis.
Take the time to pull codes just in case its something that the computer recognizes as a problem too. It will save you time on the "wild goose chase" method of diagnosis.
Last edited by greystreak92; Feb 14, 2006 at 12:49 PM.
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the iac is that cylinder looking module on the right of the throttle body that has the electrical plug in it right? should I just replace it, or is it actually possible to clean it properly? I imagine even a ford part would not be too expensive, thanks for the help btw 
Junior User
Joined: Oct 2004
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From: China Spring, TX (Waco)
I have had the best luck with motorcraft parts. The dealer wants too much for them and in reality buys them from a local auto parts in my area. You can go online to O'Reillys to check prices.
cds
cds
Lead Driver
Joined: Sep 2002
Posts: 9,182
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From: Gateway to the West
It can be cleaned with throttle body cleaner and a soft brush (I use an old toothbrush). The carbon and soot buildup is usually the problem which reduces the bore diameter and doesn't allow the plunger to seat properly.
Replacement is a $70-80 investment and if you really want to know if it has failed you can test it before you just replace it.
With the engine at normal operating temperature, the A/C and heat off, and the engine idling in park, disconnect the IAC from its electrical connector. If its working properly, the truck will sputter and die within a couple of seconds. Make sure all of the criteria are met before disconnecting the IAC but that is the test for a malfunctioning IAC. If the truck does indeed die, shut the key off, disconnect the battery for about 5-10 minutes, reconnect the IAC and then the battery. You MUST disconnect the battery or the fault code generated when the IAC was disconnected will remain in the computer memory and "confuse" it.
Replacement is a $70-80 investment and if you really want to know if it has failed you can test it before you just replace it.
With the engine at normal operating temperature, the A/C and heat off, and the engine idling in park, disconnect the IAC from its electrical connector. If its working properly, the truck will sputter and die within a couple of seconds. Make sure all of the criteria are met before disconnecting the IAC but that is the test for a malfunctioning IAC. If the truck does indeed die, shut the key off, disconnect the battery for about 5-10 minutes, reconnect the IAC and then the battery. You MUST disconnect the battery or the fault code generated when the IAC was disconnected will remain in the computer memory and "confuse" it.
Last edited by greystreak92; Feb 15, 2006 at 11:07 AM.
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Joined: Dec 2004
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From: Edmonton, Alberta
My 89 did the same sort of thing last summer. I changed the fuel filter, PCV valve, air filters and added some injector cleaner - that's it. The problem went away shortly there after...
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