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  #1  
Old 05-28-2011, 05:28 AM
appletonbill appletonbill is offline
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Default Engine racing

Recently my truck has been experiencing a "racing" engine. It has been happening sometimes when I have my foot off the accelerator and slowing down for a stop sign, or stopped at an intersection. Almost like the throttle sticks. I have used cleaner on the throttle body and linkages. Any ideas?
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  #2  
Old 05-28-2011, 06:37 AM
Clinton Clinton is offline
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Default Re: Engine racing

Check the IAC, I ruined mine when I cleaned it so I had to buy a new one and it wasnt racing anymore

Last edited by Clinton; 05-28-2011 at 06:58 AM.
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Old 05-29-2011, 07:23 AM
appletonbill appletonbill is offline
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Default Re: Engine racing

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Originally Posted by Clinton View Post
Check the IAC, I ruined mine when I cleaned it so I had to buy a new one and it wasnt racing anymore
How did cleaning the IAC ruin it? I took mine out and cleaned it with carb cleaner a couple months ago and just lately started having trouble now.
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Old 05-29-2011, 07:26 AM
Clinton Clinton is offline
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Default Re: Engine racing

I used a cleaner with a solvent, somehow it wrecked it. When i put the new one on it ran fine. I should have read the chilton manual first, thats how I realized I messed up.
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Old 05-29-2011, 07:31 AM
appletonbill appletonbill is offline
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Default Re: Engine racing

Quote:
Originally Posted by Clinton View Post
I used a cleaner with a solvent, somehow it wrecked it. When i put the new one on it ran fine. I should have read the chilton manual first, thats how I realized I messed up.
Is there a way to test it first, before I replace it? Like with a volt meter or something? Oh yeah, another thing. Sometimes it will backfire through the TB on the first start-up but it hasn't seemed to jump timing.
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Last edited by appletonbill; 05-29-2011 at 07:39 AM.
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  #6  
Old 05-29-2011, 12:27 PM
Gunner Gunner is offline
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Default Re: Engine racing

An IAC is a linear stepper motor I believe, so I don't know if there is a way you could test it. Maybe if you knew what resistance range it was supposed to have, you could check it with a DVOM....disconnected from the vehicle of course.

Is your engine running poorly otherwise? If so I'd also be looking for a vacuum leak.

I need a new IAC for my '90 Mustang 5.0L because I can never get the thing to idle right, and I've adjusted the throttle stop screw so many times but it makes no difference.
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  #7  
Old 05-30-2011, 07:43 PM
Badmotorfinger1 Badmotorfinger1 is offline
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Default Re: Engine racing

Unplug the IAC and see if the idle speed drops. If it doesn't, there's a really good chance you'd got unmetered air (aka a vac leak) getting into the engine.
Also, pull off your intake and make sure the throttle body butterfly is closed completely. If it's not, there's the source of your racing engine.
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  #8  
Old 05-31-2011, 11:45 AM
appletonbill appletonbill is offline
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Default Re: Engine racing

Quote:
Originally Posted by Gunner View Post
An IAC is a linear stepper motor I believe, so I don't know if there is a way you could test it. Maybe if you knew what resistance range it was supposed to have, you could check it with a DVOM....disconnected from the vehicle of course.

Is your engine running poorly otherwise? If so I'd also be looking for a vacuum leak.

I need a new IAC for my '90 Mustang 5.0L because I can never get the thing to idle right, and I've adjusted the throttle stop screw so many times but it makes no difference.
No. She runs normal most of the time. This is a sporadic thing. It happens sometimes while I am just coasting or stopped at a light.

----------

Quote:
Originally Posted by Badmotorfinger1 View Post
Unplug the IAC and see if the idle speed drops. If it doesn't, there's a really good chance you'd got unmetered air (aka a vac leak) getting into the engine.
Also, pull off your intake and make sure the throttle body butterfly is closed completely. If it's not, there's the source of your racing engine.
Ok, I'll try that, thanks.

----------

Quote:
Originally Posted by Badmotorfinger1 View Post
Unplug the IAC and see if the idle speed drops. If it doesn't, there's a really good chance you'd got unmetered air (aka a vac leak) getting into the engine.
Also, pull off your intake and make sure the throttle body butterfly is closed completely. If it's not, there's the source of your racing engine.
Is it possible that the TPS could be bad?
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  #9  
Old 05-31-2011, 12:05 PM
Badmotorfinger1 Badmotorfinger1 is offline
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Default Re: Engine racing

Quote:
Originally Posted by appletonbill View Post

Is it possible that the TPS could be bad?
Possibly, but it's been my experience that a bad TPS will throw a code. Plus, if the engine is racing it needs an air source and the TPS has no control over that.
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  #10  
Old 05-31-2011, 03:42 PM
appletonbill appletonbill is offline
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Default Re: Engine racing

Quote:
Originally Posted by Badmotorfinger1 View Post
Possibly, but it's been my experience that a bad TPS will throw a code. Plus, if the engine is racing it needs an air source and the TPS has no control over that.
Ok. That makes sense.
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  #11  
Old 06-17-2011, 01:15 PM
appletonbill appletonbill is offline
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Default Re: Engine racing

UPDATE: I replaced the IAC and I still have the problem of high idle, only now it is worse because it idles high all the time now. I have to literally
stand on the brakes when I pull up to a stop. WTH?
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Last edited by appletonbill; 06-17-2011 at 01:18 PM.
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  #12  
Old 06-18-2011, 10:14 AM
appletonbill appletonbill is offline
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Default Re: Engine racing

UPDATE: It was low on coolant and transmission fluid. I topped everything off and now everything seems to be operating normally. I do need to replace some hoses though.
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