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1987 - 1996 F150 & Larger F-Series Trucks 1987 - 1996 Ford F-150, F-250, F-350 and larger pickups - including the 1997 heavy-duty F250/F350+ trucks

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Old 04-27-2011, 06:03 AM
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Idle speed 1991 F150 5.8

I'm just curious to know what actually controls the idle speed? I understand that the TPS and IAC valve have effects on idle speed, but where is the actual value determined? Does the computer have a target RPM value?

Just wondering because my truck has idled low since I've owned it. The stock tach is reading about 350 RPM, which I know is too low for the engine to actually stay running- but it's certainly not idling @ 1000 RPM, as I hear most of you guys talk that it should be. I would guess it's running at 500-600 RPM but obviously I don't know for sure. To my knowledge there are no vacuum leaks.

I'm looking to increase the idle speed and want to know what I need to look at to make this happen. Just FYI, but the IAC and TPS are both less than a year old, and the TPS is reading 0.9v.

Thanks.
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Old 04-27-2011, 07:35 AM
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You need a more accurate tachometer. There is no way to "adjust" the idle speed without having a custom chip or tuner. The idle speed is controlled by the PCM. It uses various input signals to derive the target idle RPM. IIRC the target idle speed in drive is in the 650-700 RPM range for my 5.8L engine. In neutral or park my tach reads about 900 RPM when the engine is warmed up.
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Old 04-27-2011, 08:45 AM
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Fair enough- what all affects idle speed then? All the sensors or only a few? Again, I get the TPS would, but what about the MAP sensor or maybe the ACT?

Just to provide more info, the only code I'm trying to work out right now is 213 [SPOUT circuit open] which is triggered in KOER. But this code wasn't triggered until recently so I don't think it's related to the issue.

Is the coil the only thing the tach feeds off of, or what other connections should I check?
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Old 04-27-2011, 08:53 AM
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The factory tach is known to be less than accurate. The signal comes from the negative side of the coil. Ryan has a good write-up of the different modes the computer works in. Look at the section under Warm Idle: Ford Fuel Injection » Strategies

Code 213 could be a contributor if the circuit is not allowing the PCM to control the timing. With the SPOUT in and out can you see the timing change with a timing light?
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Old 04-27-2011, 03:45 PM
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Quote:
Originally Posted by rla2005 View Post
The factory tach is known to be less than accurate. The signal comes from the negative side of the coil. Ryan has a good write-up of the different modes the computer works in. Look at the section under Warm Idle: Ford Fuel Injection » Strategies

Code 213 could be a contributor if the circuit is not allowing the PCM to control the timing. With the SPOUT in and out can you see the timing change with a timing light?
Thanks for the info- I'll get back to you when I know.
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Old 04-27-2011, 03:45 PM
 
 
 
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