High idle after warmup
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Freshman User
Joined: Aug 2011
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From: Houston
High idle after warmup
Hey all, I have an issue thats frustrating the hell outta me!!!! 1999 F-150 5.4L. I just removed the clutch fan and installed two electric fans about two weeks ago. The same evening, I took a short drive and noticed the idle was way higher than it normally is. Its idling about 800 to 900 RPM. Now, the wierd part.....Startup is normal, as in the idle jumps up for the warmup, then drops to the normal 600 to 700 range. after a short drive, the idle is high and noticeable at a stop. Sometimes, if I let the truck idle, after about 10 mins or so the idle will sloooowwwly start creeping up a little at a time till it hits about 900 or so and will level off. I have tried the IAC, I have tried disconnecting the battery and letting the PCM reboot, I have checked ALL my vacuum lines, I have checked my TPS sensor and voltages, checked my MAF sensor and voltages, fuel pressures, everything I could think of....I hooked up my scanner (Auto X-Ray 6000) and checked all my running data, nothing really sticks out to me as outta whack. No codes set....help me please!!!!!
Lead Driver
Joined: Sep 2006
Posts: 7,907
Likes: 143
The idle is controlled normally by the IAC as vairable bypass air.
The only way for idle to increase is with an air source from some other location same as if the IAC was commanded to open more or you throttled the engine up with your foot.
If the leak is large enough, eventually a codes 171 and 174 would be set when the fuel tables go out of limits from the OX sensors' detection of to much air in the exhaust.
I'm sure you missed a faulty rubber part at some point on the intake system that is opening up with advancing engine bay heat.
Good luck.
The only way for idle to increase is with an air source from some other location same as if the IAC was commanded to open more or you throttled the engine up with your foot.
If the leak is large enough, eventually a codes 171 and 174 would be set when the fuel tables go out of limits from the OX sensors' detection of to much air in the exhaust.
I'm sure you missed a faulty rubber part at some point on the intake system that is opening up with advancing engine bay heat.
Good luck.
Thread Starter
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Freshman User
Joined: Aug 2011
Posts: 49
Likes: 0
From: Houston
The idle is controlled normally by the IAC as vairable bypass air.
The only way for idle to increase is with an air source from some other location same as if the IAC was commanded to open more or you throttled the engine up with your foot.
If the leak is large enough, eventually a codes 171 and 174 would be set when the fuel tables go out of limits from the OX sensors' detection of to much air in the exhaust.
I'm sure you missed a faulty rubber part at some point on the intake system that is opening up with advancing engine bay heat.
Good luck.
The only way for idle to increase is with an air source from some other location same as if the IAC was commanded to open more or you throttled the engine up with your foot.
If the leak is large enough, eventually a codes 171 and 174 would be set when the fuel tables go out of limits from the OX sensors' detection of to much air in the exhaust.
I'm sure you missed a faulty rubber part at some point on the intake system that is opening up with advancing engine bay heat.
Good luck.
Lead Driver
Joined: Sep 2006
Posts: 7,907
Likes: 143
Remember the intake vacuum level changes with throttle opening.
Upon decellerstion the vacuum can go as high as 22 inches of merc.
This may be enough to open a faulty rubber part then closes again.
To get deeper into this, as the leak gets larger the PCM will try to close down the IAC attempting to get the idle back within the table spec.
It's knows the rpm is out of control by the crank sensor RPM reading the PCM uses to hunt for the correct idle by compairing to the software table.
Additionally, on decell the PCM closes down the IAC to just enough air to keep the motor from stalling as well as close off fuel injection, so the issue can present some other drivabilty results mixed in you may not notice right away such as extra braking needed to stop the truck..
Keep at it, it's there some place.
Good luck.
Upon decellerstion the vacuum can go as high as 22 inches of merc.
This may be enough to open a faulty rubber part then closes again.
To get deeper into this, as the leak gets larger the PCM will try to close down the IAC attempting to get the idle back within the table spec.
It's knows the rpm is out of control by the crank sensor RPM reading the PCM uses to hunt for the correct idle by compairing to the software table.
Additionally, on decell the PCM closes down the IAC to just enough air to keep the motor from stalling as well as close off fuel injection, so the issue can present some other drivabilty results mixed in you may not notice right away such as extra braking needed to stop the truck..
Keep at it, it's there some place.
Good luck.
Thread Starter
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Freshman User
Joined: Aug 2011
Posts: 49
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From: Houston
Remember the intake vacuum level changes with throttle opening.
Upon decellerstion the vacuum can go as high as 22 inches of merc.
This may be enough to open a faulty rubber part then closes again.
To get deeper into this, as the leak gets larger the PCM will try to close down the IAC attempting to get the idle back within the table spec.
It's knows the rpm is out of control by the crank sensor RPM reading the PCM uses to hunt for the correct idle by compairing to the software table.
Additionally, on decell the PCM closes down the IAC to just enough air to keep the motor from stalling as well as close off fuel injection, so the issue can present some other drivabilty results mixed in you may not notice right away such as extra braking needed to stop the truck..
Keep at it, it's there some place.
Good luck.
Upon decellerstion the vacuum can go as high as 22 inches of merc.
This may be enough to open a faulty rubber part then closes again.
To get deeper into this, as the leak gets larger the PCM will try to close down the IAC attempting to get the idle back within the table spec.
It's knows the rpm is out of control by the crank sensor RPM reading the PCM uses to hunt for the correct idle by compairing to the software table.
Additionally, on decell the PCM closes down the IAC to just enough air to keep the motor from stalling as well as close off fuel injection, so the issue can present some other drivabilty results mixed in you may not notice right away such as extra braking needed to stop the truck..
Keep at it, it's there some place.
Good luck.
Thread Starter
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Freshman User
Joined: Aug 2011
Posts: 49
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From: Houston
Oh, and Bluegrass, thank you soo much for your help on this. First thing tomorrow Im gonna try to look at it and see what I can find. Your knowledge is a blessing, and thanks again!!
Lead Driver
Joined: Sep 2006
Posts: 7,907
Likes: 143
Powering down the PCM is the only way to reboot.
What this does is clear all learned data back to the origninal software as written.
That means all tables and begins the diagnostics all over again..
Once this is done, you may want to start the motor from total cold start. with the headlights on the A/C fan on full and let it run to full warm up.
Shut off, then restart and do a drive about 5 miles.
Do several normal hot restarts with normal alternator /battery recharge loads and return back.
If and only if there are no faults, the cold and hot starts should be perfect under all conditions.
What you done using this procedure is fast learn the start/idle stratigy for the most part.
Winter time cold starts will still be in learning later on.
Also load variations during normal running such as power steeing loads, A/C loads, A/C compressor coming on and off changes the idle load so the IAC has to vary to keep up with the load changes to keep the idle from hunting and stalling etc.
Good luck.
What this does is clear all learned data back to the origninal software as written.
That means all tables and begins the diagnostics all over again..
Once this is done, you may want to start the motor from total cold start. with the headlights on the A/C fan on full and let it run to full warm up.
Shut off, then restart and do a drive about 5 miles.
Do several normal hot restarts with normal alternator /battery recharge loads and return back.
If and only if there are no faults, the cold and hot starts should be perfect under all conditions.
What you done using this procedure is fast learn the start/idle stratigy for the most part.
Winter time cold starts will still be in learning later on.
Also load variations during normal running such as power steeing loads, A/C loads, A/C compressor coming on and off changes the idle load so the IAC has to vary to keep up with the load changes to keep the idle from hunting and stalling etc.
Good luck.
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Junior User
Joined: Jun 2012
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I had the same issue a few months back with my 01 (4.6), one time all voltages checked out and the next the TPS would be out of spec. Changed the TPS and haven't had a problem yet.
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Thread Starter
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Freshman User
Joined: Aug 2011
Posts: 49
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From: Houston
well, nothing has helped so far.....new tps also.....would the idle be affected like this by the IAT or MAF? seems that when the IAT reads high is when it idles high. It will stabilize around 950 to 980. It takes about 2 or 3 minutes to actually climb from the normal 700 to 720 idle to the 950-980. Im about to drive it off a cliff Im so frustrated........anyone ????? At this point I would pay ya Bluegrass for a phone consultation lol!!
Fleet Mechanic
Joined: Jul 2010
Posts: 1,354
Likes: 31
From: Winnipeg
Just a suggestion. I have no idea what mods are required for an electric fan conversion, however, what have you wired to for the fans to sense and operate? Maybe a new connection is affecting/backfeeding to the pcm somehow. Maybe a diode is required? Did you have to install a new coolant temp sensor?
Thread Starter
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Freshman User
Joined: Aug 2011
Posts: 49
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From: Houston
Just a suggestion. I have no idea what mods are required for an electric fan conversion, however, what have you wired to for the fans to sense and operate? Maybe a new connection is affecting/backfeeding to the pcm somehow. Maybe a diode is required? Did you have to install a new coolant temp sensor?
Thread Starter
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Freshman User
Joined: Aug 2011
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From: Houston
That was one of the first things I performed. I have exhausted all other thoughts and testing. I really don't want to just start changing parts and guessing.....
Freshman User
Joined: Dec 2009
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A way to test the EGR valve is you can actually block off both passages by cutting gasket material to shape of the EGR covering both ports and installing it that way turning the vehicle on and seeing what or how it acts that way
Thread Starter
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Freshman User
Joined: Aug 2011
Posts: 49
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From: Houston
True, except, Im not thinking its an EGR issue, as all passages are clean and the vacuum is good and the valve is operating as intended. Plus, the issue is an up and down kind of thing. Starts out idling fine, then sloooowly creeps up to 950 or so during about 3 to 5 minutes, then, after about 10 or 15 minutes, slowly drops back down to the normal 700 to 750 range. If I turn off the truck during this high idle period and let it set for about 5 minutes and then start it back up, it will begin the entire idle up process again.