Idle speed
Senior User
Joined: Dec 2007
Posts: 162
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the o2's sensors do adjust idle by way of closed loop operation (injector timing - the amount it is held open or closed)
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Laughing Gas
Joined: Dec 2003
Posts: 750
Likes: 30
From: North Carolina
Well..the idle does seem to be smoother under load of the AC, my biggest concern now will be to see if the CEL comes back. The "fix" now has about 750 miles on it...so , finger's crossed. If it comes back and still shows O2 sensor, will replace asap and continue on.
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Laughing Gas
Joined: Dec 2003
Posts: 750
Likes: 30
From: North Carolina
The CEL indicated upstream 02 sensor on drivers side. Just now getting around to replacing it today. Cleared the codes ...but noticed something strange. IF I put on parking brake, and put it in D, the idle hovers around600-700rpm, however, this is with my foot off the brake and only holding against the Ebrake. When I put my foot on the brake, the idle drops to 500RPM or so, and does get slightly rougher. The truck stops just fine, in fact, I'm used to it but whenever anyone else drives it, they always comment on the strong brakes...been this way since new. I did take a pic of old sensor after I removed it, may post it later, but it was nearly chalk white. Advice, opinions, and any general $.02 is welcome, thanks !
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Laughing Gas
Joined: Dec 2003
Posts: 750
Likes: 30
From: North Carolina
Removed vacuum hose to brake booster today , after running up to temp. Side note, when running higher idle at first start, there is no RPM drop issue under any conditions. Now, for the strange part, with hose removed and blocked off, at warm idle holding against the e brake in Drive, the RPM STILL drops down to 500RPM or less when I press the brake pedal. It's as if the CPU is commanding a lower idle for some reason ?! Anyway, waiting patiently for the arrival of my Bluetooth adapter for my Torque App so I can see the throttle angle and engine RPM etc in real time as the CPU sees it, not just from the tach.Stay tuned boys and girls.... 
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Laughing Gas
Joined: Dec 2003
Posts: 750
Likes: 30
From: North Carolina
More food for thought for the experts here to chew on. RPM only drops when up to temp, in D, and foot on brake. Foot off brake and RPM is "normal". How does it know I'm pressing brake ? The brake light switch ! Got truck warm, in D , holding against parking brake...and removed the brake light swtich from the bracket and pressed the brake...no RPM drop !! Funny thing, after I monkeyed with it and put it back on...the RPM drop is now intermittently fluctuating with the above conditions. The switch has 4 wires from to it..assuming power, ground, brake lights (and maybe shifter interlock) or shifter interlock is the 4th wire ?! My Haynes manual does not show me this much detail...what say you folks ? First, why would the computer drop RPM intentionally in D with zero speed ? Maybe to keep down torque converter heat ? If not normal (and I still say not) anybody have more detailed wiring schematics ? Maybe I could pull one wire and stop this ? None of the local places have a replacement switch on the shelf....will have to wait and order it. Opinions welcome !
Cargo Master
Joined: Nov 2011
Posts: 2,946
Likes: 11
From: Northern California
I actually did a search on your condition and I came up with your user name on a different forum. Good thing you found your tank imploded like that! Hope you have the issue resolved finally.
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Laughing Gas
Joined: Dec 2003
Posts: 750
Likes: 30
From: North Carolina
Sadly , yes and no. It is a product of the signal when the brake pedal is pushed and brake lights come on and the shift interlock is released. I feel it is to either reduce driveline shock when shift into D , or to minutely increase fuel mileage by reducing use at idle in D, or both. As someone who deals with PLC logic and this type of thing all the time, I would think that if it's for driveline shock, it would be momentary and once it saw D engaged, would revert back to "normal"RPM. Trying to find some detailed electrical schematics to see if it's possible to bypass this little feature. Fingers crossed...
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Moderator
Joined: Jul 2011
Posts: 6,506
Likes: 655
From: Sioux Falls Area
Sadly , yes and no. It is a product of the signal when the brake pedal is pushed and brake lights come on and the shift interlock is released. I feel it is to either reduce driveline shock when shift into D , or to minutely increase fuel mileage by reducing use at idle in D, or both. As someone who deals with PLC logic and this type of thing all the time, I would think that if it's for driveline shock, it would be momentary and once it saw D engaged, would revert back to "normal"RPM. Trying to find some detailed electrical schematics to see if it's possible to bypass this little feature. Fingers crossed...
your idle speed is nearly identical to mine. (see post #9)
i think you are trying to solve a problem that does not exist and your truck is doing exactly what it is supposed to do.
if it is not stalling, then what problem are you trying to fix?
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Laughing Gas
Joined: Dec 2003
Posts: 750
Likes: 30
From: North Carolina
The problem I am trying to fix, is the fact that with the AC on at a redlight, it feels like you yanked a plug wire off. 500RPM at idle in D with AC on is too low to meet the demands placed on engine by the AC. Sadly, I spoke with a Ford tech and he says it's a function of the ECU "Brake Over Accelerator" , to prevent a runaway vehicle should there be a malfunction in the servo throttle body. The unfortunate part is, they put this little function in , but were not smart enough to put in an exception , so that when the ground speed is zero, revert back to the standard 600-650RPM. I deal with PLC logic everyday and it's not that hard to include...or rewrite if you have access to the code, but I do not , so I cannot. Imagine if you would, an older vehicle with the IAC valve, that is malfunctioning....and the engine does not idle up properly to meet demands of AC, or defrost which also uses AC compressor, etc....it gets very stumbly and rough. You clean out or replace the IAC valve...problem solved. With the servo throttle body, we do not have that option.
Moderator
Joined: Jul 2011
Posts: 6,506
Likes: 655
From: Sioux Falls Area
must be a unique difference between the 4.6 and the 5.4.
mine idles like a sewing machine at 530 rpm with the air on.
i'm sure a custom tune is an option, but beyond that, i think it is doing what it is supposed to do.
mine idles like a sewing machine at 530 rpm with the air on.
i'm sure a custom tune is an option, but beyond that, i think it is doing what it is supposed to do.
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Laughing Gas
Joined: Dec 2003
Posts: 750
Likes: 30
From: North Carolina
Well, considering mine is the 4.6 2v, there is at least a 50hp difference in the 4.6 3v, and an even larger difference with the 5.4...so at any given RPM, even idle the 5.4 has a lot more grunt to deal with the load. The old 2v is pretty bulletproof...but it's no powerhouse
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fairlaniac
1980 - 1986 Bullnose F100, F150 & Larger F-Series Trucks
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