Hello ladies and gents,

First post on this forum, yet member of many different Ford forums related to trucks. My beast of burden is a '10 F150 Supercab with 4.6 2v (I like em plain and cheap ) . I put on between 40-50k a year in mileage, so naturally preventative maintenance is a big deal with me, and I do everything on all my vehicles, except those things I do not have the equipment to do like alignments etc. Now, the problem or question....the beast has suffered from a low/rough idle when up to temp, in D but sitting still and foot on brake and really evedent with the AC on. Have eliminated vacuum leaks on engine , with the booster, plugs are new, MAF is new, throttle body clean , new O2 sensor that was the source of a CEL for a bit...I have run the table. New revelation this weekend I want to share, it's being done apparently on purpose by the ECM based upon the brake being pressed. I learned this while fiddling with the brake light switch due to no brake lights (must have kicked it loose with my foot) . When the switch was removed from bracket and pressed by hand to simulate no brake application, there was no RPM drop when I actually pressed the brake. Put the switch back in the loop, brake lights functioning, RPM drop is back. So, it's on purpose by the ECM, but why ? Reduce shock load on drivetrain when shifting into D ? Fuel savings at idle ? Both ? Lastly..can I bypass this feature and still have functioning tail lights ? Whew..how's that for a first post ? Thanks to all in advance for any replies !

So there's nearly 200k miles on the truck? . Or you didn't buy it new.....and the idle on my truck (2013) doesn't change with the brake lights. You have a gremlin.

Bought it new , but in March '11, it was a leftover (cheap) . It has about 145k right now, and I wish it were a gremlin. Pretty much everything else has been addressed and it surely and exactly follows the circumstances above. In the other forums, there are people with trucks from about '08 up through '11 complaining of the same thing..I'm just too stubborn and wont give up on it. If you take out the brake light switch, it will not idle down at all. I have the Torque app for Android and I can confirm this via the app, the RPM will drop to as low as 502, from a normal idle of 600-650. Main thing I'm looking for now, is a detailed and accurate schematic I can follow to see what the 4 wires in the switch are for. I assume ground, power, lights...and maybe shifter interlock, but still curious if I can get around this without buying a programmer, which I'm not opposed to, but may be trading it at the first of the year anyway.

Welcome to the site. We have a few ford techs that should be able to answer your questions.

Welcome to the old FTF

Its called "brake over accellerator function". Dont make too much about the brakelight issue. Long story short, its normal. Your rough running when warm is a different issue entirely. A high end scanner will show which cylinder is missing. Thats the first step.

Awesome, at least it has a name and it's not so much a miss...just like it's struggling at that low of an RPM with the AC on primarily. Why does it not revert back to the higher RPM with the AC on ? When it was still under warranty, I took it to local dealer to replace an airbag control module, or whatever was making my airbag light come on, and I mentioned it to them then...naturally, they said "feels fine to me, no codes, no problem"

Some details I left out that may clarify that last question. Without the brake pressed, the RPM, per Torque app is between 600-650. With brake applied, it goes down to as low as 499-510. I understand the need for the brake over ride in a vehicle with a servo driven throttle, but why is it going lower than the normal idle speed ? Hope that clears up any confusion...thanks for all the input !

travlnman said:

Some details I left out that may clarify that last question. Without the brake pressed, the RPM, per Torque app is between 600-650. With brake applied, it goes down to as low as 499-510. I understand the need for the brake over ride in a vehicle with a servo driven throttle, but why is it going lower than the normal idle speed ? Hope that clears up any confusion...thanks for all the input !

Click to expand...

When the brake is pushed, it actually overrides the accellerator (people that ride the brake). I've had customers that complain about "no power", go for a ride with them, yep, foot on the brake when driving.

I'd be looking for a vacuum leak. Particularly around the PCV valve elbow. Vacuum controls idle speed.

Like I said, I get that part...why is the RPM lower than the "normal" idle by nearly 150RPM ? Why not just drop to the standard idle speed and not allow any more increase from there ?

Not a leak...strictly associated with the brake switch input.

travlnman said:

Like I said, I get that part...why is the RPM lower than the "normal" idle by nearly 150RPM ? Why not just drop to the standard idle speed and not allow any more increase from there ?

Click to expand...

Again, its supposed to do that.

The only time that would be the root cause of your symptom is if the switch is sticking "on".

I hear ya...and no evidence of sticking on. I still feel that the code controlling this should have an exception , so that if ground speed is zero, the normal idle RPM of 600-650 that I have observed , should take precedent to keep the engine from struggling under AC load....but I guess not.

blacksnapon, just out of curiousity, what is proper procedure to calibrate the servo throttle body, or, is it even necessary under any circumstances ? Thanks !

Its in the pcm programming. Another thing you might try is take carb cleaner, spray it in a rag, then manually open the butterfly then clean the bore and butterfly with the rag.

It's already cleaned, wanted to make sure there was not a procedure I was missing. Thanks !