Brake and RPM question

[another Todd]

A small application of EGR could cause that drop. Brake application could cause an electrical anomaly I suppose in the PCM and make it call for EGR.

Friend had a Dodge diesel with a trans issue of TC clutch locking and unlocking just driving down the road...long, long story short..it turned out to be a transmission wire that ran close to the alternator was picking up interference from the alternator and sending errant signals. Rerouted wire, problem solved. Only took 1.5 years to find it.

 

[travlnman]

Well in the spirit of that thought, I turned to my new toy , the Torque App. It has an indicator of EGR flow, I am assuming from the DPFE sensor. At idle with the rpm down to as low as 502rpm, there is no indicated EGR flow (0%)showing in the App. If , it is monitoring flow via the DPFE sensor, it would show flow of any kind, no matter if it was intentional or not...however, I may be wrong about how it is measuring and displaying this data...

 

[jrustles]

Other people have said they get it, and it's normal, nothing wrong with the EGR from what you can tell so far, it's probably the logic.

Perhaps when you apply brakes in Drive, the ECU expects a slowdown and stop, the RPMs may drop slightly, intentionally to reduce heat in the torque converter, and when you let off the brakes, the idle bumps up to assist crawling? A totally wild guess if it is intentionally programmed in.

 

[travlnman]

Could be...it seems more and more likely that its a part of the "program" ....good reason as any to buy a programmer and change it up a bit....where's my piggy bank..?

 

[MolaKule]

I had a similar problem with my S-10, and it turned out to be porous vacuum hoses going back to the EVAP unit.

 

[travlnman]

Would not hurt to try...changing oil today anyway !

 

[Colt45ws]

Have you tried running it with no MAF?
It'll run using the MAP and speed density.

 

[travlnman]

Yes...and MAF was replaced last yr....no change with new MAF

 

[travlnman]

jrustles may have the winning answer

My thought process brought me to this question to myself, "how does it know I'm pressing the brake pedal ?" If it's not vacuum related, which fairly sure now that it is not, by what means does it know ? The answer is painfully simple....the brake light switch. Looking at the switch, it has 4 wires leading from it. Assuming one is power, one is ground, one will trigger the brake lights and possibly the interlock for the trans shifter, what is 4th one for ? Got the truck running and warm, parking brake on tightly, put the truck in D and let it hold on the parking brake, removed the switch from the bracket. Press the brake now....no RPM change !! Unfortunately, my Autozone Haynes manual does not show me enough detail to sort out which is which. Even funnier thing, after I put it back to normal...with the same conditions as above, the RPM drop is now fluctuating , as if the switch may be intermittent and a signal is fluctuating and changing the RPM. Could a simple brake switch replacement solve this whole headache...stay tuned to find out

 

[CurtisB]

Wow, I'm just going to put this EGR valve on my spare parts shelf for now

... Clever catch on both yours and jrustles account, and it appears you have isolated the problem.

It makes total sense too. The ECM needs to know when the vehicle is stand still idling in drive, which under normal circumstances the driver is on the brakes. The ECM needs the signal from the brake switch to know when to adjust engine operation, likely because of torque converter anomalies. Intermittent signal from the brake switch = intermittent engine adjustment and the result is a sporadic idle.

 

[Colt45ws]

The RPM drop maybe to...give a vacuum boost?
Or maybe its dropping engine speed to a low level so you dont psuh through the brakes with the engine. Maybe some sort of secondary safety.

 

[travlnman]

CurtisB...wouldn't you know it...none of the local parts houses have a switch in stock

Will search high and low for a local source, if not will order from RockAuto....

 

[travlnman]

Colt45..not sure why it does it, not for vacuum boost, as that would actually reduce vacuum (reducing idle speed at zero ground speed) I have another theory I would like to voice. I think it's for one or both of two reasons, my #1 theory says it's to reduce driveline shock when you shift into drive, and #2 it's to reduce fuel consumption at idle in Drive or a combo of these two. The reduced fuel thing would not even be measurable by the everyday driver...but every little bit over the millions of trucks Ford makes gives them a higher avg mileage for the fleet , which helps them meet tougher and tougher CAFE standards. On a side note, was able to "fix " the switch and make it work predictably. Now , if I only had a detailed schematic so I could possibly bypass this annoying little feature....

 

[travlnman]

Well, spoke with a Ford senior tech....it's the Brake Over Acceleration feature, designed to override any throttle inputs when the brake is pressed. The fact that it goes lower than the"normal" idle speed seems to be lost on them...says it's normal.It's a knee jerk over reaction to the lawsuits from all the unintended acceleration incidents with the servo throttle body. Seems like a little more effort could have been put into the code written keep it from defaulting to anything below the otherwise normal RPM.

 

[MinamiKotaro]

Originally Posted By: travlnman
Has anyone noticed if your idle drops when you apply your brake ?


Every car with power brakes that I've ever driven has been like that.

 

[travlnman]

Not here...and mine does not if the brake switch is out of the loop. It is intentionally being done by he ECU, even confirmed by the Ford tech I spoke with. Still investigating ways to bypass it through a wire removal or wire jumper.