Surging Idle When Brake Depressed.
 

Hey guys, I was out in the truck (2002 F150 5.4L) last night, came upto a stoplight, and as I was waiting for the light to change, the idle started surging, light changed, kept driving, was fine, until I stopped again, then same thing.

the next light after that, I put the truck in neutral, and held the brake, the idle stayed at normal #'s. put it back in drive, and held it, and sure enough, surging again.

about 10 minutes later, check engine light comes on, finally get the truck home and park it, today I go outside and have the girlfriend press on the brake while in drive, and try and listen for some whining or any other odd noise coming from the brake booster, but nothing.

also gave the vacuum line from the booster a quick check for a leak, couldn't see anything wrong.

so, I went out and picked up a code reader, and its throwing me code P0306 (which is a CYL #6 misfire)

I find it strange, because if it was cyl #6 misfire, wouldn't it affect the truck performance no matter what, and not just while stopped? what should I be looking for?

thanks in advance.

Sounds like you have a COP that's went bad. Very common problem for a 5.4.

Or you got water under the COP

ok, but if a COP went bad, wouldn't it run bad the whole time? not just at idle, with the brake pressed?

When you are in D while the brake is depressed, the engine is being put under a small load from the torque converter. Hence when you release the brake your truck starts idle rolling when in D.

So when the engine is under load, it will usually cause misfiring if a COP went bad or a plug is flat out shot.

I'm guessing either a COP is bad or else you need to get some new Motorcraft plugs. If you do decide to replace the plugs, remember to replace the fuel filter and run some fuel system cleaner like BG44k through your system.

How many miles on your rig?

ahhh ok, I guess I'll take a look at that coil and see what I can find out.

#6 cyl is the 2nd from the front, on the drivers side, correct?

and run the fuel system cleaner through it AFTER I change out the plugs?

thanks

But, to pick up on r0yal's question; wouldn't the thing run badly under acceleration if a COP is bad and is misfiring under load?

I'm not saying the COP is not at fault, but it seems odd to me that the surge is just when idling with the brake on in drive.

On an older vehicle, I'd have suspected a vacuum leak too, just like r0yal.

- Jack

Switch that COP with another one that's easy to get to and if it throws the light again, read it and see if the code has changed to the cylinder that you swapped to. You should have your answer.

well, changed the plug on #6 to a new one, and swapped the COP with the one from #5, the odd time the idle would stumble when I stopped at a stop light, but it didn't throw a code yet. so I'm not too sure what to think just yet. :/