Rick's Free Auto Repair Advice

P0171 Hard brake pedal

Fix Hard brake pedal – P0171

Hard brake pedal on 2005 Ford F-150 pickup with the 5.4-litre engine, no power brakes, P0171

P0171 System too Lean (Bank 1). This vehicle is with vacuum assist power brakes. The power assist is provided by a vacuum booster. The booster is a dual chamber unit with a rubber diaphragm separating the two chambers. When the engine is running, engine vacuum is applied to both chambers. Then, when you press on the brake pedal, a valve allows atmospheric pressure into one chamber. With atmospheric pressure on one side and vacuum on the other, the rubber diaphragm is pulled towards the master cylinder, helping to push the piston into the master cylinder bore. If there’s a vacuum leak, you lose power assist from the booster and you’re left with manual brakes. The booster receives its vacuum supply through a rather large rubber hose that’s connected to the rear of the intake manifold on this particular engine. Check to see if the hose is still attached. If the hose is off, that would prevent the booster from working. And, it would cause a significant air leak that would result in your rough engine problem and the resulting trouble code. You’ll have to remove the upper intake plenum to gain access the vacuum port and to install a new hose and clamp. Get a shop manual for the removal and replacement steps. You’ll also need a torque wrench to reinstall the intake plenum.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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