I have a 2002Ford F150 and am having unsteady
idles urging when in drive and brake is applied,
stalling also when in drive and brake is applied
then released. Have also noticed hissing from
front and rear of truck along with a pumping,
putting sort of noise as well. Not sure whether this
is a fuel pump issue, air flow sensor, vac. line leak,
or some other issue causing all of this. Appreciate
any help that is given as I am able to perform
certain repair &maintenance on truck but am
limited in my mechanical skills.
It is possible that there is an issue with the power brake booster. The booster is supplied with vacuum from the intake manifold. This vacuum is used in the booster to amplify the pressure you put on the brake pedal when stopping. I would first locate the booster vacuum line that runs between the manifold and booster and make sure it is not cracked or otherwise degraded. If it looks good I would suspect a faulty brake booster and replace it. The loss of vacuum is likely the issue as to why it is not running correctly properly. The vacuum leak could be from somewhere else under the hood also.
I have the exact same truck with a 5.4l that has this issue. The plastic intake manifold is known to crack which causes a vacuum leak. Dorman makes a replacement manifold with upgraded O-ring gaskets for about $200. It takes a couple of hours for a home mechanic to replace it.
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