Follow the main vacuum coming from the intake manifold and it will right next to the vacuum check valve......
Check the blend door under the dash on the F150. Check the vacuum system as well. The vacuum controls the blend door and controls the heat as well as air from the defrost.
The vacuum diagram for a 1998 Ford F150 is located in the service manual. It details how each vacuum line is routed, where the line is attached, and its function.
where is the crankshaft position sensor a bank 1 circuit malfunction on a 2003 f150 at
how do i find the neutral, or clutch switch on a 2003 ford F150?
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Ford Dealer can help you. And tell you if there is anything wrong with your 2003 F150.
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go to following link you will get the answer: http://www.wonderhowto.com/how-to-replace-brakes-2003-f150/
The vacuum at idle should be a steady reading between 17 and 22 inches ( in-Hg). for most automotive engines
Most modulator valves are on the rear of the transmission. It should not be that hard to find due to the fact that it has a vacuum line attached to it. It controls the transmission shifting off engine vacuum. Also if it is bad it will pull transfluid out of the transmission and burn it off thur. the engine, this is noted with white smoke from the tail pipe.