I'm going thru the same issue on an 03 FX4. What i've found out so far is that it is a vacuum system. You need to run a vacuum check on the line that runs from the vacuum solenoid, mine is on the fire wall behind the battery, down to the vacuum pump on the front diff, if that holds, the solenoid may be bad. I have already replaced the solenoid on mine, about $50 at the dealership only. If you go to a parts store and buy a $20 repair guide ( Haynes or Chilton), it will show how to check all of this. You can also get a vacuum tester at AutoZone ( loan-a-tool ) for a $50 deposit that you will get back when you return it, good luck.
Monday, April 12th, 2021 AT 10:51 AM