How to find a vacuum leak on 1999 ford expedition?

Answer 1

Spray (water) or some kind of spray liquid or near or around the vacuum lines. Another item to check is your vacuum brake booster. A vacuum leak here can cause a P0171 code as well. The point about a small vacuum leak not causing a code is dead-on. I could hear my unit leaking and noticed my mileage drop but had no code get tripped. It was only after it got really loud that it finally tripped the code. It started as just a light hiss inside the car. Best way to diagnose is to sit in the car with the door closed and have someone disconnect and re-connect the vacuum hose from the brake booster - if you here something change, you know what it is! code as well.

The point about a small vacuum leak not causing a code is dead-on. I could hear my unit leaking and noticed my mileage drop but had no code get tripped. It was only after it got really loud that it finally tripped the code. It started as just a light hiss inside the car. Best way to diagnose is to sit in the car with the door closed and have someone disconnect and re-connect the vacuum hose from the brake booster - if you here something change, you know what it is!