Vacuum leak in the brakes?

[Jonas]

I've been trying to diagnose my intermittent high positive fuel trims and I think I've narrowed it down. I thought I had it resolved until I got p0101 and a p0171 lean codes last night. At idle in park my fuel trims look good, but when a press and release the brake pedal all the way, the fuel trims jump to +16 and then come back down. Any idea what that could be?

 

[mk378]

That is normal, since the booster works by air, it will let unmetered air into the intake while the pedal is moving.

It should not flow any air if the pedal is all the way up or being held down at a constant position.

You could test for leaks in the booster by disconnecting and plug the hose to the booster. Of course don't drive like that since the brakes will have no power, the pedal will need to be pressed very hard in order to stop.

 

[Jonas]

That is normal, since the booster works by air, it will let unmetered air into the intake while the pedal is moving.

It should not flow any air if the pedal is all the way up or being held down at a constant position.

You could test for leaks in the booster by disconnecting and plug the hose to the booster. Of course don't drive like that since the brakes will have no power, the pedal will need to be pressed very hard in order to stop.

Ah, thank you. Yes, the fuel trims go up as I press the pedal, but normalize while the pedal is still. After holding the pedal all the way down, the fuel trims shoot up when it's released, then go back to normal. That's ok?

 

[mk378]

Yes that's OK.

 

[t1snwrbrdr12]

The booster can leak. If you suspect it leaking you can monitor your fuel trims and spray it down with brake cleaner.

 

[Jonas]

The booster can leak. If you suspect it leaking you can monitor your fuel trims and spray it down with brake cleaner.

Thanks. It's the only thing I haven't really checked/ruled out. What's weird is my fuel trims gradually go up throughout the day. They start out close to zero, but after driving all day they are as high as +12 total. Driving me nuts.