Hello, I have a 2004 F150 5.4 3v 155,000 miles with a P0174 vacuum leak code and cant find the leak!! I have checked all vacuum lines, that I can think of, checked MAF cleaned it. It sounds like it is coming from the driver side but I cant locate it!! This is driving me crazy. If anyone has any information, that would be great. It cause my truck to run rough when accelerating, does ok up to 45 then rpms drop and shakes really bad. If i barely touch the gas it will go, but not very fast. It does not stall, however when warmed up, at stops it idles really rough. Please, if anyone can help that would be great.

 

Use an unlit propane torch to find the leak. With the engine running spray a little propane on the vacuum lines until the engine runs better. That is where the leak is. Be careful.

 

Try checking the brake booster line(s) near the middle of the driver side head.

Is your truck 4x4? Check the 4wd vacuum lines. All of them, including the one behind the battery leading to the vacuum box.

 

My 07 5.4 did same thing and got worse and worse. I changed the vct soidnoids and switched from 5w20 to 10w30. Runs like new again. And it shook so bad at a stop to the point of embarrassment then stalled sometimes

 

Did you have a lean code?

 

Just po345 and po349. Which are camshaft position sensors if I'm not mistaken. Those were changed and still threw the code. My phasers are prob off but the vct solenoids fixed the shaking part. So it runs good just some light ticking. And idles perfect. I would at least check those before u dive into a phaser job. My opinion though. I just broke 100,000. Never changed plugs or wires. Never needed too. My codes are intermittent too

 

Just po345 and po349. Which are camshaft position sensors if I'm not mistaken. Those were changed and still threw the code. My phasers are prob off but the vct solenoids fixed the shaking part. So it runs good just some light ticking. And idles perfect. I would at least check those before u dive into a phaser job. My opinion though. I just broke 100,000. Never changed plugs or wires. Never needed too. My codes are intermittent too. I never had a vacuum prob. Sorry if I jumped subject a lil. Just sounded like my problem

 

A visual test would be the best way. There are a bunch of videos on making a DIY evap smoke tester. Some of them are pretty cheap. The leak could even be one of the evap lines back to the fuel tank. Smoke and a bright flashlight makes the leaks very apparent

 

+1 to checking the brake booster line(s). I had this issue with P0174/P0171 codes and a sucking wind sound coming from the driver side of the engine bay. Took me ~9 months to figure it out (mostly I ignored it and actually found it accidentally).

I break in this line would also affect your 4WD (if equipped) and your ability to brake fast. Had a few close calls where I was nervous that I wasn't going to stop in time, even slamming the pedal down as hard as I could.

The rusted metal tube in the blue square has a small nip on it that is under the electrical connector at the red arrow tip. The red circled hose is what is supposed to connect to it. Second picture is how the nip rusted through and fell off INSIDE the hose, lol. Had to cut the hose, then reattached.

Your leak might not be exactly where mine was, but check that general area and all hoses leading off of that metal tube (to brake booster and to 4WD "parts and hoses").

 

Thanks for your time and specificity with the brake booster advice and photos. My husband's '98 f150 has been driving us crazy with the same hissing sound & slow-to-stop braking. He's outside now checking things out. Thanks again!