I have a 2004 f150 lariat crew cab 4x4 with the 5.4 3 valve. It now has 113 000 miles. Since i got it at 85 000 miles its had a hiss when the engine was under load. So like light acceleration or driving into wind. The guy i bought it from said that it had a manifold leak so thats what i figured it was. I got the manifold fixed and it still hisses. There are no check engine lights. It almost feels like my engine lags when it starts hissing. It sounds like its coming from right on the other side of the fire wall. Anybody else have experience this? Or have an idea of what it could be?

 

Check valve/elbow in the power brake booster damages or the grommet is dry, hard and or cracked. Inspect the vacuum line from the elbow and clip to the metal vacuum line from the back of the engine closest to the fire wall. Inspect connections from the rubber vacuum line to the metal/steal vacuum line also make sure there is no pin holes rusted spots on the steal vacuum line and inspect where this is attached to the engine as in the fitting could be loose. An engine is like a big air pump that sucks and moderate speeds, idle or coasting your vacuum will be possibly between 12 to 18 inches of vacuum on acceleration it'll drop to zero, you are looking for a small hole, cracked hose, or grommet or just a crappy brittle hose with a poor connection. I'd also suggest using a mechanics mirror or you'll be stretched out over the engine bay. Hope this give you something to look for
Robert in Canada

 

Thank you for the feed back! This should help a lot. Im not really a mechanic, but i like to think I am! I can fix something if i know what to fix. So ill give it a look!

 

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