Hiss from engine, 171,74 DTCs

[philkb]

I have the notorious lean fuel trim codes like so many others have on their engines. I already did the EGR port cleanup on the unit but did not have the funds at the time to do the isolator bolt and gasket fix. I cleared the codes for the misfires that I had before doing the port cleanup. The engine has run great after that job and gas mileage went up quite a bit too. However, a week later, the CEL came on and it was the lean bank codes. Now, with much colder, often blustery weather upun us as well as limited funds, I may have to put off further the isolator bolt fix.
I did take the time last night to listen to the engine and I noticed a hissing sound from the "front" of the engine. With all the noises in the engine compartment, especially in the front end where the belt, alternator, and PS pump are located, I'm not sure if the hiss I'm hearing is normal or not. I tried moving my hand around the area but of course I'm limited where I can cup my hands while in there in an effort to isolate the noise. Strange as it may appear, I'm hoping the hiss is the "sound" of my problem, coming from underneath the lower half of the upper manifold. If it is, then I should be good-to-go with the isolator bolt fix. Otherwise, I'll never be totally sure that that is what needs to be done. I've read more than one post about people who have done the bolt repair and still get the lean bank codes.
So, can anyone tell me if that hiss I hear is normal, or is it actually the manifold leak? Putting my fingers close to the IMRC shaft did not make any change in sound so hopefully that means they are intact and not leaking along the bushing.

[busboy4]

Hi
I don't have any really specific advise other that to suggest you use a length of hose (around 18") to your ear to isolate sounds. You can purchase a car stethoscope as well. I have been using a 5/16" length of semi-rigid hose for a long time and it is remarkable how well you can pinpoint sounds under the hood this way.
As you have said the sound is near the front of the intake, do check carefully around the vacuum hose on the fuel pressure regulator. It is detailed as a possible problem in the TSB for P171/174 and could be a source of vacuum leakage.

Good Luck

[tripletdaddy]

To add to the previous post, the small hose is an excellent noise finder, but a mechanics stethescope is for noises like bearings and other things that you firmly put the end of it on if it is not equiped with a hose like a Dr. I recently isolated a noise that was only detectable with a hose coming from a bearing like noise. Boy I thought that was strange. Anyway, I have read a number of people suggesting using starting fluid sprayed around your mystery hissing sound. One brave soul uses his unlit ropane torch. If there is a manifold leak, the sucked in fuel will cause a surge in the engine rpm. Good luck

[philkb]

I might try that next time I get the chance, although as you say, it's risky, and not recommended by Ford to use that method or the starting spray. Still, I wonder if an open-ended stethoscope might narrow it down. Again, I hope the hiss is the sure sign of the trouble and not just sounds from the PS pump or some other unexpected source.

[squeaky3]

actually you can use carb cleaner or something similar and spray it..i work at a dealership and this is how we try to diagonis a lean code

by the sound of where you said the sound was coming from i would venture to say intake gaskets and the bolts that hold the upper to lower intake to be the problem