I have a '99 F-150 4X4, 4.6 L w/automatic and 104,000 miles on it. It doesn't seem to have much power and the fuel mileage is down quite a bit. I started noticing it over a year ago when I started getting the dreaded 0401 code. I cleaned the throttle body and replaced the dpfe valve and that seemed to help a little, but the power was still down. I changed plugs and wires and tuned it up, adding a K&N intake in the process, but still no power. The light came back on after about a week with the same 0401 code. I was sent overseas for a year, so I got right back on it when I got home. I pulled the codes and got both the 0401 and 0402 codes. I cleaned the MAF sensor and the codes haven't been back. I checked all of the vacuum hoses and connections and didn't find any leaks. Here are the symptoms the best I can describe them:
There is a loud sucking noise coming from the engine compartment during part throttle. It seems to happen when the engine load is increased. I can cruise along at a comfortable pace and as soon as I give it more gas I hear the sucking noise until the RPMs catch up and then it fades away. It doesn't matter if I'm going 25 or 55, it happens. It seems like it's coming through the IAC valve, but I can't be sure.
It also seems to take longer to upshift than it used to when the engine is under load. If I go up a steep hill it acts doggish for awhile and finally downshifts. The RPMs climb slowly but steadily but at the top of the hill it acts like it doesn't want to upshift again. Before, it used to upshift 3/4ths of the way up the same hill. It acts like a bad modulator valve, but there ain't one to go bad.
Anybody got any ideas?

 

Replace the PCV valve and check the hose that goes from it to the back of the intake manifold. The elbow at the rear of the intake manifold tends to rot out creating a large vacuum leak. Good luck!

 

Sounds like a big vacuum leak.

 

I agree, it sounds like a vacuum leak, but I sure can't find one. I checked the PCV elbow and it's fine. The truck idles and cruises fine with no misses, it just feels gutless.

 

Clean the MAF sensor again. If the K&N is too oily, it can contaminate the MAF. Try replacing the fuel filter too. Good luck!

 

The "loud sucking sound" was new? How did you search for your vacuum leak?

I've heard of mechanics using a variety of flammable aerosols to find vaccum leaks. The idea being that the flammable vapors get sucked into the leak & raise the RPMs. Obviously its a slightly dangerous task spraying around the running motor. Another mechanic I know uses a propane plumber's torch. He justs opens the valve a bit & uses the tip to probe likely areas for vacuum leaks, hoping to find an increase in RPM where the propane gets sucked in.

 

The noise has been there since the poor performance has been. I used a propane torch (bernz-o-matic) to check for leaks. I also did a pretty thorough visual inspection. The fact that it doesn't happen at idle or cruising conditions kinda steer me away from vacuum leaks. I'm almost certain it's air rushing past the partially open IAC.

 

The noise is the reference point on this fault.
The IAC has little effect at part throttle because the throttle plate open area far exceeds it.
If you had a full time air leak, codes 171/174 would be present.
Look at the mounting on the TB and the elbow for being loose or some malfunction related to mid throttle movement..
The trans shifts from a combination of throttle position, govenor speed and load.
Since you use more throttle on a hill to overcome the fault, the trans will upshift later.