Vacuum Issues

[Hogitall]

I have a 1998 Explorer with a recently rebuilt motor 4.0 SOHC V6 (intake manifold was not replaced) 2 wheel drive that about 75% of the time sucks air so hard from the line (small 3/8" or 1/2" id hose) that goes from the air filter housing hose (large 3" id duct style hose that's preformed) to the intake manifold that it sounds like a fog horn under my hood most of the time. It settles down and stops now and then but starts back up when I touch the gas pedal. I have pulled it out and it's sucking very hard like it's starved for air...? I have replaced the IAC valve a few weeks ago but the plastic cap that goes over the top port came off and is lost. My service engine light is and has been on for long before the motor was replaced. The new motor has about 10k on it.

FYI...The truck has 347,000+ miles and I've replaced nearly everything in the drive train the past few years so I figured I'd replace the motor instead of buying someone else's problems. I bought it with 21,000 miles on it.

Thanks in advance for any and all input towards getting her quieted down again...

 

[my98nnj]

An engine is technically an air pump. The more air you get through it, the faster is spins. Is the stock air filter box in place or are you using an aftermarket cone type?

 

[Hogitall]

Stock filter box is in place.

 

[Hogitall]

No help on this one I suppose...?

 

[Hogitall]

I guess I'm the first to finally stump you all...Hmm?

 

[1992fordgreen]

Not the first... I'd check every single line around the intake and underneath the TB and such for cracks, lacerations, leaks, ect. If you find none, it seems like it would be something internal. 347k on an original intake manifold?