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I noticed that the black vacuum distribution box is connected to the large flexible intake hose just before the throttle and also to the intake manifold just after the throttle. There is a lot of vacuum present on the prethrottle hose. It seems like this is causing a lot of air to bypass the throttle. Is this normal? Is a lot of air just supposed to flow from the "accordion" through the vac distribution box and into the intake manifold bypassing the throttle?
 

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I noticed that the black vacuum distribution box is connected to the large flexible intake hose just before the throttle and also to the intake manifold just after the throttle. There is a lot of vacuum present on the prethrottle hose. It seems like this is causing a lot of air to bypass the throttle. Is this normal? Is a lot of air just supposed to flow from the "accordion" through the vac distribution box and into the intake manifold bypassing the throttle?
Normal!

Small low flow clean, metered flow of air passes through the box and back into the intake. Actual flow is small, but still attached to intake vacuum. Tiny flow compared to the PCV which is supposed to be significant flow.
Assuming the PCV is working as designed, and OEM design, not some afermarket cheap thing.
As long as all the air into the intake is 'metered' and 'filtered' all is per design.

-chart-
 

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Discussion Starter #3
I only replace parts on the Taurus with Motorcraft parts.

That hose to the distribution box is pulling a lot more air when the engine is at idle than the crankcase breather hoses that are part of the PCV system. Are you saying that hose should have even less vacuum than the breather hoses? Is the box just a box with a bunch of hoses that connect to it or does it have some kind of valve that moves inside? It would seem as though even if it is metered air, bypassing the IAC solenoid and the throttle would make it difficult for the IAC solenoid to do its job. But if that's the original design, and the box isn't supposed to restrict the flow, then that's cool I guess.

I'm not experiening high idle, and my LTFTs are only slightly elevated but less than 3%. I actually got the P0171, and P0174 warnings for lean mix the other day. I replaced the hose from the PCV valve to the IM (which I did previously 5 years ago) and the PCV valve itself while I had the hose off (which I replaced 2 years ago), (all parts Motorcraft), I cleared the DTCs and the PCM hasn't stored any codes since I did this (several days, several drive cycles, and over 200 miles). However it does seem to idle a bit rough and the RPMs dip close to around 500 at times and I'm trying to figure out why.

I had the dealership replace my harmonic balancer a few months ago because I just didn't have the time. I also had them replace the front seal while they had the balancer off. I'm wondering if the rough idle is from something they did, me being paranoid thay they messed up and noticing things I didn't notice before, or just that the car is getting old and engine parts are getting worn (193K).
 
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