Well, this one may be pretty difficult to explain: Just a quick mention on the very basics of "positive crankcase ventilation." The PCV valve is the device that draws the fumes out of the crankcase, using engine manifold vacuum. The return air is just that: it is the fresh (filtered) atmospheric air that must be returned to the crankcase, replacing the same volume that the PCV valve is removing. (technically, the volume is a little less, due to cylinder "blow-by", but that is getting way more technical than necessary for our purposes.) As a side note, if atmospheric return air was not provided into the crankcase, then you would soon be pulling manifold vacuum in the crankcase, and introduce all sorts of bad issues.
Okay, you can see this fresh air return port as it goes through the throttle body gasket, by referring to the fourth photo posted by mkaake on June 19th at 7:02pm, on the topic of "Upper Intake Gasket" (page one of the responses). You can see that he is holding the plenum (on it'* side) such that you see the entire red silicone throttle body gasket. (note that you can see the EGR stove pipe on the right side of the phote as we view it.) Okay, the two sort of odd shaped ports adjacent to that stove pipe are the coolant ports to the throttle body base plate. Now, the perfectly round port (maybe 3/8" diameter), in about the one o'clock position of the photo as we view it, is where this "return air, or fresh air" passes from the throttle body into the plenum. The air, after entering the plenum through this port, makes a 90 degree turn, and goes a port in the bottom of the plenum, (again surrounded by red silicone gasket) into the lower intake manifold. Actually, if you refer to the second page of the same topic, and the last photo shows the port in the plenum gasket. (-at that point it is an oval shaped hole adjacent to the one round coolant port.
After it is in the lower manifold, I can't tell you it'* exact path, but it finds it'* way down into the lifter valley area. Wow, now does this sound like a real mess, or what.
Hey, I sure hope I don't "lead you down the proverbial garden path" here, and get you into someplace you don't want to go. I would hope that a good shop mechanic could help you investigate this return air path, but I'm betting that most of them are not even aware of it. Also, I certainly don't have a good feel for how likely or not that anything could be blocking those passages.
I did go out to the garage to my '99, and pulled off the air filter and box assembly, removed the throttle body inlet screen, and looked down into the throttle body to see where this passageway enters the throttle body. I found it, but again, hard to explain.
I almost wish I hadn't started trying to explain this one, and again I'm very sincere when I express my concern that I may have only made things more confusing.
Can someone else help out here ?? -maybe with some photos ?