Looks like I was describing it wrong mosesburb. I looked at the line from the tank on Wednesday and it is a smaller diameter than I remembered. A 1/4" hose fits snugly on it. The fitting on the air cleaner is quite a bit larger. If the fitting on the underside of the air cleaner turns out to be unnecessary (since I have a filtered breather to the open air on the driver side valve cover) it'd be easy to put a rubber plug in its place instead.
The earlier guess about line routing was based on a couple of my older carbureted cars. On one of them it has a separate vapor recovery tank which sits above the fuel tank, and a line runs from the vapor recovery tank up to the engine bay. From there it connects to a valved tee where one end leads to the air cleaner and the other leads to the crankcase.
The '88 engine I've got originally had a charcoal canister and A.I.R. system, but I've removed them. I'm trying to imagine what the original engineers intended and replumb accordingly. Unfortunately I don't have a manual which describes the emissions system in any detail. I'll probably buy the fuel & emissions manual from an Ebay seller just to make sure I hook everything up right.
I have left the MAP sensor, PCV valve and EGR pressure valve solenoid hooked up to the proper ports on the TBI unit. That left two extra unused (small) ports which I blocked off. I assumed that the unused ones had lead to the air pump and charcoal canister. The instructions on the Edelbrock intake manifold say that the MAP sensor port can be teed from as well if the transmission has a vacuum modulator, but I don't believe I need that.
Still gotta figure out where that small fuel tank vent line was meant to go. I do have the old fuel pump from the '88 truck handy for reference.
From left to right we've got 1. vent to fuel filler neck (large diameter) 2. fuel feed 3. fuel return and 4. mystery vent line (small diameter).
Here are the three that run along the frame, near the transmission. The vent line is the one with the rubber hose on it.
I guess it depends on what the manual says. Without it I'm assuming that either it'll get vented to the open air with a check valve, or, more likely it'll lead back to the air intake path somehow so that the fuel vapors are burned instead.
Oh, also a very special thanks to mosesburb for graciously digging through his old reciepts for me. With his help I now have longer rear parking brake lines for my Suburban (ACDelco DuraStop 18P266).
