What is this supposed to do? Vacuum port on TB (FSM owners)

[Guest bastaad525]

When I still had my stock turbo manifold on the motor, there was a vacuum hose that connected from the intake manifold to a small port on the bottom of the stock turbo throttle body. I also noticed that I had a strange problem, that when accelerating, when I would let off the throttle to change gear, the rpm's would stay up for a few seconds, usually causing a rough shift if I was trying to shift fast. I noticed this more whenever I was slowing down, approaching a light, as I always put it into neutral when coming to a stop, and I would notice the revs would hover for a while before slowly dropping back down to idle.

 

Since then, I've switched over to a '75 intake manifold, which has a few less ports and things on it, and only had enough ports for my BOV, FPR, Boost gauge, and the brake booster. I didn't know how important or not the vacuum connection was to the TB, so I just blocked off the port on the TB and let it be. I noticed is that the revs stopped 'hovering', they would immediately drop down whenever throttle was released, as I felt they should.

 

However, I also lately have been noticing something else. When accelerating quickly, I notice that the car bogs slightly between shifts, even though the boost comes back up almost immediately, it feels like it takes a second to 'come back on'. So hard shifts always sorta feel 'mushy'. Strange thing was that my N/A motor, which had 30 less hp and nearly 70 less ft lbs of torque, had no problem chirping 3rd and sometimes even 4th gear, whereas with this motor I can rarely even get it to chirp second, because of that 'mush' after the shift. My BOV is recirculated, so I dont' think it's a matter of running rich between shifts from air venting. I dont seem to remember it feeling like this when that vacuum port was hooked up before but I'm not sure. I find it hard to believe that on quick shifts the turbo is slowing down so much as to cause lag after each shift. It may be an issue with the EFI, I dunno.

 

So anyways, my real question is, does anyone know what the ACTUAL purpose of that port on the TB really is? I browsed the haynes manual and didn't see anything specific about it. Maybe one of you guys with the FSM can see what it's for? I'm wondering if I should hook it back up... my only real option would be to install a tee in the vacuum line that goes to the FPR, as it's the only port that is the right size... would there be any problem with this?

[Guest bastaad525]

Well... no replies so I decided to go buy a tee and some hose and hook it back up.

 

 

 

Nothin... it did absolutely nothing. Well, wait, not entirely true, I notice the idle jumped up around 100rpm. Otherwise, the car seemed to run exactly the same. The revs didn't hover like they used to, either. Which is odd, because I remember that it only stopped doing that when I disconnected and blocked that port initially. Surely one of you guys with an FSM can look this thing up and see exactly what it's supposed to do?? To be honest, I'm not even sure that it's supposed to connect directly to the manifold, maybe it used to connect to one of those control valves or something on the original turbo manifold.

 

Come on guys someone solve this mystery for me it's driving me nuts!

[SleeperZ]

I use it for the fuel vapor cannister, so it vents only under throttle tip-in, to make sure the vapors go into the engine. AFAIK, that's it's purpose on an FI car. If no one knows for sure, I can venture to the garage, I know the FSM is buried somewhere there...

[Guest bastaad525]

That does make sense. Can anyone else confirm this? The guys who swapped the motor into my car musta put it wrong then because I remember for a fact that it was hooked up to the intake manifold before... I thought it was something specific to motors that were hooked up to auto trans', though I dont remember what gave me that idea, nor did I ever have a clear idea as to what it was for. I just remember that when I had the stock zxt manifold, that hose went to a regular port on the manifold, and that when I removed it and blocked it off, and used the port for the vacuum reference on my BOV, that the motor would stop 'hovering' at whatever revs whenever I would let off the throttle. Which of course, having that problem at all only makes more sense that the guys had hooked the thing up wrong in the first place.

 

I dont think I'll hook mine up to the vent tank line though, as I know for a fact that my fuel system is not airtight, and having an open line at the TB would surely introduce unmetered air into the system.