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Okay, my CJ with MPFI had been idle badly. It would drop to almost dying then come back up the back down. If I applied some gas quickly it would die. This would go away once it warmed up. So while cleaning the engine to get ready to fix that lovely leaky vavle cover I noticed a rubber hose the ran from a port on the intake manifold had come off of somewhere and was just lying there. I look around and see a round silver colored round thing with a nipple coming our of the center. This is on the end of the fuel rail. I believe this is the fuel pressure regulator and the rubber hose is vacuum. I reconnected it and my baby runs fine now. Even seems to have better acceleration.So, if my assumptions are correct, what does the vacuum do here?

You basically answered your own question. The vacuum hose is for the fuel pressure regulator. Under high vacuum (deceleration), the regulator will drop the fuel pressure. Under heavy acceleration (low vacuum), the regulator will increase the fuel pressure in the fuel rail. Normally, you don't really notice a difference if the hose comes off. On my Cherokee, I can't see any difference in idle. I would suspect that in your case, you had a little too much pressure and too much fuel was squirting in...and/or the vacuum leak was causing you a problem as well.
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