If you are talking about the fuel pressure regulator, it receives a vac signal to let it know how much to regulate the fuel pressure. Essentially the vac come from the manifold and controls how much fuel flows through the regulator. At idle, hi vc, the regulator will flow less fuel. At WOT little to no vac, the regulator will be wide open allowing max fuel flow. If the vac line is not connected you are likely seeing more fuel flowing at idle and low speed than you need causing you to run really rich, so it is important to have the line hooked up for your fueling to work properly.

Hope that helps

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Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)