Name that part 89 300E
 

I don't have a pick yet. The part in question is the little canister that is located just in front of the fuel distributer, with fuel lines running to it, then to one of the ports on the FD. There is also a little vac lin that runs into it from the engine side. What does this do? I as because upon disconeccting the vac line that runs to it, I lost the connector. I made due with what I had, and just used a vac butt conecctor, but I'm not sure that the conection is tight enough. Please advise, thanks!

OK. I see that its a fuel pressure regulator. Does anyone know what role the vac plays in that part. It seems that the vac connector is actually part of the unit. Does anyone know if it is just stubbed into the part? when it wasn't connected, it didn't look like it actually connected to anything inside the part...

normally the vac regulates the fuel pressure
 

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

And this would be the case whether it was in open- or closed-loop, right?

Generally that's true. We are talking about two related systems, the pressure regulator is generally just a mechanical device controlled by the vac signal. Open or Close loop will be determined by the ECM based on a host of electronic signals namely engine temp and O2 sensors, but generally will be used to adjust ignition timing and in injector pulse.

Usually fuel pressure regulators fail/safe is to be wide open so that you always have fuel available so by not having the vac line connected you are likely in a wide open mode. Still at max fuel pressure you will likely be burning rich and the ECM will treat this as open loop, but if your engine is at temp and the O2 sensors are still reading rich then your ECM may try to compensate with timing advance.

Best bet is to install a fuel pressure gauge and take a reading after the regulator with no vac signal applied. Then apply vac and watch the gauge, your pressure should adjust downwards accordingly. If you don't have any of these tools available to you try the old sniff test. With the engine at temp and no vac signal carefully sniff the exhaust and see if it smells rich. Find the unconnected vac line and plug it into the regulator, give the car a few minutes to adjust and sniff again, it should smell less gassy.

If you cannot find the vac line look at a schematic for your car and see where it should have been connected and install one there. Also if your vac line has come off and was not capped at the line you also have a vac leak now that could be causing some idle fluctuation, so it is important to fix this and connect it to the regulator.

the pressure regulator on ke jet is regulated by a small line from the fuel distributor.i believe that line is not vac as such but a depression line from the crankcase vapor line at the rear of pass side valve cover.i think it is more of a safety conn in case the diaphram ever breaks in the regulator.i might add that i have never seen one break which doesn't mean it will never happen as we drive older and older cars.