Quote:
Originally Posted by MattBelliveau
And this would be the case whether it was in open- or closed-loop, right?
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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.