My 96 grand marquis wont stay started unless the vac hose us unpluged

Hello, There are several Vacuum lines, but I would say that you should disconnect the Vacuum line you are talking about and plug it. Then see if the engine runs even better. I am thinking it may be an EGR line and about the only difference between what you did and leaving the line unplugged is the EGR did not move.

Now there are Solenoids on the engine to control the Vacuum flowing to the EGR and one of the Solenoids may be open when it should be shut. So you can not verify the EGR is bad unless you have a Vacuum test gauge or press up on the EGR diaphragm with your fingers and see if the diaphragm is ruptured. A ruptured diaphragm will not hold Vacuum. Also the EGR is hot when the engine runs and the Solenoids switch off and on so rely on the on-board computer.

You should be setting off "check engine" lights if any of the Solenoids are bad. If you have found a way to make the engine run, it will give it time to diagnose the problem. There can be a few vacuum controls under the dash which can operate the A/C and heat flaps. Trace the Vacuum line.

If you are disconnecting the big Vacuum line to your Brake booster you need to reassess what you are doing. You would not have power assist for the Brakes.

I owned a 1994 Crown Vic and currently have a 2002 Marquis. So I have a good idea where the Vacuum lines go as your engine is the same. Autozone and Oreillys and Advance will scan for free. They may not want to do it without an engine light on, but if the problem is not the EGR then you need a scan. I hope I have helped you.