Starts with starting fuel, but will not stay running?

Hello BLAKE GERMEROTH,

I would start at the pump and make sure you have sufficient power and ground.

I would use a multimeter so we can test for a voltage drop or an insufficient ground.

If you don't have one, you can get one at your local auto parts store or hardware store.

Here are some instructions on how to use it if you haven't. You don't need an expensive one, but I wouldn't get the cheapest one I could find either. You get what you pay for.

https://www.2carpros.com/articles/how-to-use-a-voltmeter

1) First set the multimeter to volts DC and connect the multimeter to the battery, and make sure you have 12 volts. Then go to the pump and test at the connecter with it unplugged. Put BLACK lead to chassis and use RED lead on connector terminals until you see voltage. Do you have the same 12 volts you had up front?

If not, there is extra resistance in that wire and causing a voltage drop so the fuel pump isn't receiving enough current to operate the pump correctly.

2) Next check the ground by placing the multimeter on Ohm's. Place the RED lead on ground on the connector again placing the black lead on the chassis.

Looks like there is 4 wires, ground should be all the way to the right on the harness connector. See what you get for a reading. It should be pretty close to zero.

If not, you have a ground issue.

3) If it is, plug the pump in, and perform the same test only this time touch the fuel pump housing with the RED lead and put the black lead to the chassis. You should get the same reading. If not, there is an issue with the ground connection on the pump.

4) If everything is good there, I would move to the front and test fuel pressure. There should be a Schrader Valve on the driver's side fuel rail.

You should have 35-40 PSI with key on engine off.

If you don't have a fuel pressure tester, you can rent one at your local auto parts store. Just keep in mind they get banged around and might not be as accurate. I have added a picture of one for you as well as a common Shrader valve.

Once fuel pressure has built up and appears to be within spec, leave it and watch it for a while and see if it is dropping of at all. This could indicate a bad Fuel Pressure Regulator.

5) Next, I would test your ECT sensor. It should be a single wire sensor. If it is, you should be able to unplug it and the gauge in the car will go all the way to the hottest position. I apologize I don't have really any wiring diagrams for this so let me know if you are seeing something different.

6) Another thing it could be a vacuum leak either from a hose or intake gasket introducing too much are into the engine, so it's leaning out the mixture way too much.

Use this video to find one. Best method in my opinion that I have used since I was a kid.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

7) I also thought I saw something about a Dual Fuel Tank Switch, but I can't find anything now on it. We had the Grand Marquis when I was a kid. Awesome car, so comfortable and powerful. I thought it had a fuel tank switch on the left of the driver's seat by the sill but that was a long time ago. Do you have one on yours?

I hope this helps. Let me know how you make out and we can go from there.

Thank you,
Brendon

Hello BLAKE GERMEROTH,

Awesome. Happy to hear you found the issue.

If you need anything else let us know or just start a new question.

Have a great night.

Thank you,
Brendon