I noticed my 99 Passport started idling rough when the fuel level dropped to 1/4 tank or less. When I would fill up, the rough idle would go away. Now it has gotten worse. Even with a full tank, it will start normally the first try of the day, and run at normal driving speeds, but will not idle, and if it dies it will not restart. I tried injecting a small amount of fuel directly into the throttle body intake and it will immediately start and continue to run normally at high idle and above, but if I let it go to normal idle speed it will die and will not restart. If you let it sit for an hour or two, it will restart, but will still not idle and then it will die and not restart. It seems like a fuel problem, but where to begin?

Jack

is the CEL coming on?

No CEL.

Same thing this morning. When I first turned on the key to ACC, it sounded like the fuel pump ran for a second, then the I went ahead and started as usual. Engine started immediately, then died and would not restart. Now when I turn the key to ACC, I do not hear any fuel pump sounds. It seems as if the fuel pump is bad, but why does it start on the first try after sitting for hours/overnight, then not afterwards? I replaced the fuel filter, didn't help and the old one seemed fine. Pushed in the valve on the fuel rail (before changing filter) and not much pressure, fuel just dribbled out.

Pushed in the valve on the fuel rail (before changing filter) and not much pressure, fuel just dribbled out.
Sounds like fuel pump to me. Electric motors can do odd things. It might be ok for a couple seconds when cool then over heat quickly.

Maybe fuel pressure regulator? Not sure since I've never had one go bad I dont know the symptoms.

I am thinking it has to be either the fuel pump or pressure regulator. I'm not sure how to check the pressure regulator, though.

I am thinking it has to be either the fuel pump or pressure regulator. I'm not sure how to check the pressure regulator, though.one check is to pull off the vacuum hose. if it's got fuel in there, or heavy fuel vapor, it needs to be replaced.

you can get a fuel pressure gauge for not a ton of money at a bigboxpartsstore and screw on that schraeder valve and test for sure what the fuel pressure is.

also, check the connections to the fuel pump at the fuel tank. they are known to corrode and/or the wiring starts breaking down at the connector causing intermittent conditions.

//bc

Fuel pump relay comes to mind, also. It would be a quick, cheap thing to replace. Sometimes when relays go bad, they'll work at first but then they heat up a little and the coil won't pull in.

Thanks for the suggestions, I will give them a shot when I get off work this week. I did try switching out the relay with another, but no success.

Well, after sitting for a week, the Passport started right up and idled fine. Drove it around for a few blocks, then let it idle in the driveway, no problem. Then I turn off the ignition and try to restart. Then engine does the same as before, it turns over but does not want to fire. No CEL. I'm about to pull my hair out as to why it will start after sitting for a day or so, then runs fine, then will not restart.

Are you turning the key to the on position, waiting 5-7 sec and then starting it or are you just putting the key in and starting it?

I tried turning the key to "on" to see if I could hear the fuel pump (I didn't), then tried starting, but nothing. Then tried just turning the key to start immediately, but that doesn't work either.