So my brother-in-law's Mustang idles rough. But not when it's got a full tank. Put some gas in there and it runs fine, but let it get to about 1/4 tank and it will shake and miss. Weird. It's a '98 3.8 liter. Anybody got any ideas? First guess is something in the tank.
Does it run like this with the gas cap off??Could be a plugged fuel pick up sock or a weak pump(not completely submerged) at a quarter tank.Or a faulty vent valve not allowing the tank to stay at ambient pressure level(not likely)but possible.Or as spork 101 stated, cracked pump housing.The crack is uncovered at a quarter tank causing the pump to cavitate(suck air)Good luck.
Ah, I hope it ain't the pump. But at least it's not that bad of a job to replace the fuel pump on these things. Not like an '82 up Camaro/Firebird. I just finished replacing the fuel filter, and it was very nasty. Poured it out and it looked like Coke. It's too late to test drive it, but I'm crossing my fingers...