91 Vic won't run when hot

Hi, I have a 91 Crown Victoria ex-police cruiser with 320,000 km on it. On warm days it stalls after about 45 minutes of driving. Just before it stalls, the fuel pump makes a high pitch whine for about 10 seconds. After it stalls, the pump still runs for about 2 seconds. The fuel pump will work every time the key is turned on, but it makes a growling noise instead of a soft buzz. I have replaced fuel pumps several times on this car and I also replaced the fuel strainer in the tank every time I replaced the pump. With a new pump, the car seems to work for a while; but then it acts up again. The fuel filter is replaced as recommended. I also replaced the fuel pressure regulator. I checked the ignition and there is lots of spark. The injectors are firing as I tested them with a test light.

What engine?

Hi, It's a 5.0L engine. I managed to borrow a fuel pressure gauge today and hooked it up and ran it until it stalled. While it ran, the pressure was 35 psi. When it stalled, it was 18 psi. I thought that the fuel pump went out again, but when I pressed the bleeder valve on the gauge, there was a lot of air mixed with the fuel as it came out. I turned the key on and bled it again and there was still lots of air mixed with the fuel. I then bypassed the fuel pump relay and ran the pump till I got all the air out of the fuel system. Now the question is, how is all that air getting into my fuel system? It also looks like I've been replacing fuel pumps for nothing.

next time it stalls...check to see if you have spark....i'll bet $10.00 you don't

I checked for spark yesterday when it stalled and I discovered the air in the fuel system. I pulled the coil wire off the distributor cap and had a friend crank the starter and there was lots of spark present. About 2 years ago, I did have a no-spark condition with this car. The coil was defective, so I replaced it and no trouble with the ignition since. Last night I ran it for about 15 minutes and checked the fuel system again and it had some more air in it. If I could figure out where it's picking up air, I would have it solved.

Drop the tank and look at the hose that goes to the pump. If it has fuel over the hose then it will probably run ok. If there is a hole or crack in the hose, as the fuel level drops below the crack or hole, the pump will try to pull air along with the fuel.