Just changed the fuel pump in 99 Chevy Ex PU Z71. Ran great, then would start later in the day. Replaced battery and battery cables and still won't start. Any suggestions?

If the pump is starting, and you can hear it, then something in the tank is either intermitently blocking the inlet to the pump or something is blocking the line to the engine. It is also possible the pump pickup is not within the the fuel. However, it is not unusual for older vehicles to have corrosion in the metal fuel line caused by water or other contaminent in the fuel that breaks off and intermitently plugs the line or the filter.

before you do anything check you fuel line at the engine and see if you are getting fuel. sounds like its not getting any fuel.

Well there is no shutoff relay, but there is a fuel pump relay that powers the pump.
I would check it first. The computer grounds the relay for 3 seconds when you crank the engine to boost fuel pressure. Once the engine starts the pump is powered only if you have oil pressure.

Check fuel pressure with a gauge

Sounds like the pump is going bad & stops
mechanically or electrically

You don't need codes to have cylinder misfires,bad
egr valve or O2 Sensors with over 100,000 miles

You may have loaded a fuel line or connector up with contaminants. What else is happening during the second or 3rd day of running? Is it running low on fuel? Try to make a list of your gauge set! It only makes sense.

check distrubutor timing may be off because distrubutor is worn out also sounds like the fuel pressure regulator is bad

ecm fuse is probly blown also, this happens alot when you replace the fuel pump

Remove air filter and unplug the mass air flow sensor and try again. if i cranks clean the MAF. check sensors for skewed readings that won't set codes but will upset computer.

BAD FUEL PUMP

spark plugs known problem when cold