I have a 99 silverado z 71 with the 5.3 engine with 235.000 miles we had a problem with starting it when it was warm found out it was a bad fuel pressure regulator replaced it now it starts all the time but has a problem at idle it just idles up and down only about 5 to 6 hundred rpm but bad at redlights and stop signs kind of lunges at person in front of you

sounds like a fuel filter issue is there an inline filter

Have you checked the fuel pressure regulator to see if its leaking?

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

check high tention coil wire, coil wire going to dist cap with test light, if test light light up, it good also check dist rotor, and if that good might be pick up coil , the module might be bad, let me know what happen Bob

you have a cracked head most likely there was a batch of heads with a castech casting mark that caused some problems just like what you describe. here are some pics of what to look for.

spark plugs are bad

It sounds like the fuel pressure check valve on the fuel pump has gone bad and your losing fuel pressure when engine is turned off. Try priming the fuel pressure twice, that's when you turn the ignition keys to the run position with out starting the engine for 10 seconds and then turning the key's to the off position and on again for 5 seconds. After the second prim the engine should start, to check this you can lone out a fuel pressure gauge and check the fuel pressure at the shredder valve. You should have 62 to 65 PSI during prim and once the engine is running, it should drop by 5 psi. As for your the accuracy on reading the right amount of fuel in your gas tank, the sensor is apart of the fuel pump sending unit and the float may have gotten bent and nothing you can do to fix it unless you take the fuel pump out again and replace the sending unit. Good luck and keep me posted.

perform a fuel pressure test at the fuel pressure port.
if is low pressure remove the fuel tank and check return pressure line clips conections

I would have the computer scanned ( no SERVICE LIGHTS right ? ) looking for an errant coolant sensor not telling the computer to give your vehicle a bit RICHER ( a little bit more fuel ), since it does start. Just has POOR cold drive away. Hope this helps

try replacing your EGR Valve, or your Oxygen Sensor

I had the same exact problem at 175000 miles on my '99 Silverado..after paying the garage several times to read the computer codes to me and ex-plain that 1st I needed an $899.00 head gasket repair, and then just a $250 injector replacement, I decided to run some injector cleaner throguh the system...guess what?............I cetainly am glad I didn't have the head gaskt repaired or the injector replaced...problem fixed