99 Cavalier Turn the key you can hear the pump kick in and the car starts fine. The next time you turn the key and can not here the pump but you also will have no spark. Would a bad pump cause no spark? or is this a sensor thing? thanks for any help

Air in fuel system

Check the connection from the battery cable to the starter. Might need cleaning. Solenoid may not be kicking in. Is the whirr only when the key is in the start position?

does it crank? do you hear the fuel pump come on when you turn the key to the run position? do you know if you have any spark?

Usually the sign of a bad fuel pump check valve that allows fuel pressure to bleed off over night. Try this in the morning. Turn the key to ON for 2-secs, then OFF. Repeat 3 times. Then try to start. If it starts right up, you need a new fuel pump. By turning the key on and off you're filling the fuel lines and building pressure that you lost overnight.

Tap on the bottom center of the fuel tank while someone cranks the vehicle. If it starts you probably have a failed fuel pump assembly.

Try this next time you start it. Turn the key to the run position, do not start the engine. Wait 3-5 seconds and turn the key off. Turn the key right back to the on position and wait for 3-5 seconds. Turn the key off. Repeat the above one more time. Now try to start. If it starts right up you have a fuel supply problem. Either the fuel pump is bleeding off or the fuel pressure regulator is leaking. Pumping the pedal is doing nothing besides buying time for the fuel system to pressurize. Post an engine size and I'll help you locate the fuel pressure regulator. Let me know if you have any questions and I'll do my best to answer them. Thank you for using FixYa.com!

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

The relay is located in the underhood power distribution box, it is relay number 2, to see the picture of the box go to the web link of your owners manual, the info is on page 359

http://www.motorcraftservice.com/pubs/content/~WOTF12/~MUS~LEN/36/96f12og1e.pdf

Fuel pumps are also a common proble with the cavalier/sunfire. can you hear it running immediately after turning the key on (not trying to start)

Sounds like a after market pump? I did the same thing.Go Ac/Delco for the best case.Note no sound no fuel.Try the old turn the key on and give the tank a kick.This kinda kick starts the pump.Idone this a few times.If you are stuck and your pump stops and it did start by kick don't stop the engine till you are home or a shop.

Prime the pump. Put the key in, turn it to full electrical power (right before you'd start the motor) - you should hear the fuel pump kick on for several seconds. Once it stops, turn the key off for 20 seconds or so. Repeat this three to six times - that should be enough to build up fuel pressure. Then try cranking the engine over.