3.1 Liter Idle Problem

Have a '99 Century with the 3.1 vin M engine. Idle varies RPM and hard to start. Have to give gas to start. No codes have been set. Replaced the idle air control valve but still not working properly. Once underway runs great. Any ideas?

Check the fuel pressure with the key on engine off. It may not be at the proper level as the fuel pressure regulator may not be holding enough pressure at start-up or idle.