Starts fine cold then starts and shuts off then starts very hard but will start . could this be fuel pressure regulator what what happens to make for lean air fuel mix during cold start in 80's had a cold start injector what happens with obd2 this is a 97 gmc sonoma 4.3 vortec

Try swoping fuel pipes round on your fuel pump

Just because you replaced the fuel pressure regulator does not mean it is working correctly; which is the most common cause of your problem. Hook up a fuel pressure guage and watch what happens when you are starting it cold - it should go up to about 47 PSI and once started it should read between 40-47 PSI, then when you turn the engine off: Does the pressure drop? There is a check valve built into the fuel pump also; that can be faulty and if the pressure is generally reading low and the check valve is bad- this can also create your problem. We have run into this before also; When the fuel pump is cold the internal pump seals are sealing great, but after a period of running time the pump seals lose sealing power and if you try to start it again it will be hard to start. The biggest thing with this situation is getting the fuel pressure readings at the following points of time:: Start-up, running cold, running at normal operating temp, engine off, engine re-start.
Pete/ Bambenek Auto LLC

Usually a sign of leaking fuel injectors or a fuel problem.
Starting with a wide open throttle turns off the injectors which would clear out the cylinders of fuel. Could also be a leaking fuel pressure regulator if it is getting too much fuel.
Does it eventually clear up once you get it started ? Is the check engine light on ?

Fuel Pressure Regulator is located on the fuel rail.

1. Disconnect the negative battery cable.

2. Properly relieve the fuel system pressure.

3. Remove the air cleaner outlet tube(s).

4. Remove any necessary components to access the pressure regulator.

5. Disconnect the vacuum hose from the pressure regulator.

6. Remove the pressure regulator attaching screws.

7. Remove the pressure regulator from the fuel rail.

To Install

8. Discard the O-rings on the pressure regulator and replace them with new ones.

9. Lubricate the O-rings with clean engine oil.

10. Install the regulator into the fuel rail and tighten the retaining screws.

11. Connect the vacuum hose to the regulator.

12. Install any components removed to access the regulator.

13. Install the air cleaner outlet tube(s).

14. Connect the negative battery cable.

You probably should perform a fuel pressure test to see if the fuel pressure is bleeding off of the fuel rails when the engine is shut off. If it is, you could have a bad fuel pressure regulator or a bad check valve in the fuel pump module (which will require replacement of the module).

One way to test for this condition without any tools is like this:
cycle the key ON, wait about two seconds, then OFF, wait about 2 more seconds, and back ON again. Repeat his 3 or 4 times before turning the key to the CRANK position. If the engine starts immediately every time you do this, then the hard starting is definately caused by the fuel pressure bleeding off.

Does it hard start when warm or only when cold? could very possibly be a fuel pressure problem, like fuel pressure regulator. Also could be cold start injector.
Thanks,
Lee

Sounds like a Solenoid might be bad on the starter if it spins freely and a high rate of speed without engaging.

Also, the relay could be bad.

I am ot sure if the fuel filter and pressure regulator would cause this problem.

No code will show with a bad starter or bad relay, I don't believe.

Good luck with this one.

sounds like the fuel pressure regulator,also make sure the fuel pump and sending unit are working properly. happened to 99 olds of mine! good luck

The most likely cause is the fuel pump unit. The fuel pmp has a check valve that is supposed to hold fuel pressure on the fuel rails after the engine is shut off. when this check valve leaks, it allows the fuel from the fuel tank to drain back into the tank. Then, when you go to start uour car, the fuel has to travel the full length of the car, and build up enough pressure to spray through the injectors before the engine will start.

The fuel pressure regulator can cause similar symptoms, but is not as likely to fail as the check valve in the fuel pump and will be much worse after a short hot-soak. (Shutting the engine off hot - for about 5 to15 minutes)

hi there i do believe you have a faulty or leaking injector because it should stay pressurized when you shut off the engine