1999 Ford Taurus V6 OVC gas SE trim. Issue: when cold, cranks for about 30 seconds then starts. Have to press accelerator slowly to keep running. After it warms up for about 3-4 min. it will stay started it will run and idle. It will run and drive at slow speeds. When you try to get to hwy speeds 45-55mph it bogs down and wants to die. If you pull off road and wait for a few seconds and slowly press the accelerator it will finally “catch it’s breath” and rev up. Put in back in gear and same thing happens again. I suspect either a fuel pump or fuel pressure regulator. Attached a fuel pressure test kit at shrader valve. KOEO reads 12psi. KOER reads 21 psi. Removed vacuum hose from fuel pressure regulator PSI stayed the same (did not increase). So, fuel pump or Fuel pressure regulator or something else?
I would start with the fuel filter first and then re-test your pressures. If the filter is clogged or hasn’t been changed, there is a chance for a significant increase in pressure. This would make a difference across your readings. If the readings do not change, replace the fuel pump and fuel pressure regulator. Applying vacuum tot he regulator should increase the pressure. @0 psi is fine at idle and should increase toward 30 psi on acceleration.
Of course if there is a check engine light on you would need to scan the codes and address them also.