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Sounds like fuel pressure is too low-either a weak , old pump, or you need to change the fuel filter-most likely, the pump, though. Check the fuel pressure specification for your car, most mpfi systems require 40-50 lbs. of pressure.
If the pressure holds steady than the regulator is likely not the issue. It is very common for the fuel pumps to be the source of this issue. Many times you can start the engines with a shot of carb spray and the engine will continue to run after until you shut it off.
Get a quality fuel pump. Either a delphi or an A/C Delco. These pumps are worth the extra money as they are more reliable than most of the other aftermarket brands. One in particular to stay away from is a brand called Airtex.
You have two pumps,I guess you know that ,if you already did one. Did you check fuel pressure at the rail and check out the electronic pressure regulator? What codes are coming up and do they stay set?
the control unit is no good.i had a similar situation with a 98 tracker,and installed a switch to give it a ground for the compressor relay that comes from the a/c computer module.
it works good and goes on and off with the a/c switch.
try restting the inertia switch in the trunk. When you turn key to "ON", the fule pump will run for only 5 seonds if the engine does not start. Listen at the fuel tank for humming of the pump.
The PCM, Powertrain Control Module controls the fuel pump relay. So, when you turn key on, the power relay powers up the PCM and the PCM powers up the fuel pump relay.
On a 4.0, 40psi is OK cranking...it should drop about 10lbs running . If not check regulator. Also do a drainback test. often the check valve in the pump fails requiring that system reach pressure before it can start. Also check fuel delivery. if filter is partially plugged pressure may be OK but fuel flow will not be.
it is not the regulator if it has 40 lbs of pressure try to pinch off the return line and if pressure drops quickly it is either the check valve in the pump or the rubber seal between the pump and the sending unit