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Have you checked the fuel pressure and especially how fast it bleeds off when when the pump shuts off?
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So it cranks slowly you mean? I did not get that from your first description. In that case it sounds like it might be bad battery cables, use this procedure to check them.
Have someone sit in the vehicle to be ready to crank the engine. Set the multimeter to measure voltage.
Test the positive battery cable by connecting the red lead from the multimeter to the positive battery terminal. Connect the black lead of the multimeter to the engine starter's positive terminal. Have your helper crank the engine and note the reading on the meter before the engine starts. The meter should read close to zero, and any reading of more than 0.3 volts is reason to replace the cable.
Test the negative battery cable by using the same procedure as above for the positive cable, but this time you will have the red lead of the multimeter touching the metal housing of the starter motor, and the black lead of the multimeter connected to the negative battery terminal. Again you should see a reading of 0.3 volts or less to be confident that your cable is good.
Avoid using a multimeter or ohmmeter to directly measure the resistance in the battery cable. Automotive quality ohmmeters do not have the required sensitivity to directly measure the very small resistance differences in battery cables that make a very big difference in performance.