Fuel relay problems - had fuel relay switch replaced last week. This week - the car wouldn't crank and had to go back to the shop where they unplugged it and reconnected it - the car now cranks. It happened again today - same senario. Unplugged the relay and reconnected it - now it works!!! What is the problem!?

Anti-theft system has nothing to do with it . Battery cables , maybe , Park neutral safety switch possibly ! With anti-theft system problem you wouldn't a click or anything , that is if the anti-theft disables fuel or the starter . Plus the SECURITY light would be on if it were a anti-theft system problem . Your best bet , take it to a qualified repair shop that knows how to do starter circuit testing >
Starter Voltage Drop
How to voltage drop test starter motor circuit

And what's this identity anti theft system ? No such thing ! The vehicle has a content theft an vehicle theft deterrent system !
This system disables fuel not cranking of the engine. The VCM/PCM will not disable the fuel due to any BCM message for the remainder of the ignition cycle.
If the VCM/PCM does not receive the same password from the BCM as the last learned one, the vehicle will start and then stalls due to the Fuel Lockout.

You basically had a intermittent problem,If anything it probably was not the fuel pump, In order for the fuel pump to turn on the ECM, needs to see a crank value, of 250 RPM or the fuel pump will not turn on, what gives this it is the crank sensor,The crank sensor does age and when it goes out you can be driving down the freeway and all of a sudden the car stalls,and it doesnt start up,

Did it jump time. Have u checked fuel pressure with a gage, With a fuel pressure tester u can see if the pressure bleeds off right away. Should have fuel pressure around 45 to 50 psi. Also a bad fuel pressure regulator can cause that also, What about the mass air flow sensor.

Have you ever change the fuel filter
Even the air filter.

crank position sensor

probably the crank sensor, above the starter on the firewall side-no crank signal, no fuel pump activation.

Hello, Check your Interlock switches under the dash. At the present time, you say the engine does not crank. The Interlock should be on the clutch pedal for a stick and the brake pedal for the Automatic. The clutch Interlock would kill the starter current.

The switches often get kicked by shoes when you enter or exit the vehicle. But, since everytime you go to start the car, unless it is in Neutral, you operate the clutch or brake pedal to start your car. If the Interlock switches are broken or loose, the movement can jiggle the switches so you get different results at random.

The other things I would look at are the battery level. Ford products are very sensitive to low Voltage and it activates the Antitheft, reducing juice to the starter. And then put a pressure meter on the test tap for the fuel pressure. Your Pressure Regulator could be making the thumping and you have changed components which can need adjustment.

It seems like you are floating around several problems; no crank and then cranking and no spark.
You may have a bad Ignition Relay too. The Ignition switch sends power to both the PCM and fuel pump Relays, but you should have an Ignition Relay too. If you look at the terminals under the Relays, the Relays have about 4 terminals each. The Relays are dependent on each other. 2 terminals of the PCM can connect to the Fuel pump or Ignition Relay and so on. So if one Relay fails to trigger another, the car won't start.

You may need to replace only 1 more Relay to be good or the Interlock switches. I would like to know what the final cause was found to be. I hope my suggestions are very helpful to you.

try checking the fuel injection module out, ( "spider injection module"). My 2000 blazer did the same thing, fouled out the plugs and all, i hooked the fuel pressure gauge up and had good fuel pressure, just no start. I took it to the shop and they found the injection module to be the culprit. you'll need to replace the plugs if it was the problem since you cranked and cranked it over.

Check the crank position sensor. I had the same problem in a 1989 cherokee 4.0, and the solution was simply to acomodate the sensor wiring. No replace was necesary

Not likely a fuse those you can check
If a fuse blows you don't replace it only
to blow again,you find out why

Relays should last for 25 years & you can
switch them with other to check that

The fuel pump siezes mechanically or electrically
& that can go on for 6 months so check fuel pressure
at the motor with a gauge

Also there are at least 3 other tests that have to be done at
a repair shop with the correct tools