1999 Cadillac EL Dorado Not starting

I doubt a regulator, fuel pressure-certainly. Try spraying some starter fluid or carb cleaner in the intake and see if it fires. This will tell you fuel is absent. Check the fuel pressure, should be around 40-45 psi. From there, if you have no fuel pressure, the next check would be power and ground to the fuel pump.

While you pull a wire, you might pull a plug as well, and check it. See if it is wet with fuel.

I have to assume you checked spark properly, from a plug wire with a short gap to a ground, or with a spark tester. First let's check for codes, and to see if the PCM is capable of displaying. Press 'off' and the passenger side to 'warmer' on the climate panel until all segments light. After that codes should start display, record any PCM codes or messages. The first step for no spark is to check power and ground to the ignition control module. At the module there is four connectors, we are interested in the three wire gray connector, it will only have two wires in it, a pink and a black. Check with a test light across these pink and black wires, with key on it should light. If it does not light, check from the pink wire to an engine ground. If it still don't light, check the DIS fuse in the underhood fuse block. If it does light, check with an ohmmeter, the black wire to battery negative(do with key off). We would be looking for a low resistance reading(less than.5 ohm). If you have continuity to ground, it may need an ignition module, but let me know the results first. While we are there, let's check the resistance of the crank sensors. There is a 6 wire gray connector with four wires, two on each end. These are your crank sensor wires, sensor A is yellow and purple, and sensor B is black/white and lt blue/white. Measure the resistance of each sensor at that connector, report your findings. I am not sure off the top of my head, what the value is, but I can find out if it don't seem right. First though, check for codes, it may lead to a more specific area to focus your diagnosis.

Are you disconnecting the lines at the filter?

That is a good indicator that you have a failing fuel pump, which is in the tank. Obviously it can't be trusted, needs replaced, I would recommend doing filter at the same time. When you said no spark, I was a bit surprised. These ignition systems are rather bullet proof, and other than some secondary misfires due to an occasional coil, you don't see many problems. I don't believe I have ever seen a no start/no spark on a Northstar, in the 12 years that I have been with Cadillac.