Engine is spitting and sputtering?

Hello, it sounds like you may have a wiring issue that went from bad to worse. What happens to the fuel pressure when cranking the engine over? Does it drop off when cranking?
40psi is a bit on the low side, should be 48psi key on engine off.
You may want to pull at least the front spark plugs out and see if they are soaked in gas or what their condition is. If you pull the spark plugs out and they are all soaked in gas and fouled out,
Pull some of the relays out of the power distribution center and check the pins for corrosion on the relays and inside the pins where the relays sit. Sometimes these fuse panels in the engine compartment get corroded from underneath and cause low voltage/high resistance issues. Keep an eye out for any green crusty corrosion in any connectors or on any Ground locations going to the engine block or body.

Instead of pulling timing covers right now, you could do a compression test on the cylinders you can reach, I know on some of these the back bank isn't always easy to get to. But a compression test would give you further reason to check timing by the marks, are you hearing the engine crank over faster than usual?
Are you seeing a check engine light on with just the key On? I assume since you were able to check it for codes you had communication with the PCM, this should be an OBD1 system,
Can you get any type of live engine data with the current scan tool? Checking for a crank and cam signals.

I would also check for fuel injector control, you can do that with a basic incandescent test light, each injector should have one wire that lights the test light brightly (12v), then hook the test light to battery positive and probe the other injector wire, while cranking it should have a dim pulse to it meaning the PCM is grounding the injector. Here's a guide on checking the fuel injector,

I'll post the wiring diagrams for you, they're not great, but will help, what are you using to check for spark? The adjustable spark testers are best because they will stress the coil somewhat. The "light up" ignition testers don't really give you a good idea of how strong the spark is. I'll get these wiring diagrams together for you, that way you can verify a full 12volts on critical components.

https://www.2carpros.com/articles/how-to-test-a-fuel-injector

https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester

https://www.2carpros.com/articles/how-to-test-engine-compression

Okay, if your fuel pressure is below 43 PSI. They have this as part of the Crank No Start flow chart of theirs, they want you to check fuel pressure at the fuel filter. If you can't blow air through the fuel filter it's no good, and most of the time a clogged filter will stress a fuel pump and overwork it causing it to eventually fail, with low volume/pressure output. So, I would start here.

Here's your firing order and cylinder location, along with fuel pressure spec, ignition coil connector and the PCM connector if needed.

The fuel pressure spec is from All Data, and using a test light on the ground side of the fuel injector is to test if the PCM is actually controlling them, if it's not pulsing the ground side of the injectors that tells you the PCM has stopped controlling them for some reason, such a bad cam or crank position sensor. Testing the resistance of the injectors is fine, but the test light is to check for PCM control. On some systems, especially these old ones, if there is a fuel injector shorting out when hot, for example, the PCM might shut them all down due to excessive current flow, on newer system, cylinders are controlled independent of each other. You still have 40psi and a no start condition, if you have good spark and you're sure of that, then you are most likely losing injector control.
Go ahead and do a resistance check of the injectors, they should be 12-14 ohms each, but if you have a test light hooked to B+ and there is no pulse happening on the ground side of the injector connector, there's no fuel injector control happening.
An old school trick is to put a long screwdriver up to the injector and listen for it clicking away while someone cranks the engine for you, just using the screwdriver as a stethoscope.

Not necessarily on a vehicle this old, this is an OBD 1 system, all systems 1996 and after are OBD2 and much more advanced, The PCM in this vehicle might not be picking up the fault, there's something wrong if it won't start but you're still not getting any trouble codes, so in this case you need to go back to the basics, if you have fuel pressure that's good, and 40psi is a bit low, but if it's not dropping off when cranking, we will assume, (and that's a terrible thing to do with automotive diagnostics,) that fuel pressure is not your issue, so you need a good ignition spark, compression, fuel injector pulse, etc., for this thing to run, using a test light is a super basic test we would use all the time now to check for injector control from the PCM, if that is missing, then we have an area to focus on,

Here is a 38sec video, watch this real quick so you understand what I'm saying, if you are missing an injector control pulse from the PCM, then you could be dealing with a bad crank or cam sensor, and yes it would be great if a code set every time there is a fault, but that's just not how it always works, a regular old basic test light draws very little current flow, so it's a good test to quickly tell if that's the direction we need to go in.

https://www.youtube.com/watch?v=jJ7wtkuJxEk

The injectors will all have a common wire the same color, which is the power feed, the other wire is the control wire, you need to back probe the injector connector with a back probing pin or a T pin, back probe example in diagram 1, back probe pins diagram 2. A cam or crank sensor can fail when hot, when cold, or just all together, it's always one way or another, since this issue has progressively gotten worse, I would say you had a sensor on its way out, a wiring issue, or a failing PCM. Yes, it should be battery power at the coil connector (12v).

This is the power feed to the fuel injectors and ignition coil. It starts at Fuse #3 at the Power Dist Center fuse box in the engine compartment out to the ASD Relay, so if you take out the ASD relay you should have power on 2 of the 4 pins of the relay socket with the key On and relay out, the PCM grounds the ASD relay and Fuel Pump relay at the same time if these diagrams are correct, so if you have fuel pressure this may just be a bad ASD Relay, but following the below diagrams power goes out of the ASD relay to Splice A142 and that goes to this Lite Grey Connector which in turn sends power out on the 1st diagram to each injector and the coil pack.

Also, in the first diagram marked in blue, is the ASD Relay goes to the PCM at pin 51 and is supposed to be a "Sense" but also looks like the Relay Control wire as well, the diagrams are incorrect in some places. The PCM should be setting a code if power is not sensed on this circuit, and since you're not getting any codes there might be an open circuit further down from the relay. You'll need to trace this Dark Green wire with an Orange Stripe.

I will try to find to find this Lite Grey connector in diagram 2 below, which looks to be 10 pins. You may be able to find it in the engine compartment and check for power there on the same color wire, this dark green/orange wire seems to feed power to all the injectors and coil pack so it should be easy to identify.
At least now you know what the issue is, make sure you don't have any blown fuses as well
Diagrams 4, 5 are the ASD Relay operations and testing.

Okay. But these wiring diagrams are completely horrible, thats because theyre older, and I can see quite a few issues with them,
Dont bother with resistance/continuity testing now, right now you are looking for voltage, if it lights the test light up bright, you can verify its a full 12volts with a meter,

using a test light is faster and it puts a small load on the circuit,
But with the ASD relay out, you should have 12v on two of the pins where the relay sits with the key On, If this is correct, the control side of the relay should be pins 85 and 86, one of those pins should have 12v with the key On, Pins 30 and 87 should be the Load side of the relay, so one of those pins should have 12v.

(And pin 87A is a normally closed contacts with pin 30 until the relay is activated. For now, just verify the 2 hot pins, the PCM should set a code if it doesnt sense power.)

on the Red wire, it should be Red/White. These diagrams have the relay contacts listed backwards in some cases and upside down in other,
So we will just go with basic ASD relay operations.

The Red/White wire going to pin 30 is the constant power feed with the key On, , thats coming from the Fuse #3. But if you have 12v on pin 30 and it lights the test light bright, it should be ok for now.
Pin 87 is where the power is going to exit the relay and head out to the injectors/coil etc. So those pins are the Load side. That should be the Green/Orange wire, So key On the relay should click and there should be 12v on the Green/orange wire (pin 87).

In the wiring diagrams they have these relays pins marked 29, 30, 31, 32 so thats useless here.
Right now all we know is that youre missing power to the coil and injectors so we need to solve that first.

There should also be a Blue/Yellow wire and a Blue/White, those should be the other 2 wires to the relay, One will have power, and the other goes to the PCM.

*(note here, the PCM is using the Green/Orange wire for its "Sense" going to pin 57 of the PCM, thats telling the PCM that power came through the relay. So it might be ok,
I think youre going to find a broken or very corroded connector somewhere. And there's voltage being lost there.

You have green/orange wires at each of your injectors, correct?

Thats where it goes, it passes through a few connectors but shows the same color coming out of them all. This is common with older designs, especially when that one wire powers a section of components such as coils, injectors, etc.

Here you could check continuity from pin 87 with the relay out to one of the injectors or the coil plug on the same green/orange wire. See if theres a high resistance reading or an open circuit, let me know what the ohms are, and check a couple of the front injectors. There's a few splices that could be corroded.

So, you have a green/orange wire going to each fuel injector, correct? As well as one going to the coil plug, correct? I just want to be clear on this.

If you have 60-80 ohms of resistance on that power wire from the ASD relay location to the fuel injectors that's way too much, you should have around 5 ohms. Depending on how many connectors are between the ASD and injectors, it sounds like you have a corroded connector somewhere.