Missfire I have A 97 tacoma with all 4 cylinders miss firing .I have replaced the o2 sensor ,pressure regulator ,fule filter,pvc valve, plugs and wires

Does it have good spark at plugs, good fuel pressure to injectors and injector pulsing, good cylinder compression ?

The internet is filled with all sorts of OBD1 Code 44 info to read

Bad Egr Valve --letting unmetered air in
Vacuum Leaks in the Exhaust Before the O2 Sensor
Bad Oxygen Sensor or corroded wiring
Vacuum leaks anywhere
Fuel Pressure too low
& more

You'd have to tee into the fuel filter to use a fuel pressure gauge,
if you got no shrader valve on the motor

You changed a fuel pressure regulator-- in the throttle body?

You won't see much with a scope on that system, if you scope
the O2 Sensor,their sort of slow compared to OBD2

That's Random or multiple missfire code. Have you checked for proper fuel pressure? Improper fuel pressure will trigger this code if it's too low, thus causing a lean mixture in multiple cylinders.

have you checked the injectors

your injector spider it look like a spider under the planium its the injector pipe that are broken if you check everything else that it you need to remove the planium you will see that injector module be carefull loosy plastic clip holding those injector in they break easy let me no and thanks for using fixya

Check the fuel pressure regulator. Pull the vacuum hose to see if it has fuel in it. If it does that is your problem.

Pull codes from OBD computer go from there. Could be other problems, check fuel pressure, All electrical connectors- loose, water, dirty, ground connections, etc. Could be a failing sensor. How do you know its missfiring on 3-4? one or two cylinders would be a really bad shake. Creek is to deep stay out of creek. Good luck!

Fuel injector cleaner commonly has a low success rate, and will not help if it's more than one cylinder that is missfiring. The fact that you have a "rattle" noise, leads me to beleive the engine is not receiving fuel on one or more cylidners. If it is all the cylinders, than the fuel pump may be the culprit, or the fuel pump circuit. But if only one cylidner is missfiring than it is a possibly an injector. A code P0300 refers to all cylinder missfires, and any code between P0301-P0308 distinugiushes a particular cylinder.

Misfiring
1- a general electrical tune-up (spark plugs, wire set, cap & rotor) is a basic start to cure this issue. Wiggle the dist shaft while you are in there, if you have one.
2- Then a good carbon cleaning / throttle-body cleaning.
3- Vacuum leaks at any of the hoses that connect to the Intake manifold; especially the MAP sensor connection.
4- If that does not help then the O2 sensor could be bad or drifting.
5- Funky coil/coil-packs
6- Theoretically a microsecond glitch in either the cam or crank shaft sensors would cause this, but I am not aware of this as a typical cause.
7- Lastly a funky PCM but again it is not common for this to cause miss-firing.

Note for the V8 then also:
a) bad erg valve (this dilutes the combustion gases and hence raises the combustion temp
b) a leaky plenum/Intake manifold gasket. Leaking Intake gasket symptoms

Also can check the TSB 18-48-98 Ignition System Cross Fire/Secondary Ignition Wire Induction (Complete Version )

Hope help with this (remember rated this). Good luck and have a nice day.

check intake for vaccum leaks,the gasket also and egr valve, thats alot of parts to have replaced without diagnosing first, if there are no fuel leaks and you said all injectors were replaced then you have a defective fuel pump, also these engines had misfire problem because of dirty injector but you replaced them so count that out. good luck.

I doubt if it is the pistons or PVC valve. It is either a fuel or a spark problem. First check the spark and make sure it is good and strong, if you have a strong spark the problem has to be fuel flow related, maybe the fuel pump or the injectors.