My 96 v6 auto recently left me stranded it runs and drives however once in gear even with foot on brake won’t rev above 1k and is super super slow like barely moves but will rev higher although delayed if in neutral. Manifold has less vaccum than it should at about 14 inHg max and fuel pressure at rail is 45 psi

 

How is it running when it won't rev? Smoothly? Sputtering?

I assume you'd mention an CEL, but check it for codes anyway, not all codes set the light.

 

it misfires pretty bad throws a code for almost every cylinder when cold it really won’t rev at all but once warm it revs pretty normal as long as it’s slow if you blip the throttle it will barely rev that’s all in neutral once in gear when revved it won’t climb rpms and sputters out

 

Misfire codes?

The symptoms could sound like a plugged convertor/blocked exhaust, but that usually doesn't lead to sputtering or misfiring, the motor just sort of smoothly chokes down on the exhaust blockage.

If it was a plugged fuel filter/failing pump the engine could have good pressure at idle/low power, but quickly run short of flow and have a pressure drop when the demand goes up. Did you check fuel pressure when it's struggling?

Could be a bad MAF and/or leaks in the intake system after the MAF. When trying to make more power, more air going in, but if the MAF doesn't read it, it won't inject more gas. You'd normally see some O2 sensor related codes in that situation though, lean mixture codes.

Could be a failing component in the ignition system. Or a failing sensor on the engine. Or bad plugs.

Hard to say really, it's hard to diagnose based on a few sentences without seeing how it's running.

 

If you want to check if it's a blocked exhaust you can pop out an o2 sensor. It'll throw an o2 code, but if it revs higher then you'll know.

How did it leave you stranded?

 

Multiple missfires also could indicate bad injectors, mind you the old style injectors are not as common to fail but they do. I would clear the codes, fire it up again, if misfiring that bad the CEL should flash, shut it down and see if it flagged a specific cylinder, then try swapping plugs and coils to see if the misfire moves to determine the offending component, while confirming proper wire orientation. It would be nice to log the fuel trims aswell.