Jerking while Idle?
 

I've got a 1992 Ford Ranger 4.0 V6 that for some reason starts jerking after running for about 10 minutes/ driving for about 5 miles or so. It's like I can stop at a red light and it starts bucking and "scooting up" in a sense. It also struggles to initially crank up but I'm not sure if that is related to the jerking issue or not. Any ideas as to what could be causing this or how I might could fix it?
I've tried doing some research on my own and things have popped up like it being the motor mounts, mass airflow sensor, idle control valve, throttle body plate, or pcv valve but I'm honestly clueless on if any of these could be correct or where to start on figuring out what it could be.
I can take a video of it when I get the chance and post it here!

How many miles are on your 1992?
A 34 year old machine might have more than one problem.

What shape are the plugs/wires/cap and rotor in?

Have you physically inspected what you mentioned?
> Motor mounts with a helper to look as the engine torques?
> MAF or cleaned it?
> IAC valve by doing the unplug test?
> Throttle body clean?
> PCV, the valve itself and the piping or hoses which carry the fumes to the intake?

It’s currently got about 431,694 I believe 😅. Honestly I wouldn’t doubt it needing a new rebuild after all this time, I’m just hoping it doesn’t have to yet. And I’ve checked the throttle body plate but I think that’s it.

I was going to suggest checking the MAF readings but I forgot the '92 YM is pre-OBD II. I'm sure there's someone here that could tell you what the voltage readings should be. Definitely clean it if it hasn't been, 432k means that truck has been good to you! Videos galore on how to clean it.
IAC unplug test is very simple, do it while the vehicle is warm. If it engine starts chugging and feels like it'll die, the IAC is working properly. Also listen for a howl - my OHV had the howling issue a while back and a new IAC fixed it. This *could* contribute but what do I know :laughing:
Definitely check the PCV & hose - it's at the rear of the driver side valve cover. It should pop right out. Look at the hose for cracks and shake the PCV. Plenty of videos online for checking that little part. It's only a couple of bucks too, so easy fix.

So, I've attempted replacing the Idler, the PCV Valve, and have checked the IAC and the jerking is still happening. However I've been able to pinpoint the symptoms more and it seems to happen after about 10-20 minutes of driving and it only occurs when pressing on the brake. My grandpa said he's been able to "bypass" it by pressing on the gas and the brake at the same time. My grandpa believes it could be a vacuum hose but I wanted to hear more opinions on it before I toss more money into it.

Maybe the diaphragm in the booster itself?
I don't know exactly how to check a brake booster....perhaps listening?

Yea I’ve been planning to get that replaced anyways, I’m pretty sure the brake booster is the source of a “whining” sound coming from the engine bay on the passenger side. I’ll definitely be taking a closer look!