Recently, I replaced the driver side valve cover gasket on my brothers 2001 4.0 because it was pouring out so much oil. I got the gasket and cover back on and replaced all the wiring and vacuum hoses like I took them off (i’m almost 100% sure but I am wrong often). I started the truck up and now it idles around 500 rpms and bogs down when I try to accelerate last 1100-1500 rpms. It does this during idle and while moving. I’ve cleaned the mass airflow sensor, idle air control valve, and egr valve. I’ve also sprayed carb cleaner on my vacuum lines to see if they’re leaking too. I’m at a loss and about to take it too a shop. Help me please :(

 

Is the PCV connected? On the 01 4.0SOHC, it's located at the back of the valve cover. really hard to see.

 

We actually replaced the pcv valve because it was soooo gummed up, made sure it was connected correct but i’m going back today to make sure everything is as it should be.

 

Welcome to the forum

On cold start engine should idle at 1,000rpms
This is done by the computer using the IAC Valve, 2 wire "can" on the throttle body, unplug it and make sure prongs are not bent over
As engine warms up computer start to close IAC Valve a bit to lower idle RPMs, 700rpms would be lowest but with manual transmission, automatic would be 750-800rpms

No IAC Valve control should set a code and turn on CEL(check engine light)

Engine would bog down at lower RPMs if IAC Valve is not working, computer counts on this extra air

 

I checked the IAC valve and it had no bent prongs, i cleaned it out the best i could while i had it off too. when I unplugged it, the truck wouldn’t start and the CEL came on but when i plugged it back in and started it up, the CEL went off and the truck started up and idled around 500 RPM’s again. It sounds like the engine surges a little but the rpm’s don’t move from about 500. It kinda sounds like it had an oversized cam in it.

 

It reads like the IAC Valve is opening a bit, but not enough based on engine temp, cold gasoline engines need to be Choked, they need RICH fuel mix and high idle
Computer sets Choke Mode based on coolant temp, which it gets from the ECT(engine coolant temp) sensor

I might change the ECT sensor on speculation, its on top of the thermostat housing has a grey/red stripe wire, NOT a Red/white wire(that's the dash temp gauge sender)
If ECT sensor is "telling" computer engine is already warmed up(a restart) then engine would run poorly until it did actually warm up

And you said it runs fine above 1,500rpm or so, like at highway speed, no power loss runs like "normal"