i just joined the other day haven't got any replies so ill' try to explain again my 2009 xlt has 203000 miles, i replaced the engine at 183000, had to use my sensors and it was just a long block. it is not showing any codes but will pulse the idle and stall at stops alot it just started bucking at differnt throttle positions and stalling under loads.I have done the obvious things like cleaning MAF replaced IAC CKS TPS. replaced fuel pump about 2 years ago,replaced fuel filter twice,cleaned throttle plate. It seams electrical to me with the not all the time/on and off. the truck has a dealer installed remote start(in Hawaii? I know ) the ignition does not chime when I leave the key in and open door and the remote will not go through the entire sequence and start truck just flash lights honk horn and thats it no start. it sometimes fires up to 2000 rpms and back down to 0 maybe 5 times before it catches and runs. HELP stuck in Hawaii BTW used my old knock sensor, manuel says must replace didnt see that as i was working on the swap????

 

First thing, never replace sensors or controls
A little fact
90% of sensors or controls replaced by DIYers were not bad, 90%
So don't replace, test if possible, unplug and plug back in for sure, most problems are just dirty connectors

But don't feel too bad, 50% of sensors replaced by Pro Mechanics were working fine, may be a little Bill padding there, but I think most people, Amateur or Pro, just find it easier to replace things.
If you take that to the extreme, then you should just buy a new car when the ashtray is dirty, sounds silly and it is, but it is the exact same thing just carried to the absurd, lol.

Only sensor that ever needs to be replaced are the O2 sensors, they use a chemical that wears out, change at about 150,000 miles, but only the two front O2s, the rear lasts much longer.
I have a 1994 4.0l OHV engine, all original sensors, except O2s, on second set, 350k miles


Fuel injected engine are computer control, yes of course

And so, the idle is also computer controlled
Computer use the IAC(idle air control) Valve to allow more or less air into the engine, to set stable idle RPM, so basically a controlled vacuum leak.

When you first turn on the key the computer will open the IAC Valve all the way, for starting.
After engine starts RPMs should go above 1,500, IAC valve is open all the way
Computer will then start to close the IAC Valve so RPMs will drop, until computer gets to it's "target" RPMs
1,100 RPMs for cold engine
650 warm for manual trans, 750 for automatic

IAC valve is controlled by the computer Pulsing the Ground on the IAC valve motor.
IAC Valve gets 12volts with key on, on 1 wire
Computer grounds it's other wire so it opens all the way.
After start up computer pulses the ground so IAC valves spring pushes it closed and the pulsing holds it partially open.
You can uses some sewing pins to pierce the IAC Valve wires, and a volt meter set to DC Volts, turn on the key
You should see 12volts(battery voltage, 12.3 to 12.8volts)
Start engine
Voltage should now be under 12volts, the pulsing is very fast so you won't see it, you will just see a lower voltage, but it should be a steady voltage.

I think you have a vacuum leak
After engine is warmed up it should be idling at around 700
Unplug the IAC Valve wires
IAC Valve will then close and idle should drop to 500, or engine may even stall
If idle stays above 650 then you do have a leak
4.0l SOHC are notorious for PCV valve hose leaks, at the elbow on the under side.

Reason for the up and down RPMs is because the computer is trying to set a stable RPM
If there is a variable vacuum leak computer close IAC to lower RPM and then vacuum leak might get less and RPMs drop too far and computer has to open IAC valve more

I would spend $30 on a vacuum gauge, it works on any gas engine.

Good read here on how to use it: Technical Articles: Engine testing with a Vacuum Gauge - at Greg's Engine & Machine

If idle is swinging up and down then hold throttle open a bit while testing, also when testing IAC Valve, if RPMs still go up and down with throttle held open a bit then you do either have a vacuum leak or a fuel leak.

Whats the MPG like?

 

Thanks for the reply the MPG is not that good, have not done a real check in a while normal is 19.5,think its lower because of the half power while running and the stalling and restarts. It is not some thing that happens all the time it is random and i can not duplicate. I had a lean code right after engine swap, finnaly a P202 # 2 injector was failing when i removed it to replace with a new one found a crack in the plastic cup injector sits in! If not for the cup coming out with injector I would not have figured it out all codes cleared what do you thunk about that knock sensor?how about that remote starter ? it has a feature to kill engine remotely also

 

Remote start is off-line when key is on, so couldn't effect running, the fact it isn't working would also mean it wouldn't effect operation when running.

Knock sensor is there to modify spark timing a little when running regular gas(87 octane)
4.0l SOHC has 9.7:1 compression so will pink/knock on 87 octane gas

Advancing the spark to prevent pinging does lower the power for sure but couldn't cause a stall.

I would start with vacuum gauge tests and go from there

 

Hi RonD
If you haven't gotten the idle issue squared away yet take a look at the throttle body. Mine was doing the same thing, couldn't find any vacuum leaks anywhere. Finally a gentleman told me to remove the throttle body, and look to see if there is space around the butterfly plate inside when throttle body is closed. If so replace the throttle body. The plate is designed with a hole in the plate for proper idle, anything else causes problems