Engine replacement on 2000 Nissan Xterra

AutoZone has a lot of info on torque specs for various vehicles.
The Xterra uses torque to yield bolts which must be replaced every time the bolt is removed.
Here's the site -
https://www.autozone.com/repairguides/Nissan-Frontier-Xterra-1998-05/Engine-Mechanical-Components/Cylinder-Head/_/P-0996b43f8037f9d4

Sounds more like the EGR valve is faulty. Change this Exhaust Recirculation Valve (EGR) before you do anything else. See that your power comes back and you can speed and climb like you used to.

get a scan tool or go to auto zone or advance auto and get the code read see what the first code is and work from there

The only cure to fix squeaking belts, is to replace with the new one. When the belts start squeaking noise, then it is the time to check the status of the timing belt or chain, depending on the size of the engine.
The 2000-2004 Nissan Xterra was offered with the choice of 2.4L 4-cylinder or 3.3L V6 engine.
The 2.4-liter 4-cylinder DOHC engine was a pretty good reliable engine, but for this truck, it was underpowered. This engine has a long-lasting timing chain - there is no timing belt to replace.
The 3.3-liter V6 SOHC was also a bit slow on pick-up, but for daily use, it was pretty good, durable, simple engine. This engine has a timing belt that needs to be replaced at recommended intervals. The SE model offered a supercharged version of the same engine with more power.
To get more information, please read my tip :
When should I change the timing belt on my car?
http://www.fixya.com/cars/r6481199-when_change_timing_belt_car

possibly the O2 sensor is worn out,and how long has it been since last tuneup?

i assume you are saying need to turn light out/ service engine soon light if so there are two ways you can hook to a scanner that has the ability to turn off light or you can unhook both battery cables from battery touch the two cables together for about 10 secs. reconnect to battery if the problem that caused the light to come on has been corrected the light will go out upon completion of drive cycle by touching the cables together you have drained the kam keep alive memory and light should go out

Instrument.

9 times out of 10 it is nothing. My engine light stays on. If it really bothers you then you can take it to any oil change place and there is a sequence that they do.`

I had a problem on my Nissan Xterra 2000. Symptons sound similar. In my case, it ran fine for a while and then suddenly the engine starts to stumble and lose output. At one point it did not even have enough HP to move. Irregular idling too. Thought it was water in my fuel. Bought water remover from the store and added to gas tank. Problem did not go away. Instead, "Service Engine Soon" light came on. Had to have it towed to the dealer as there was no way the engine could keep up with traffic. Dealer replaced the "Crank Position Sensor" today. Parts $33.00, Labor: $300 (about 3 hrs worth of work). Seems to work fine now but I will wait and see. Dealer also stated that Distributor bearing appears to be noisy and will need to be replaced sooner or later. I will wait for it to go before I fix that.

The knock sensor is not a huge deal to replace and will not turn on the light. It should be dont though.

The crankshaft sensor is located directly between the engine and transmission. It is dead center at the bottom.