2 burned valves

What exactly do you mean by an "overhaul"? More information please. Which diesel engine is this? The 2.5 litre or the 2.6 litre? What year is your vehicle? Some engines can be different settings depending on the size it is.
I'm assuming this is for the clearance between the valve rockers and the spring end of the valves?
If you're breaking rockers your mechanical timing might not be set correctly risking bent valves or the valves are already bent. Even One tooth out on the timing belt/chain can cause problems. You might need to take the cylinder head off to check the valves.
When you set the clearances the engine needs to be Cold and the cylinder you're working on needs to be at Top Dead Centre (TDC) to make sure the valves are closed. On an inline 4 cylinder engine the cylinders work as pairs, 1 and 4, 2 and 3.
Set cylinder number 1 to TDC. Set the clearances for only cylinders 1 and 4. Using the crank pulley bolt turn cylinder 1 to Bottom Dead Centre, (180' turn clockwise). This should bring cylinders 2 and 3 to TDC so you can set those clearances.
Is the engine burning the oil? (a thicker, more blue exhaust smoke than normal). Burning oil can also be a sign of a blown head gasket or badly worn piston rings.

Based on the symptoms you have described, it is possible that you are experiencing issues with both the timing chain and the sensors. It is not uncommon for a low oil level to cause damage to the timing chain, especially in an engine with high mileage like yours. The reduced power and hard shifting can also be related to timing chain issues.
However, the oil leak and oil residue in the air intake tube could also be indicative of a valve cover gasket issue, as you suspected. This could also explain the brown liquid building up in the air intake tube.
In terms of next steps, it would be advisable to address the oil leak first and replace the valve cover gasket. This should stop the oil from dripping onto the exhaust manifold and eliminate the burning oil smell. Additionally, cleaning the air intake tube and replacing the air filter may also be necessary.
Once the oil leak and air intake issues have been addressed, it would be best to address the sensor codes next, starting with the camshaft and crankshaft sensors. If the codes persist, then it may be necessary to look into replacing the timing chain.
Given the age and mileage of your vehicle, it may be worth considering whether it is worth investing in the repairs. However, if the repairs can extend the life of the vehicle for a few more years, it may be worth it in the long run

NISSAN, maker< why skip makers name?
NO YEAR TOLD.
ok not told you where OIL is.
tip only
body top
the whole plugs soaked in oil as COP and spark wells love to do.
no engine true name told. nor country. no year.
how hard can you make this,, leaving out all those facts.
NP200 a mini pickup truck on AFRICA or what/
this is a www word wide forum, most countries have no such car
nor here USA.
the answer is simple. OIL does NOT belong there.
for sure.
only why matter not but we can not see where oil is EXACTLY at all.
2009 1.6L engine???petrol is sure.
one I see sold is spark HV wired to 4 plugs
if tips are soaked in oil that means
1: valve stem seals bad OR VALVE GUIDES BAD.
2; or bad piston rings, stuck or collapsed or broken.
3: or cracked head or piston.
we do a compression test , all; 4 same PSi (or bar) and 150PSI min.
if fails needs engine work.
4: PCV VALVE CLOGGED,AND CAUSES HUGE OIL BURN.
1 AND 4 ARE SIMPLE FIXES.

NO VIN OR ENGINE TOLD. SO ANSWER IS NOW GENERIC.
5 ENGINES THAT YEAR GEN2 BIT 1 OF 5 IS DIESEL SURE.
ONE IS LPG+GAS
TO FIX A CAR KNOW FIRST WHAT IS UNDER THE HOOD.
MISFIRES UPHILLS !! CLEAR AS DAY.
303 MEANS CYLINDER 3 HAS LOW POWER, ONLY
3 CAUSES

1. BAD COMPESSION #3
2. BAD SPARK #3
3. BAD INJECTION #3 NON, WEAK OR LEAKING INJECTOR

NOTE ORDER. CAN'T BURN CORRECTLY FUEL WITH #1 OR 2 BAD.
TO GET ALL CODES POSSIBLE TAKES SOME DRIVING
PARKED IN DRIVE WAY NO.
SWAP INJECTOR #3 WITH #2>?
CHECK COMPRESSION ON #3 YET?
3 WAYS TO FAIL AND INSIDE ENGINE MORE.
#3 VALVES BURNED OR STUCK. OR LIFTERS FAILING OR

I HOPE YOU CHANGED COIL PACK HV WIRES 1 AT A TIME.
DO NOT MIX THEM UP, EVER.

Hi..That is a lot of oil you are losing, The engine is definitely toast.
Blue/Gray Smoke: Blue/gray exhaust smoke is an indication of oil burning in the combustion chamber.

Valve Guides: Excessive clearance between the valve stem and the valve guide allows oil to leak past the gap into the cylinder.
Piston Rings: Worn or damaged piston rings will cause blow-by resulting in blue/gray smoke.
Worn Cylinder Walls: Worn cylinder walls cause blow-by resulting in blue/gray smoke.
PCV System: A stuck closed PCV valve will cause excessive crankcase pressure resulting in blue/gray smoke.
Is it worth fixing? Depends on how much this car means to you, Mileage of vehicle, and just how much money you want to throw into a 20 year old car..The engine will need to be completely overhauled or change out with a used engine probably close to 800- $1000 either way..Probably have a lot more into this than you can live with I'm guessing
Is it worth fixing?

Blue smoke (how many miles on engine, 50k or 500K???????
to some this means, burning oil, others steam
it that blue smoke lingers a long time that is oil burning
and smells just like oil burning.
and means 2 things,
1: engine burns oil.
2: if car had autotmatic trans the vacuum modulator love fail
and suck ATF fluid to the intake plenum and burn it, and bingo burning oil like mad.
3: PCV valve packed solid,and forces engine to leak oil or burn oil. or both. this is a 50k miles service point, skipping this?
4: gas in the oil making it thin and not sneaks past valve stems.
does oil change cure this.>?

now engines.
worn out engines smoke. (burn oil)
as do bad valve guides and seals.
and bad piston rings.
100 years same story, and miles matter. History on engine too.
matters.

Until the engine is sealed you will not know how much oil is burned. Price the parts. Difference too in whether its a 4x4 or easy access 2 wheel drive.

Check rear seal type to see if it can be changed by pulling trans and flywheel. If stick shift, probably good idea to replace clutch disc to verify not oil soaked. If automatic, oil is no consequence.

Kind of matters how much is wrong. With trans out, some rear engine seals can be replaced without removing oil pan. But if bearings are bad, on 2 wheel drive you may be able to squeeze off oil pan and check bottom of bearings.

You can do poor man's bearing job with plastic-gage. Rod bearings can probable be switched 100%. Main bearings lower cap can be serviced with upper 1/2 left in place. Not ideal, but can save from pulling motor.

Value depends on whether your Labor is free. You have a 14 Year ol truck. Depends on what you already replaced to maintain it or what else it will need. You could need rings after working on the bottom end even with a poor man's bearing job. Plastic Gage can tell you the size you need for each bearing. Then keep track and only buy individual sizes.

If you could find a wrecked truck with a good engine, places like carparts.com have a good multilist. Rule of thumb is different for each person. I would look hard at spending 1/2 of what truck is worth to fix it, knowing other things are wrong.

Given the 22 year vehicle age I would let it go to
truck heaven

Motor is worn out as you know

Rings,valve guides,valve seals
not worth fixing

Sounds like the Valve Guides are **** out oil is seeping down valve stems and burning in the cylinders. Some times STP oil treatment can slow the oil down because of its thickness. I have put about a cup of dexron2 transmission fluid in my oil to swell up the O-ring seal on the valve stem to stop this problem and a cup will not hurt anything. Like i have put STP in Automatic Transmissions to correct leaky valve body problem work like a charm.Can give you 3 to 5 more years of life to a transmission.

It sounds like oil is seeping into the cylinders while the car is stationary and then being burnt off in one big puff of smoke when you start the engine. You're probably also burning oil when the car is running. You may notice that you have to constantly add oil to keep the level up. It's not uncommon for old engines to burn oil. The proper fix would involve either a ring job or a valve job both of which involve quite a bit of labor. If you had to pay a mechanic to fix it, it may cost more than the car is worth. It not a great solution from the air pollution standpoint but you could probably continue driving the car for years by just adding oil as needed. You may have to replace the spark plugs more often because the burning oil will tend to foul them.