Hey guys. 04 4.0 165k
Im starting to uses some oil. I put 30k on the truck pretty much problem free in the past year with no issues at all.

Then I noticed oil pushing out the air filter and smoke at start up.
I changed the PCV and put in new air filter. No more pushing oil.

But I every so often I had smoke at start up.
More recent oil change I went with synthetic oil.
On a 4 hour trip I had engine light come on but then went out. Checked oil at fuel station and it was 1.5 quarts low. Topped it off
Then a couple week later it was low a quart. Topped it off again.

2 weeks ago it threw a code for #6cyl miss. I cleared it

This past week when changing oil I went back to conventional.


here is my thinking...

PCV was a fluke thing

when i went synthetic, oil was bypassing valve guide seals and that was my smoke at startup and consumption.
Think maybe I fouled a plug?

OR is a bigger issue than am willing to let myself think?
Bad valve guides, bad rings?

 

And under side of truck shows no signs of oil leaks?

Couple of things may be going on
PCV Valve wasn't a fluke, should be changed every 3 oil changes in the North or 4 oil changes in the south, its has to do with temp and oil thickness when its cold out

Yes, cylinders 3 and 6(on V6) are the ones that are first effected by valve guide seals leaking oil, the engine sits at a angle and oil from the valve train pools at the rear of the heads, so............

When that starts to happen you get a double whammy, obviously it burns a bit of oil, but that burnt oil starts to coat upstream O2 sensors, causing them to read a bit Lean, so computer adds extra fuel which along with the oil causes misfire and plug fouling.
Upstream O2 sensors should be changed every 100k miles, if you are not sure on yours then change them, downstream O2 is your choice, they tend to last twice as long as upstream since they see cleaner air being after Cat Converters

Spark plug tips should "tell the tale" on whether or not your valve guide seals are leaking

You can do a Leak down test or DRY and then WET compression test, that will tell you if Rings are an issue, but I doubt it

I don't think you can change the valve guide seals in the 4.0l SOHC with heads on, like most OHV engines
There are oil additives that will swell the rubber guides helping them to seal better, I think "high mile" oils have that in there already but not sure

 

Good to know about the PCV, my last 04 got 1 in the 260k I drove it

 

Yes, PCV(positive crankcase ventilation) is there to suck in Blow-by.
When a cylinder fires some of that Super HOT exhaust gas is "blown-by" the piston rings, hence the name "blow-by"
These HOT gases vaporize some of the oil that's coating the cylinder walls and pistons as it blows-by
This is where ALL the oil vapor inside the crankcase and valve covers comes from
Valve train and bearings don't run anywhere near hot enough to vaporize oil, they spray oil which causes no vapor at all

In the old days there was a vent tube on upper oil pan or lower block and blow-by exhaust gases and oil vapor just exited the engine, dripping oil all the way, lol
PCV system was added to prevent that emission
So engine vacuum sucks in the blow-by and oil vapor to be burned in the engine

AS the PCV Valve gets dirty it sucks in less blow-by but the engine area is sealed so blow-by has to go somewhere
There is a Vent hose, breather, on opposite valve cover on a V engine, this is normally used to pull IN filtered fresh air because PCV Valve is usually set to pull out more blow-by than generated
But when PCV Valve is dirty then Blow-by gases and oil vapor go OUT the Vent hose, this causes an oil coating in the air tube, throttle body and upper intake, and also on under side of air filter in some cases, after engine is shut down the air in the intake system warms up pretty fast so has to exit and it flows OUT the air filter, carrying the blow-by and oil vapor with it

OOPS, I see your a Diesel guy, so already know about blow-by, diesel engines are the worst for that, dirties up the oil pretty fast, because of the high compression used

 

Well, looks like I am going the reman engine route