98 4.0 OHV oil loss.... cause valve seals?

[Vwmat]

So my 98 4.0 OHV with 160k on the clock it losing oil rather quickly. Seems to be 1qt every 250-300 miles. I have no leaks (quadruple checked), but do get a pretty decent puff of white smoke on start up. It has to have sat for a bit, it won't puff after being off for 5 minutes. It does not smoke while driving, at least nothing you can see. When I first got the truck, a cylinder wasn't firing, and upon replacing the spark plug, found it was filthy. So oil is getting into one cylinder.

Doing my research, it seems to be bad valve seals. Can that really let a qt in there after 300ish miles? Is there a way to visually see a bad seal with the valve cover off? I'm quite the hands on (mostly vw) mechanic, and the truck isn't my main car. Are there any good DIYs for the seal replacement? I'd gladly spend the $22 for seals and a day of my time to fix this. I'd also not pull the heads to do this.

 

[RonD]

Leaking valve stem seal is on the intake valve so in the intake side of the system, replacing all the seals would be easier than trying ID one seal or even if it is a seal that is leaking.

Intake manifold always has lower air pressure when engine is running, vacuum, so if valve stem seal(s) are leaking then oil on the upper stems(valve train) will be pulled down passed the seal onto the back of the intake valve, and then be pulled into the cylinders to be burned with gas/air mix.
Common test for leaking stem seals is to go down a hill in lower gear, using engine as the brake, this increases intake vacuum above 28" so leaking seals would suck in alot more oil.
Then accelerate, the extra oil would come out as a puff of blue smoke.
But this can also be failing piston rings as well, same vacuum can suck oil up passed rings on intake stroke.

A compression test would ID if a cylinders rings were the issue so you could do that first.
Dry test
Then add oil and do wet test to see if Rings are the issue.

You can change valve stem seals with head(s) on
You need a smaller valve spring compressor to get into tight spaces.
An air compressor is optional but many prefer that method.

You rotate engine manually to put cylinder at TDC, both valves closed
If manual, put trans in gear to hold crank in position
If auto strap crank pulley or put wrench on crank bolt to prevent turning
Add compressed air to cylinder via spark plug hole
Remove valve spring, air pressure holds valve closed
replace stem seal
replace spring
move to next valve, you should do both intake and exhaust seals

Move to next cylinder

You can also use a smaller size rope that fits into spark plug hole
Put cylinder piston down 1/2 way in its compression stroke, feed in the rope so it coils up inside cylinder
Rotate crank to push rope up against head/valves
Secure crank so rope holds valves closed

Remove springs change seals
rotate piston down and pull out the rope

Repeat for next cylinder

In either method you need to put towels down around the working area
Valve spring "keepers" will go flying if released with pressure, towels will help you find them
Also not a bad idea to have a few spare "keepers" on hand, nothing worse that needing a critical part and having to make a trip or wait until parts store opens the next day.


Valve stems seals don't usually wear out that fast but............yours is a bit lower mile vehicle than average so that could be the reason.
Valve cover area does have fresh air circulation via vent hose and PCV hose, in normal operation an engine is used at least a few times a month if not daily.
This keeps valve train area coated in oil.
If engine sits for extended periods, especially in dry warm climates, the oil coating dries up and the valve stem seals dry out which causes them to crack and then leak.
Low mile vehicles are great but there can be downsides to it as well.