What makes you think that?

Adding oil to a cylinder for a compression test is about meaningless. When rings wear, excess oil is naturally left on the cylinder walls so adding more isn't going to help the rings seal better. If enough oil is added, you have now artificially increased the compression ratio and compression pressure will rise.

You didn't get any / minimal leakage when you did a cylinder leak down but have 150 PSI during a compression test of the same cylinder? Pick one, you can't have both.

( In theory valve guide clearance could be tight / carboned up and cause the valve to hang open a bit when running then close when the engine is static but, you don't report a valve tap when running. You could have a broken valve spring but should also have a valve tap. )


So the car is in good shape and used at a rate 2x of the typical yearly average but you don't want to fix it? In that case run to fail then scrap the car.

Because I've rebuilt multiple om603 heads that have been run on straight diesel and WVO. WVO exhaust valves are badly pitted and won't hold pressure. Sometimes I can lap them back to life, sometimes I can't. The seat in the head usually comes out unscathed but the exhaust valves are always ****ed. Most of the heads I've found in the JY are WVO victims.

Then there's all the built up crap in the bottom of the prechambers and blocked holes.

One's experience is not conclusive. Definitely not my experience. Cars running on wvo are few and far between, may be one in 10000 or more. Most JY cars ran on diesel. I will open up my car one day and let you know.

Sounds like a valve isn't seating. Most likely a guide. The compressed air method goes something like this.
1) remove valve cover
2) crank engine over so both valves are closed on cylinder #3.
3) remove injector and install compression tester fitting with an air hose coupler. Attach compressor air hose.
4) listen at exhaust pipe for a hissing noise. Then move to the intake and listen for a hissing noise.

This test will identify the source of the compression loss.

When you added oil in cylinder 3 and the compression didn't go up indicates the problem isn't the rings. Which points to a valve/guide problem for cylinder 3.
I'd buy a guide (maybe 2 dependent on compressed air test results) and valve stem seals. Focus on cylinder 3 and fix the valve seal problem(s). Replace all the valve stem seals and put it back together.
When my 85 300sd had a bad guide for cylinder 1 exhaust valve it had a miss and gray smoke. Once the head was repaired the gray smoke and miss went away.
Once you've completed the compressed air test post the results.
Roddy

Sent from my SM-G900I using Tapatalk

This is not so...

If adding a splash of oil changes the readings at all, the rings are suspect.

If not, then either there is a hole in the piston, or the valves are the culprit.

A hole in a piston or valves not sealing would leak down rather quickly. I'm thinking something is suspect in regards to the information being put forward.
If the rings and valves are good then a bent rod seems likely.
Really need compressed air results.

Sent from my SM-G900I using Tapatalk

If it was the Pistons and or Rings and Cylinders issue adding oil would have brought the pressure up. You results are an indication of a valve or head gasket leak.

"Introducing oil into a cylinder with worn piston rings increases the cylinder's compression. The oil fills the gap between the worn rings and cylinder wall. If there's no increase in compression after running the test, an intake or exhaust valve is faulty."

https://www.freeasestudyguides.com/compression-test-injecting-oil.html

Also your gauge not holding pressure is an issue with what ever valves are in side of your gauge if you are using a real compression tester.

No one should expect the actual Cylinders/combustion area to hold pressure because the Piston Ring Gaps provide a path way for air to bleed off.

Another issue not mentioned is that Mercedes diesels with aluminum heads have incidents of pre-chambers leaking due to warpage around their sealing area and occasionally cracking in that area.

Well, I opened up the engine by taking the head and the sump off. I pushed the piston of cyl 3 out and it is stuck rings. Two of the rings are stuck to the grooves, jammed tight.

I also did 2 tests.

1) I did an ambient cold compression test, car has not been driven for almost 2 months. I was surprised that the reading is exactly the same as the hot compression test I did. It leads me to believe hot and cold test makes NO or very little difference.

Cyl 3 = 150 psi.
The others cyl range between 340 to 360psi. Almost identical to hot engine.

2) I poured SAME amount of oil into cyl 3 and 4, on top of the piston. I found cyl 3 drained into the sump much faster than 4. It is a very crude test but it gave me a clue that the rings may be bad before I removed the sump and pushed the piston out.

Also I remembered I changed out the injector on cyl 3 a while ago, may be a year. It was over dispensing. I think it was the reason that the ring was baked into the grooves. So much labor to fix a stuck ring. So test your injectors every 50K, no more than 100K miles. It will give you grieves if it over dispenses.

Found The Problem,
 

So what is the prognosis ? .