Negative engine blow-by ??
 

I was trying to do a tea-pot test by removing the oil filling cap. I loosen the cap and I did not find any pressure or vapor coming out. Instead I felt the cap was being sucked slightly into the opening. It did not dance around or move. Engine has 250K miles.

Good, bad or indifference?

Good, sounds like you have minimal blow-by. When you change your air filter every 30K miles there should be absolutely minimal amounts of oil on it.

Dkr.

Is your air filter plugged up?

Which engine (in your extensive list)?

(Disconnect and plug all by-pass crankcase gubbins and try again!)

It is a 87 300D OM603. Car runs fine. I have not taken it to the open road yet as it is registered as no-op. What can cause negative pressure inside the valve cover?

Turbo?

Yes.

Indeed - alternatively perhaps air being drawn from the crankcase into the intake as some form of emissions control (gone wrong / malfunctioning)

In general, some diesel engines have a check valve that allows air to exit the crankcase but not reenter. I don't know if you engine has that or not.

Re-read post #3 & 4. The PCV sucks on the crankcase. Gas engines suck pretty hard (via intake manifold vacuum). In your diesel, the only vacuum is from the pressure drop across the air filter + ducting. I first disconnect the PCV and leave the larger oil fill cap on. If I see minimal puffing from the little PCV port, that suggests the piston rings are excellent.

Removed the air filter completely. Still negative crank case pressure, it is sucking the oil filler cap down. I think the engine is sound and runs well. Just wondering what can cause negative blow-by. Anything I can check or just be happy?

You don't seem to be reading the replies

I think I did. What have I missed?

I think Ah-Kay is reading the prior threads which seem to want to point to a PCV issue. I'd think a "PCV" type problem would be a "positive" crankcase pressure issue. Ah-Kay is talking about a "negative" crankcase scenario.

On the OP's 603 I don't think there is a traditional "PCV" type valve. There is an oil vapor condenser built into the valve cover to return collected oil to the crankcase and a "breather" pipe that runs from the valve cover to the intake system between the turbo and the intake crossover pipe. If it is clogged it would result in increased crankcase pressure.

I wonder if the crankcase mounted mechanical fuel pump could have a cracked internal seal that is drawing on the crankcase environment as its pumping fuel to the primary filter causing a negative crankcase environment.

I simply cannot imagine what would cause negative crank case pressure... if the air filter is off, there is nothing that would pull enough volume of air to put crank case in a vacuum...

verify the turbo inlet hose is not plugged with something... is the turbo inlet hose disconnected?

on many 603 motors there is a emissions valve under the air cleaner... if it is stuck closed, and you have the air cleaner assembled... it would create a vacuum...