Oil breather check valve? (1987 auto)
Is there supposed to be a checkvalve in the rear hose from the oil fill neck or is integral in the plastic fill neck??
If your model year is configured the same as the 90 S4 one of the two hose connections has a one way valve built into it [the neck housing that is] and I have not seen one with a valve in the hose as in earlier [pre-S4] models [I believe].

That being said there appear to have been a number of variations on the theme so best confirmed by an 87 owner.

I've seen plastic filler necks without the valve. I don't pay attention to when and where this started and stopped....I'm not trying to keep track of every single little "model change"....I'm just trying to fix cars with problems.

Regardless....there has to be one there...otherwise there is a direct 1/2" hose from the filler neck to the intake. Above about 4,000 rpm, that hose gets filled with oil from the crankcase...pumping oil by the cup fulls into the intake manifold.

Note that the check valve frequently gets stuck in the hose and gets overlooked....not put back in....in both the metal filler neck models and the plastic filler neck models.

Funny story...928 International sells each and every metal filler neck that they can find...they have a waiting list....but they don't sell the check valve as a part of the filler neck (which it isn't). They also were not aware that the check vavle was even there, because it is not listed in PET (or I've never been able to find it.) Anyway, the last time I had a car that didn't have one, I went over there to find one...and we had to take one out of an assembled engine.

The point is....there's a bunch of cars out there that don't have this valve...and have the 1/2" hose hooked directly to the intake system.

If you one of the people that are possibly caught in this little issue, it would be good to fix it.

greg brown
It's a very simple PCV valve, like in any 1970's American engine, except it is not hooked directly to intake vacuum pulling it open, like in those American engines. It depends on excessive crankcase pressure to push it open.

There is also an interesting additional factor involved in opening this valve....the velocity of the incoming air through the connecting boot between the MAF and the throttle body pulls a slight vacuum on the hose attached to this valve....allowing it to open at lower crankcase pressure.

The simple fact remains....many are missing/leaking and the result is a direct 1/2" oil path from filler neck to intake.

While this will not be a problem at lower rpm, at higher rpm the windage inside the crankcase virtually fills the filler neck area with oil...and the vacuum created at the pre-butterfly attachment (as mentioned above) draws copious amount of oil into the intake.

The check valve needs to be there....and it must have a very positive seal. Any leakage will result in oil being "vacuumed" up and drawn into the intake.

This is a huge "free clue" for pre-GTS 32 valve owners that are having excessive oil drawn into the intake! It is very common for the valves (both the "loose" version and the captive valve in the plastic filler neck) to become "gunked" up and leak past the spring seat....allowing oil to be drawn to the intake.

My "updated" passive ventilation/breather system eliminates all of these issues and assures that no oil from the crankcase or heads can get to the intake....on engines without crazy crankcase pressures from wear.

Remember that removing oil from the intake system increases the effective octane of the fuel....increasing performance.
...which brings the question why did they do away with it in GTS? That hose breathes openly from the filler neck into the MAF elbow in the GTS without any check valve. Seems like they had a lot of fate in the higher port in the filler neck resulting in a lot less oil mist in the crankcase gas. I think that the higher pickup location is good, I use it, but is it really that much better? ;-) Reading dead people's minds is difficult.
Don't forget that the left the oil return paths from the oil control rings out, which seems like it was an effort to reduce crankcase pressure....tough to think of any other reason for that idiocy.
Indeed- I figured that one out by the time I realised that I had been a dumb ass when converting my S4 breather system to the GTS variant [so it looked like a GTS motor- durrgh!] and oil consumption went south. Worse still when I modified the filler neck I kept the GTS item and modified the S4 neck!

How that check valve in the S4 filler neck body can help anything breath is beyond me given how small it is.