Extreme vacuum under oil cap after PCV replacement?

[JJDubya]

I had the PCV system replaced under warranty on my 2014 C7 S6. I recently noticed when going to top up the oil while the engine was running that there is an extreme amount of vacuum now pulled under the cap -- to the point where it actually required force to remove the cap. The idle speed changes abruptly when removing the cap (as would with any vacuum leak), and the ECU intervenes. And, when pouring oil in, it is actively sucked into the engine.

Is this normal behavior? I've had the car for 40K+ miles and did not notice this behavior before (and, indeed, on any of the hundreds of cars I've checked/added oil to in my lifetime)... certainly not a vacuum of this magnitude under the cap.

[3g_bishi]

%120 normal.


Sent from my iPhone using Audizine (http://r.tapatalk.com/byo?rid=87676)

[ItsLarryG]

Why are you opening the oil cap while the engine is running?

[wuplur]

I’ve never heard of anyone topping off their oil with the engine running and creating such a vacuum leak by removing the cap would probably set some faults. When the 2.0 came out, this was actually an easy way for us to diagnose a bad pcv when it was causing extreme vacuum that you couldn’t remove the oil cap.

[WatchMeSpend]

My PCV threw an error code 12 times in 3 years. My mechanic said to leave it, but when it does go, if I remove the oil cap, it would suck my hand down forcefully. 3.0, not 4.0 but your description and his description match...

[JJDubya]

Why are you opening the oil cap while the engine is running?

Because when you back out of the garage leaving for work and the oil level warning flashes and you have your 5 year old already in the car seat and it is 95F out and you want to leave the AC on so they don’t suffocate...... you open the oil cap when the engine is running. Why would you do it?


Sent from my iPhone using Tapatalk

[JJDubya]

I’ve never heard of anyone topping off their oil with the engine running and creating such a vacuum leak by removing the cap would probably set some faults. When the 2.0 came out, this was actually an easy way for us to diagnose a bad pcv when it was causing extreme vacuum that you couldn’t remove the oil cap.

I’m confused... first reply was “120% normal” to vacuum under the cap. But two subsequent posts suggest one way to diagnose a bad PCV is by extreme vacuum under the cap. Not normal, then?


Sent from my iPhone using Tapatalk

[Valpo A7]

You will NEVER get an accurate oil level in the engine with the engine running.

In your situation that you described I would add only enough to make the light go away and finish filling when you can do so when its safe for your family and you can mess with the engine while it is shut off.

[JJDubya]

You will NEVER get an accurate oil level in the engine with the engine running.

In your situation that you described I would add only enough to make the light go away and finish filling when you can do so when its safe for your family and you can mess with the engine while it is shut off.

Understood... oil circulating in the engine won't show up on oil level. Everyone seems fixated on the part about adding oil with engine running; it is irrelevant, and I realize it isn't typical. I wasn't checking oil level, I was adding oil due to message in MMI, engine happened to be running due to circumstances, and I removed the cap. The relevant part is that the vacuum was, in my opinion, extreme. Work was done at the dealer and wouldn't be first time dealer didn't repair properly. PCV should regulate vacuum level, correct?... should neither be too low or too high. I believe it is very high on my engine right now. I have a vacuum gauge at home and will measure it.

[ItsLarryG]

Everyone is “fixated” because it takes one minute to shit the car off after you have the oil in your hand, pop the hood, then add it like you’re supposed to. Sorry that your daughter was in the car seat, but one minute with the door open will not harm anyone.

[15 Phantom S6]

PCV should regulate vacuum level, correct?... should neither be too low or too high. I believe it is very high on my engine right now.

I'm no mechanic so maybe I have it backwards, but my understanding is that high vacuum when you take the oil cap off is a good thing and means everything is in proper working order. A leak in the PCV would pull air on its own so when you take the oil cap off there would be no vacuum since it's pulling air from the leaking area instead..

Also, my limited understanding of a PCV is that a PCV doesn't regulate the vacuum level up an down to keep it in check, it only releases excess pressure..

[3g_bishi]

Yes, there is vacuum and there is supposed to be. But if you can not get that cap off and there is TOO much vacuum then the pcv is bad. If small amount of vacuum then you are ok.


Sent from my iPhone using Audizine (http://r.tapatalk.com/byo?rid=87676)