Silly question ... When removing Oil fill cap while running should it feel preasure

[cpu77]

I was topping off the oil while my car was running. I could feel as the though there was positive pressure while installing oil cap. Normal or plugged PCV?

[ZipMeUpJ]

Im thinking thats normal. When i had a bad prv pulling off the cap was more difficult than normal because their was a loud sucking noise

[87supraman]

My 3.0 definitely sucks the cap down. PCV is on my list to do, 145k miles on the original.

Sent from my SCH-I645

[MacFady]

+1 sucks down, may even be difficult to remove, same with the dip stick. Car will want to die once you get it out/off.

[Elliott]

This is not a dumb question. It's a good barometer (quite literally) of how the PCV system is functioning.

[old guy]

Removing the oil filler cap should not change your idle speed. If it is under a lot of suction you have a problem with either the PCV valve pulling excessive air from the block or the PRV is clogged up and not letting purge air into the block.

When you remove the oil cap at idle there should be a fluctuation of air moving in and out of the opening. This is caused by the sump air being displaced by the bottom of the pistons as they move up and down. You should be able to hold a piece of card stock over the opening and see it move up and down. If it simply blows up and stays there you either have a nonfunctioning PCV valve that isn’t pulling any vacuum or excessive blow-by at idle as a result of worn components.

[AlyAudi03]

Removing the oil filler cap should not change your idle speed. If it is under a lot of suction you have a problem with either the PCV valve pulling excessive air from the block or the PRV is clogged up and not letting purge air into the block.

When you remove the oil cap at idle there should be a fluctuation of air moving in and out of the opening. This is caused by the sump air being displaced by the bottom of the pistons as they move up and down. You should be able to hold a piece of card stock over the opening and see it move up and down. If it simply blows up and stays there you either have a nonfunctioning PCV valve that isn’t pulling any vacuum or excessive blow-by at idle as a result of worn components.

ok, wait this is interesting didn't know this. so it shouldn't ever blow out steadily. only in and out. ive noticed it blow out, but haven't really paid enough attention to see if the air flow was moving in and out. ill have to check that. if its worn components. which ones do you think are more than likely the issue?

[ECS Tuning-Audi]

Going off what old guy said usually a simple test with out the use of paper or card board is if you loosen the oil cap and leave it in the opening it should hop around a bit. If it either gets sucked down and stays down or blows out your PCV system is not functioning properly.

Jason

[AlyAudi03]

Going off what old guy said usually a simple test with out the use of paper or card board is if you loosen the oil cap and leave it in the opening it should hop around a bit. If it either gets sucked down and stays down or blows out your PCV system is not functioning properly.

Jason

yea definitely doesn't suck down. i noticed that with the dipstick test, when my pcv valve was a goner. fixed that. i do let it bounce occasionally to check it, and it does dance. but i've really only paid attention enough to notice it blowing out only. PRV and pcv were replaced about a year ago or so, as well as other hard plastic valves, check valves. etc. replaced a few hoses as well. but have had trouble finding a leak.

[03a4b6ztk]

Might be a stupid question but what is prv and does the 3.0 have this. I take off the oil fill cap and it has strong vacuum and idle changes noticibly.


EDIT: I googled prv. But cant find out whether the 3.0 has one.

Sent from my SM-T2105 using Tapatalk

[road_tripper]

my 3.0 also has very strong vaccuum. no codes either.

[Luxus Panzer]

looks like I am good :) (1.8t)
(CLICK TO PLAY VID)

[vtince]

Removing the oil filler cap should not change your idle speed. If it is under a lot of suction you have a problem with either the PCV valve pulling excessive air from the block or the PRV is clogged up and not letting purge air into the block.

When you remove the oil cap at idle there should be a fluctuation of air moving in and out of the opening. This is caused by the sump air being displaced by the bottom of the pistons as they move up and down. You should be able to hold a piece of card stock over the opening and see it move up and down. If it simply blows up and stays there you either have a nonfunctioning PCV valve that isn’t pulling any vacuum or excessive blow-by at idle as a result of worn components.

Old Guy, does this test also work with the 3.0L? When I loosen the cap, there is a slight vacuum sucking the cap down in place, not dancing around. And when I remove the cap, idle changes ever so slightly. Does this seem like a bad PCV?

[old guy]

Hopefully one of the 3.0 guys will respond with their experience. I would expect a very slight vacuum would be normal for a 3.0. With 6 pistons going up and down the pulsations are greatly reduced and consequently you don’t get the oil cap dance thing going on.

[ECS Tuning-Audi]

Sounds like normal operation to me!

Jason

[vtince]

Thanks guys, I'm trying to figure out why my driver side valve cover gasket keeps leaking, it's been replaced twice already in the last 7 months. Passenger side is fine. It's leaking on the side toward the rear bolt under the coolant tank, any ideas, I thought it may be a bad pcv. I did remove the pcv to see if it was clogged but it looked ok. I was able to blow air freely through both sides... Is that normal?

[SJorge3442]

Old Guy, does this test also work with the 3.0L? When I loosen the cap, there is a slight vacuum sucking the cap down in place, not dancing around. And when I remove the cap, idle changes ever so slightly. Does this seem like a bad PCV?

slight is fine. When its hard to remove then you're PCV is clogged. Just went through this a few months ago.

[FromS60toB61.8t]

Thanks guys, I'm trying to figure out why my driver side valve cover gasket keeps leaking, it's been replaced twice already in the last 7 months. Passenger side is fine. It's leaking on the side toward the rear bolt under the coolant tank, any ideas, I thought it may be a bad pcv. I did remove the pcv to see if it was clogged but it looked ok. I was able to blow air freely through both sides... Is that normal?

i know with my 2.7 allroad, the oil caps on them went bad pretty frequently. They didnt relieve pressure as they should and keep blowing out valve cover gaskets prematurely. All was fixed with a newer revision oil cap. Maybe the same problem?

[stres-tm]

I recently had a low oil warning (after 1000mi. Since last oil change) and when I checked the dipstick while engine was running it was under a large vacuum and the idle drastically changed tone, then I replaced the oil dipstick and tried the filler cap and the same thing occurred. From reading this it sounds like PCV needs replacement?
Also could the oil consumption and PCV be related?
I’ve seen someone replace just the PCV disk or should I replace the entire system?
2012 A4 b8 Avant
IMG_2798.jpg

[Tahoe3.0]

this thread brings some interesting things to light for me. a little while ago when I was redoing my vacuum lines I popped the passenger side breather hose off for some room to work and noticed some crud build up inside. I took note but didn't really think too much of it. Last week I was changing the oil and had the car running while adding the last little bit. There was a lot of suction/resistance removing the cap and once it was off the idle changed pretty significantly and started to sound like it was going to stall so I shut it off. My idle has been pretty "burbly" for a while, more-so recently.

I know I probably shouldn't but I am going to try this as a replacement and if it doesn't work out go with heater hose and the brass fittings.

[stres-tm]

Deleted

[old guy]

I should mention that the 2.5→3 inHg is only applicable to the 2.0T engine. The 1.8T PCV system is a totally different set up and operates with significantly less vacuum on the block.