Engine runs really rough when oil cap is removed. PCV Valve bad?

[never_comment]

a4 B7 Quattro w/ 180,000 miles. I have been burning a crazy amount of oil in the last 10k miles, like 3 qts b/t oil changes. I was told to possibly the PCV valve (which I replaced 2 years ago), and to check it by opening the oil cap on a running engine: 'If you can't remove the cap then the PCV is not working.' Well there was slight suction on the oil cap not nothing to difficult, but the engine immediately began running really rough with the cap off. Does this still indicate PCV Valve?

[Kingzilla17]

Engine will run bad once removing the cap regardless of the PCV being bad. You are creating a major leak. The PCV is bad if the cap is hard to remove due to suction. If you need to use more force than usual then the PCV is bad. Sounds like its not that if it's not difficult to remove.


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[Theiceman]

If you are using that amount of oil and it has been a drastic change from what you normally see I would check to see if you have separation or a crack in the valve cover.

oh i should point the valve cover is actually part of the PCV system , some people do not realize this and assume it is just the PCV valve. if it is cracked the manifold vacuum will pull oil out of the head right into the intake.


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[Brillo]

The cap should be difficult to remove with the engine idleing. What that means is that the PCV valve is working properly at idle. But as Theiceman said above, that doesn't necessarily mean that the PCV valve system is working correctly, specifically under boost conditions. When the manifold pressure goes from vacuum conditions (at and slightly above idle RPMs) to positive pressure (boost conditions) the PCV valve closes to the IM but opens to the pre-turbo port (right side front of the VC) to continue to keep the crankcase under vacuum to remove blow-by pressure from the crankcase. This part of the crankcase system should be assessed also to see if the PCV valve is functioning correctly at all RPMs.

[ahotboy15]

Wow such a great description brillo

the cap should be difficult to remove with the engine idleing. What that means is that the pcv valve is working properly at idle. But as theiceman said above, that doesn't necessarily mean that the pcv valve system is working correctly, specifically under boost conditions. When the manifold pressure goes from vacuum conditions (at and slightly above idle rpms) to positive pressure (boost conditions) the pcv valve closes to the im but opens to the pre-turbo port (right side front of the vc) to continue to keep the crankcase under vacuum to remove blow-by pressure from the crankcase. This part of the crankcase system should be assessed also to see if the pcv valve is functioning correctly at all rpms.

[EvolutionArmory]

When your PCV valve goes bad, it will be damn near impossible to remove it at all. Another sign of it starting to fail is oil getting pushed through your valve cover bolt seals or having oil on your oil cap. The excess pressure pushes on the cap making it almost impossible to open and the excess pressure will try to push oil out anywhere it can, like the cap or bolt seals. 😀

Just threw in a new R revision for my early B7 for that very reason last week.

[pezgoon]

When your PCV valve goes bad, it will be damn near impossible to remove it at all. Another sign of it starting to fail is oil getting pushed through your valve cover bolt seals or having oil on your oil cap. The excess pressure pushes on the cap making it almost impossible to open and the excess pressure will try to push oil out anywhere it can, like the cap or bolt seals.

Just threw in a new R revision for my early B7 for that very reason last week.

Wait really?

I had my pcv freeze this winter and when it happened I believed it “blew” my valve cover gasket, I was planning on replacing it (the gasket) but have been debating doing some of the bolts too as it’s coming up and around a couple since then

Are you telling me it could be because the pcv actually failed??


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[EvolutionArmory]

Failed, frozen. What difference does it really make? It basically did something it wasn’t suppose to either way 😀

And I wouldn’t be able to say if it failed because it froze or froze because it failed. Sorry.

If you get oil coming up through the bolt seals or oil cap, it’s most likely PCV related.

[Brillo]

Wait really?

I had my pcv freeze this winter and when it happened I believed it “blew” my valve cover gasket, I was planning on replacing it (the gasket) but have been debating doing some of the bolts too as it’s coming up and around a couple since then

Are you telling me it could be because the pcv actually failed??


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Pezgoon: If your PCV valve froze in the position where the IM is open to the crankcase, the crankcase would be exposed to boost pressure which will try to push oil out of the crankcase where ever it can - seals, gaskets, oil filler cap, etc. The valve cover gasket is particularly vulnerable.