PCV Valve fitted but suction when oil filler removed

[Dave M]

If I may add my thoughts here. Just to clarify Dave, there is nothing new about having a vacuum in the stag crankcase; this was/is implemented originally with the Strombergs 'constant vacuum' port.

The vacuum can only occur if there is some restriction to the inflow of air. If you were to provide an intake vent there would be little or no vacuum. In that case any weaknesses in oil seals, gaskets etc IME allows oil to leak.

I believe that the current 'restricted' path for air into the crankcase is mainly via the dipstick felt seal, and lip seals such as those at the crankshaft front and rear. On my car the rear seal is not very good and leaks rather badly without the positive crankcase ventilation. I am using what I call a moderate flow PCV valve, compared to the usual big V8 ones, which in my experience are too severe. From your description of symptoms, it sounds like you have one of these (big V8) fitted.

But possibly no two engines are the same and some may work better with the higher flow PCV valves.

Hi Chris
Thanks for your reply, Im hoping your right and as you know I will be trying the differant PCV valve when it arrives, still can not figure out why having an increased vac in the crankcase would cause a follower to tap though. Thanks again for posting out the vac adapter so quickly.

[V Mad]

still can not figure out why having an increased vac in the crankcase would cause a follower to tap though.

I guess that this occurs because the cam cover, is presumably also in a *vacuum which pulls upwards on the follower when the cam is not compressing it onto the valve. It seems kind of obvious now that I come to think of it? Maybe a lesser vacuum might make a difference?

*vacuum connected to crankcase via oil drain holes in the casting.

[V Mad]

still can not figure out why having an increased vac in the crankcase would cause a follower to tap though.

I guess that this occurs because the cam cover, is presumably also in a *vacuum which pulls upwards on the follower when the cam is not compressing it onto the valve. Then it taps as the cam compresses it again. It seems kind of obvious now that I come to think of it? Maybe a lesser vacuum might make a difference?

*vacuum connected to crankcase via oil drain holes in the casting.

[Dave M]

Yes Chris the vacuum should be the same beneath each cam cover as they are connected by the oil ways etc and no 6 has always tapped slightly on my car at tick over anyway so perhaps your right about the follower, my worry was oil starvation on the NS head causing the tapping, will try the differant PCV and also check that oil is getting to the cam on the NS head, to put my mind at rest.

[wilf]

I was trying to think of something that might deflect under the load of vacuum on one side and atmospheric pressure on the other and then get slightly hit by part of the rotating assembly, but am coming up blank - any ideas anyone?

[Dave M]

New PCV valve now fitted and suction when oil filler removed is a lot less but still has suction which is a result. Tapping is still there but does not alter as much as with the previous PCV valve. Had the cam cover off and can't see anything amiss as I said before no 6 has always been prone to tapping. Interestingly when I remove the oil filler there is no change in the revs, engine just ticks over as before but can definitely hear the suction release when I remove the cap.

[SaskStag]

Which valve did you use in the end?
Tanya

[Dave M]

Hi Tanya
I bought it from Chris at Stagweber and the details I have are F6442259 for DOHC Sierra, Granada, Scorpio. Still produces suction at tickover but not as much as the other PCV so will see how it goes.