Crankcase Breather Valve

[Peaks]

Can I just check what the crankcase breather valve is supposed to do? (to avoid confusion I've also seen this described as a PCV, but it's the breather valve between the rocker cover and the inlet manifold on a Mk2 Vitesse that I'm on about).

My understanding is that it allows air/fumes from the rocker cover to into the inlet manifold, but will not allow any air from the manifold back into the rocker cover - is that correct? 

Reason for asking is that although my car runs very well, the exhaust note at idle is a little uneven, and if I clamp off the hose from the rocker cover to the vent valve the exhaust note becomes beautifully smooth. I've checked for air leaks into the rocker cover  by pressurising it slightly (ok, that is a grand way of saying I blew into the hose on the rocker cover...) and I am confident there is no air leaking in where it shouldn't. So, I do wonder if the breather valve is working correctly. 

[johny]

No, its to control the pressure inside the engine. This should be kept slightly negative so any gases coming out of the oil or blowing by the piston rings are drawn off into the inlet manifold rather than being released to the atmosphere but also not too negative that excessive air is sucked in through oil seals etc. Obviously the gas flow can vary with engine wear and also the suction of the manifold is variable so the valve opens and closes to ensure that the correct pressure is maintained.

The valve does need occasional maintenance such as cleaning and checking of both the spring and diaphragm. I recently found the latter on mine had gone pretty rigid so replaced it - cant say it made any difference but then it was running well anyway...

[Peaks]

I think we agree with each other Johny -  we just have a different way of describing how it works 🤔

[johny]

yes you can think of it in different ways but it definitely isnt to stop flow from the manifold back into the rocker box.

The vacuum in the engine means that the dip stick should have is seal pad and the rocker cover cap be a good fit. Actually interesting to take the cover off and put your hand over it to see how much suck there is...

[Clive]

PCV valves, as mentioned, are to keep the "suck" pretty constant.

If yo just connected to the manifold direct, there would be massive suction at idle, weakening the mixture and pulling too much air out of the engine. So the PCV regulates that, and actually opens as revs increase. 

The earlier method was to connect the rocker box to the air filter, giving very slight suction, but also swallowing up any engine fumes. I use that method on my Spitfire (4 throttle bodies make a PCV tricky!)

[Patrick Taylor]

The fumes from the rocker box can be a bit oily/dirty, which makes the engine run more lumpily but protects the environment. Disconnecting it reverses both these things, I believe.

[Peter Truman]

attached are the Triumph Dealer Notes re Crankcase Breather Valve hope it helps, I've done it quickly as I'm going out if they need making clearer I'll do it later

Regards Peter T

[Peaks]

Thanks for the replies. 

[Stratton Jimmer]

Thanks Pete Truman. That is what I call a definitive reply!