2 replies to this topic

[mickthefitter]

Can anyone help me identify the visible differences between an A series engine with a closed crankcase breather and one with an open breather? The reason is I've bought a new oil filler cap/breather and on fitting it, I found the old one I'd just cut the retaining loop off was a simple cap with no vent, which I didn't know existed. Now by looking at one of the online parts suppliers I can see that there are engines with open and closed breathers. My car is a '63 Hornet but I think it is fitted with a '71 or later engine, though it retains things like a dynamo, metal air filter body and a distributor with a metal vacuum pipe. I don't know if having the wrong kind of oil filler cap will do harm of affect performance, so I'd like to know how to tell if my engine should be fitted with a oil filler cap that seals the crankcase, or one that is vented. The old cap was a loose fit as the seal had shrunk, which is why I replaced it. Cheers, Mick. 

[mickthefitter]

I've just been looking at other posts on this subject. I've read references to the rocker box being vented to the air filter, and I've seen references to a PCV valve. My previous Mini, a '73 850, had a PCV valve (or at least a canister on the block that I took to be one) and a vented oil filler cap. The 998 engine in my Wolseley Hornet, whether its got a post '71 engine or not (I'm going by the serial number) does not have a canister  down the back of the block and has the metal air filter body connected by a rubber pipe to the rocker box. This engine had a loose fitting 'sealed' cap which I want to replace. 

 

When I took the vented oil filler cap off my 850 engine while running, it would almost stop. I read another post that mentioned this too suggesting this was correct for an open breather system. So if my Hornet engine speed does not alter if the oil filler cap is removed, does this suggest it needs to retain a sealed filler cap and the breather system is closed? 

[Ethel]

The reason the engine falters is because removing the cap lets more air in & weakens the mixture. A non-vented cap would a little bit worse if anything.

 

What connections are there/ The later cars allowed air in through the filler cap and sucked it out via one or more of the tappet chest,  timing chain cover , the transfer case.