Filter pipe from oil bung to air filter. query?

[DJ8226]

Gents what does the pipe that runs between the oil filler bung and the pipercross air filter do? I see some cars have a filter fitted to the oil bung. Is it simply a breather that allows air into / or out of the oil bung. Should I fit remote one or would it be detrimental. Or am I getting it all wrong. Can someone explain it all to me and if I should fit one where is best place to buy, the bay? TIA Damon

[GreenV8S]

That hose is the crank case breather hose. The crank case breather lets exhaust gases out of the crank case - they are also quite oily since the crank case is full of oil mist. Usually the breather is routed through a combined oil separator / flame trap. On all modern production cars it is routed back into the air intake so the gas/oil mist gets burned, conventionally through a positive crank case ventilation system that connects into the intake manifold. On older designs, it would be connected into the intake upstream of the throttle. Feeding the gas/oil mist mixture back into the engine is great for improving emissions but rubbish for power. On anything designed for competition, it would be common to direct the crank case breather into a catch tank instead.

[DJ8226]

Thanks for that, so it flows out of the bung and down to the air filter end so as to be reburnt? So....... if a remote filter is fitted this simply catches the oil mist and would need to be replaced periodically? Sems a long route for it to get back into the engine, plus does this mean all the mist is going into the airflow meters and so on up to the throttle butterflies?

What's the concensus remove or leave alone ?

[toast boy]

Yes, it flows back into the air flow meters and yes it will gunk them up. I've just (very carefully) cleaned my AFMs with an earbud and some panel wipe. I've also replaced the hose with a filter on the bung and blocked off the inlet on the air filter. I don't know if it has improved anything other than less gunk going into the AFMs but it certainly hasn't done it any harm!

[GreenV8S]

You want to be careful if you're replacing the breather with a filter.

The normal PCV setup has a fresh air inlet to one of the rocker covers, and a breather outlet somewhere else that goes via a check valve to the intake manifold. The fresh air inlet needs a filter to keep muck out of your engine, which is why you'll often see a little filter attached to a rocker cover, sometimes nothing more than a spigot with a bit of foam over the top but sometimes cute little baby versions of proper air filters complete with branding from K&N etc. It's important to note that what is going through this filter is clean fresh air being sucked into the engine by the PCV system.

If you take your breather *outlet* and wack a similar filter on the end, it will have oil and gunk blowing through it and will very quickly clog up. A clogged breather can very quickly cause all sorts of problems and you really don't want that happening. So it you have a simple straightforward breather outlet (no PCV setup) then either route it into the air intake or dump it (unfiltered) into some sort of catch tank. On a normal healthy engine the catch tank will remain more or less dry although it will get a bit mucky.

[DJ8226]

Right understood, its ran ok for 20 years with the original set up so it can stay as it is. Would like a long piece of red silicon hose to match the rest of the hoses though. Anyone know what diameter it is to save me going out to the garage. Thanks again.

[phillpot]

10mm

[GreenV8S]

If you're using silicone hose, remember that ordinary silicone will discolour over time if you expose it to oil/petrol. To avoid that, you need to specify hose with a fluorosilicone lining.