Ford KA / Fiat 500 Small breather hose purpose etc

[KittyKa]

So a while back I fitted a K&N 57i induction kit to my 1.2l petrol KA and it came with a small rubber bung to block the small breather hose coming from the inlet manifold that would connect to the filter housing between the throttle body and the filter element, it connects to a similar place that the crankcase ventilation hose connects to on the filter housing / air box.

I had it blocked for a few months and have noticed that typically when lifting the clutch in first gear without pressing the accelerator the car wouldn't struggle to stay above 800rpm when I brought up the clutch however now it slightly struggled before overcorrecting. I also noticed that when bringing the clutch up and pushing it back down multiple times to cause the engine to rev up, instead of the normal behaviour where it would rev up then down and up again it would now sometimes rev very high (up to 3k rpm sometimes if I did it quick enough) before slowly dropping back to idle, and trying to heel-toe when downshifting was difficult as the engine would be more unpredictable and would sometimes just rev up to 3k for no reason once the clutch was engaged while on the brakes.

All of these negatives aside, the throttle response was slightly better as the throttle body stayed further open at idle however I had all of the issues I previously mentioned and the rev hang was quite bad. The engine also seemed to not feel as torquey throughout the rev range.

This led me to do some investigating about what the purpose of the small breather hose actually does as nobody can seem to figure out what its for online and a lot of people have come up with different reasons for its existence. Some important things to note are that it is not under vacuum when connected to the stock air box as it is before the throttle body and the pipe does not smell of petrol fumes or have any visible oil in the tube as I cut a section and took it off to check.

Finally I decided to do some very "scientific" testing with a latex glove and a mini K&N filter. I trimmed the tube down to size so that it was not pressing against the pipes going to the cabin heat exchanger and then installed the small filter and placed a latex glove over the filter and blew it up with some air. When I started the engine and let it idle for no more than 2-3 seconds the glove was sucked against the filter so it was clearly drawing in air.

After turning off the engine and removing the glove I reset the ECU self adaptation parameters and did an oil and filter change to start back from what the car would've been like when I first installed the induction kit and immediately I noticed the throttle response was ever so slightly slower however the car idled a lot more stable and felt more torquey when up to temperature throughout the rev-range than it did when the hose was blocked off which has led me to the conclusion that this is either some sort of idle breather hose that lets in a restricted amount of air past the throttle body to assist with idling and subsequently lets more air in hence the feeling of more torque throughout the rev range. OR it is some sort of hose that connects to the crankcase to let in air however I don't see this being the case as blowby should create positive pressure in the crankcase so it sucking the latex glove tight to the filter wouldn't really make sense.

After a few hundred miles and driving in a very spirited manner the throttle response seems to have improved with time and obviously take this with a grain of salt but the engine feels more performant than when I first unblocked the little hose and installed the filter.

I have also noticed that also as I have removed the AC system from my Ka but still have the buttons on the dashboard, enabling AC with the compressor clutch unplugged makes the engine sound different which I believe is caused by it toggling VVT even at idle. This happens if you enable AC by pressing the button or by turning the blower to windscreen as I believe on most if not all fords putting the blower on windscreen turns on the AC and it does the same thing in the fiat 500.

I have attached a picture of the little filter connected, I think its really concerning that K&N tell you to block this hose off as the engine revving up before while I have been heel-toe downshifting under braking down a hill has made me have brake fade a couple of times, not ideal, if buying this kit please make sure you get a little filter for the small breather hose, if for no other reason I have mentioned I think it helps with performance and its only £17 extra. Ideally I think they should include this in the kit or mention what happens if you block it off.

The same will apply about everything said here to the fiat 500 if the hose isn't blanked off on your airbox like it is on some fiat 500s I've seen.

[KittyKa]

The pipe in question is fiat part number 51888291, reference number 4 in the attached diagram

[KittyKa]

Mini filter in question is K&N Filters part number: 62-1600RD-L 

[blueoveralls]

How did you mount the small breather? I have a fiat 500 where the small spigot on the airbox (for this breather hose joins) has broken and a new airbox is approx £350, there is a hack I've seen whereby a bleed nipple has been inserted but there is a potential risk with any solution like that of debris entering the MAF.... so an additional filter that doesn't complicate air filter changes sounds good to me.

Lastly it sounds like you reset the ECU?? How did you do that? 

[KittyKa]

Hiya, I have an induction kit on mine so it doesn't have an airbox fitted, I reset the ECU using an OBDLINK SX and a piece of software called multiecuscan.

If you need a new airbox btw I am literally trying to sell mine atm haha seen as I have an induction kit, it does say ford durateq on it rather than fiat 500 but they're identical other than that. If you're interested drop me a message.

Maybe this is an opportunity for you to fit an induction kit otherwise haha.

[blueoveralls]

Hi, Kind of you to offer your air box but your solution for £17 has worked for me for now.  Although....... ( I'll keep in mind}

What did your induction kit cost? I suspect it will be cheaper than the OEM part. It would allow me to move the small filter closer to the MAF too, similar to your solution as the airbox is quite large.

Thanks for posting as it gave me direction. I wonder if anyone knows why the additional vacuum is needed in the airbox??? You only need to put your finger over the hose to feel it. Perhaps for emissions so that even at tick over the engine has negative pressure???  Would be interesting to know.

[KittyKa]

I'm glad my rambling has helped someone haha, the k&n induction kit was £170 I think? They make them specifically for the fiat 500 and ford ka and still cheaper than a new airbox. At idle id imagine because its right next to the pcv hose it probably draws that past the throttle body using the little hose, I think its mainly to help the car idle and get away with the fact that the throttle body on the ka and fiat 500 is super unresponsive but it could also be a little to do with emissions, some fiat 500s don't have the mini hose at all so its hard to say haha.

Just to note, the induction noise is quite loud, definitely in a good way tho. I'll try and record mine when I go for a drive and I'll post it here.

[KittyKa]

Thought I'd post an update about this mini filter being used, since installing it I have driven about 3000 miles and I'm happy to report I have had no issues from installing it.

Not had a single issue with the engine revving up when pumping the clutch or anything like with it blocked and it drives like a dream. If you are adding this mini filter due to a snapped hose connector on the airbox just keep in mind that you will need to seal up the hole on the airbox so as not to let unfiltered air in, this can be done with tape or by melting some abs plastic over the hole to create an airtight seal either with a spare bit of plastic or some abs filament and a soldering iron or 3d printing pen.

The mini filter is a washable filter but it is not cotton and it is not oiled, to clean it you will need K&N synthetic filter cleaner, do not oil it. The 57i induction kit however needs the k&n recharge kit every year or so.

Hope this helps!