The secondary air filter, what the?

R

Rovertron

Member
On picking up some service items, I was told of a secondary air filter, a small white pot, very similar to the fuel filter. So I bought one along with a couple of new flame traps.

I didn't think anything of it, I knew the primary air filter is immaculate, also, I had no idea what on earth it was for.

At a loose end last weekend so I thought 'Well, I bought the stuff... best fit it.'

Taking the filter box off, I got a real eyeful, the white pot hiding underneath was filthy, all black inside. So replaced it, new flame traps and while I was at it removed an airlock in the heater matrix topping up through the inlet manifold output to the matrix and then all back together.

I got a bigger surprise when I took it out on a test run (more for the airlocking matrix), the air induction improvement was amazing, not only was the engine note a lot smoother under load but response was much better, the engine is also so much quieter too.

It's a sea of rubber pipes under the filter box, just to solve my own curiousity, what does that little filter do?
 
Its basically a crankcase breather, often neglected due to its position under the airbox. Most V8's will leak oil like a sieve if its blocked (along with the flame traps). It can indeed make quite a difference to how the engine runs!

Its also worth checking the breather hoses (connecting the flame traps, as you know) for cracks/splits and constrictions due to carbon build up.
 
Before we restored my P5B many moons ago, I was on the M3 when the oil light came on & the engine started running horribly. When the AA man got there he took the side of the air filter off & all the engine oil ran out of it. The crankcase had pressurised due to blocked flametraps & blown it all up through that small filter into the air filter.
Messy!
 
Cheers guys, thanks for the heads up, definitely a annual service item from now on then.

It has to rate as the best £4 I've ever spent on the car to date.

As for the breather hoses, there was the odd smear of oil come to mention it. The workshop manual recommended soaking the flame traps in petrol for a clean up so I think I might have the hoses off as well and give them a clean up in some soapy water following with a good rinse.
 
although the Haynes manal mentions cleaning flame traps, dueto their low price you are better of fitting new items; they can get very bunged up.

re: crankase filter; I have twin K&Ns on apir of HIF6 and there was a long rubber hose dangling off the back of the engine, I eventually figure out that this hose would be routedto the air filter box, whcih I no longer have. So I will be fitting a small K&N style breather filter purchased off ebay. :)
 
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