Don't Breathe the Oil Vapors

An air-oil separator (AOS) is a small component connected to the engine that, as the name suggests, separates oil vapors from the air that passes through it. Aston Martins have these and they work... in theory. In this blog post I'll be talking about the failings of the OEM air-oil separator, a common leaking problem, and how to fix both.

To get things started, let's take a look at the AOS in the V8 Vantage.  It's the smaller of the two flat-topped plastic components, located right here:

The Problem

Just kidding, it's more than a problem. There are actually two problems with the OEM AOS.

  1. It leaks
  2. It doesn't work

The AOS is held in place by a single bolt, which means it doesn't have positive pressure on both sides of its inlet. As the AOS vibrates from the engine and heat cycles from heating and cooling, it eventually lets oil seep past its single o-ring which isn't properly sealing because it's only got a single bolt on one single side keeping it in place.

The good thing is that it's a very small amount of oil that gets out. However, oil cakes the bottom half of the AOS as well as the drain tube coming from the bottom of the AOS and the oil collects dirt and grime that build up on both.

Such a thing can be considered simply an annoyance - you'd only ever notice it when you stick your hand down there... for whatever reason... and come up with a smear of thick black, oily grime on your hand. Here's how to fix the AOS. It's a quick, easy job. But more than just an annoyance is the fact that the air-oil separator doesn't actually work.

To sum up what an air-oil separator does as quickly as possible:

Hot, pressurized air is vented from the engine to relieve excess air pressure that can lead to a number of issues. That air contains oil vapors. The oil vapors can't be vented to atmosphere because of emissions. So the air goes through an air-oil separator (yes, yes, and a PCV but this isn't about that) that is supposed to remove those oil vapors and return them, as oil, to the oil sump. The cleaner air is then sent into the intake manifold to be burned off, with any trace oil being burned in the process. However, the OEM AOS doesn't quite accomplish this. Plus, the AOS is bypassed by at least one vent line. So, there's that.

Why does this matter? Because the oil vapors contaminate the intake air/fuel mixture, which affects everything from engine power to fuel efficiency to emissions.

The Solution

A few years ago I pulled my throttle body off the intake manifold to give it a nice cleaning. When I looked into the manifold I saw a depression, a hole of some kind. As I often do when I'm curious about such things... I stuck my finger in it. Oil covered my finger, which shouldn't really happen when poking around in an intake manifold.

I started investigating the source of the oil. I pulled off the EGR solenoid and diddled that. It was clean. I pulled open the plastic tubes leading into the intake duct and diddled those. Also clean. Then I pulled off the plastic tube connecting the PCV to the intake manifold. The diddle was dirty!

I decided I needed to fix the problem - I needed to come up with a way to prevent oil from getting into the intake manifold. I started doing some research, poring over parts diagrams, and looking for solutions. I eventually designed an oil catch can using a custom mounting bracket, OEM fittings, and off-the-shelf components. I tested the setup on my car and it worked really well. I posted my project on, and a bunch of people were interested. I sold a batch of the kits and then went with a more customized route. The end result is the oil catch can kit you can find at VelocityAP and in my online store. It blends in perfectly with the engine bay - most people don't know it's aftermarket! Here's what it looks like installed. (It's the black cannister down by the power steering reservoir.)

Does it work? Absolutely yes. I've drained a ton of oil from my own can as well as my buddy Justin's. Others have reported substantial amounts being collected in their catch cans as well. It's shocking to see how much oil gets through the system!

Curious about it on your own car? Remove the hard plastic hose connecting the PCV to the intake manifold. There's likely oil sitting in the inlet to the intake manifold. Pretty clear sign that a catch can would be useful!

Here's a video showing the entire process of pulling the catch can and draining it. You can see how much oil comes out after a couple thousand miles. (Well, sort of see... the black shirt didn't do me any favors in the video!) Justin's catch can was nearly full after ~7000 miles of driving. That's a whole lot of oil!

When I originally started selling these catch cans, Stuart over at VelocityAP agreed to sell them for me so I wouldn't have to deal with the business side of things. I just dealt with supply, inventory, and shipping.  That's why the catch can kit is labeled as a VelocityAP product rather than a Redpants product. Now that I've started up my own store, Stuart has agreed to let me sell them myself, which was an extremely kind gesture since our original agreement was that he'd be the sole seller! (So a huge thank you to Stuart!)

Just keep in mind that although my oil catch can kit does capture the majority of oil vapors before they can get into the intake manifold, it doesn't fix the leaking AOS - that's still something to address on its own. I did try to replace the o-ring on the AOS with a slightly thicker one. It didn't work. Not only would the AOS not seat in the aluminum housing, the o-ring actually started slicing apart when I applied extra pressure. So, unfortunately, it seems like we're stuck with the same o-ring.