The Positive Crankcase Ventilation (PCV) System in an Aston Martin DB9 – and what Sucks about it!

This is a weak spot in the DB9 engine.  If you’ve ever removed your throttle body and found a puddle of engine oil laying in the intake manifold chamber, the PCV system is to blame.   In this article I am going to give a short background of the ‘what’s going wrong’, and then in the next articles I will lay out which parts you need and how to replace them.

What is the Positive Crankcase Ventilation System Doing?

1961 MG Midget Engine showing draft tube on side

All engines have blow-by gases that escape past the piston rings and build up in the engine crankcase.  In the old days (think sixties and before) all a manufacturer had to do was leave an opening in the crankcase chamber and let the pressure blow out naturally to the atmosphere.  My 1961 MG Midget has this exact setup, just a tube connected to the side of the engine block that runs down under the car and lets the gases vent out into the slip stream while driving (and making yet another oil stain anywhere I park).  Problem is, those gases are full of nasty emissions – partially burnt fuel and other bad actors – and it’s bad behavior to dump them into the atmosphere today.

Starting in the late sixties most manufacturers had to cleanup their act a bit, and a simple solution was to just suck those fumes back into the intake system (using the natural vacuum that occurs) and burn them again as part of normal engine operations.  Nice simple solution – and they named it Positive Crankcase Ventilation (PCV).

As part of this solution they needed to worry about two things:

  1. Blow back.   Consider for a moment if there was a back fire or some situation where the intake manifolds were briefly under pressure.  You wouldn’t want to pressurize your engine crankcase as this could effect performance potentially blowing out gaskets, seals or even your dipstick.   To deal with this, they installed a one-way valve in the vent that will only allow positive pressure built up within the crankcase to flow towards the low pressure (vacuum) of the intake system.  This is the PCV Valve.
  2. Replacement Air.   It’s not possible for the system to exactly suck out the exact amount of blow-by gases as they are created.  It would also not be good to create a vacuum condition in the crankcase by sucking too hard, this would only lead to more blow-by and could effect engine performance.  The solution is to allow filtered air to bleed into the crankcase as needed – make up air.  A pipe connects the fresh filtered air from the air inlet system to the top of each valve cover.

So why is there engine oil building up in my DB9 intake manifold?

Why? Because it was sucked into there through the PCV vacuum line, and on our DB9’s this line connects to the intake manifolds right behind the Throttle Bodies, the location of maximum vacuum.   The air coming from the crankcase is laiden with oil vapor, and some portions of this entrained oil are dropped out into the manifold as they enter.  A pool of oil can form (see photos).

Photo by Old Phart

The discussion forum’s point to ‘faulty’ PCV valves as the culprits, but honestly it’s not their job to stop entrained oil from flowing through the pipe.  The PCV valve is just supposed to act as a one way valve and stop air from going backwards down into the crankcase.   There are two separate air/oil separators that the vapors are sucked through prior to reaching the PCV valves, and my guess is that they aren’t designed that well and aren’t doing a good job.   If the velocity of the air flowing up the pipes is fast enough, I can imagine a thin mist of oil vapor starting to deposit on and coating the walls of the pipe, and the flow of the air itself dragging the film of oil up the pipe until it opens up into the intake chamber.   So, the real culprits may be the air/oil separator design or speed of the air flow (which relates to the pipe diameter).  Either way it sucks [pun intended] – we get engine oil pooling up in our intakes.

If I’ve missed some subtly here, please comment to correct me below.

Is this really bad?

Not particularly.   While annoying to discover the oil when doing service work, it’s not a big deal since there is relatively little of it.  A few things to consider:

  • It’s not enough oil that you are going to run low on engine oil.  We are talking about a few tablespoons of oil over time.
  • It can run down into your air filter airbox.   When you turn the engine off, if enough oil has accumulated it may ooze forward past the throttle body butterfly valve, and then drool down the plastic air intake pipe towards the air filter in the air box.  Start and stop the car hundreds of times and these tiny bits of ooze can accumulate.  If you’ve looked inside that air pipe, you’ve probably seen engine oil buildup as well.  While messy, this doesn’t really hurt anything.  Just wipe it out when you are doing service and don’t be surprised when it comes back by the next service.
  • It can run down into the intake valves, and cause blue smoke in your exhaust at start up.  Normally there is a pool of a few tablespoons of oil laying flat like a puddle inside the manifold.  Imagine parking on a slight uphill angle, or on a sideways slope left or right.  In any of these cases the puddle can flow towards the inlet valves and will be burned off when you start the engine, but you’ll get a momentary burst of blue smoke out the exhaust when it does.   So, keep this in mind if you see the smoke once in a rare while.  [Smoking all the time at startup might mean your valve seals are shot – unless you park on a slope all the time :>) ]

How specifically is the Aston Martin DB9 PCV System designed to work?

The official Aston Martin Workshop Manual section 3.08 on Emission Controls has a nice diagram and write up about it.

Essentially there is a 2-stage system.   Since the vacuum levels within the intake manifolds vary tremendously between part throttle (high levels of vacuum) and full throttle (low levels of vacuum), they had to come up with a system that would work in both cases.

Part Load Conditions

Most of the time we drive our cars around, we are barely using any of the engines massive power capability, and we are on part throttle.  In part throttle conditions the vacuum levels in the intake manifold chamber are high, and are great for sucking in crankcase gases.

If you look at the diagram:

  • Crankcase gases are sucked up through openings in the bottoms of the two air/oil separators (item 6 in the diagram).  The front air/oil separator is linked to the rear one by a single rubber hose.  All the gases eventually leave horizontally via the hose at the back of the rear separator.
  • The gases are then split through a Y connection and are turned vertically.  They head through separate rubber hoses towards and through a left and right vertically oriented PCV valve (item 5 in the diagram).
  • They exit the PCV valves (on both the left and right banks of the engine) and turn horizontally again and run through small diameter plastic piping (item 1 in the diagram) towards the front of the engine.
  • This black plastic piping runs along under the intake manifold and then comes out (where you can finally see it) and turns towards a nipple connection on the intake manifold just behind the throttle body.
  • The gases are then sucked into the intake manifold plenum and consumed by the engine.
  • At the same time, make up replacement air is being sucked into both engine bank valve covers.
    • Clean filtered air is drawn in from the air inlet pipe connection just in front of the throttle bodies.
    • The air flows horizontally from the front of the engine to the back along a stainless steel pipe just above the exhaust manifolds (item 3 in the diagram).
    • Once it reaches the back of the engine it turns up and flows through a rubber hose connected to a fitting in the top of the valve cover.
      • What can’t be easily seen is that there is a small duck-billed one-way style valve embedded inside the hose just before the connection to the valve cover (item 4 in the diagram).
        Duck Billed Valve Inline

        [If you take this hose off , you can easily see it]   This keeps gases from flowing the wrong way in this hose should a back pressure situation occur.

Simple enough.  Air is sucked out through one set of hoses, and fresh air sucked back in through another set to replace it.

Full Load Conditions

When our cars are roaring at full throttle, one side effect is that there is a lower vacuum level in the intake manifold since our foot is mashed flat to the floor on the throttle and the throttle body butterfly valve is wide open.   Another problem is that the small plastic pipes used in the part throttle condition probably can’t flow enough gases fast enough.

In this situation Aston has designed a second stage system into only the right hand engine bank.  Under these conditions there is now a level of vacuum in front of the throttle body at the plastic air intake piping.  The line that was designed to allow fresh air back into the right hand valve cover now reverses its level of flow and lets the vacuum that exists suck fumes back out this larger diameter pipe.  But how you ask since there is a duck-billed one way valve at the hose connection to the valve cover?  On the right hand bank they have designed in a clever Y connection to the rear end of the stainless steel pipe, one branch goes up to the valve cover for the partial load conditions, and the other branch connects to a hose that circles around the back of the engine to connect just below the right hand PCV Valve at a T branch in the rubber hose (see the photo earlier).

In these conditions the partial load lines are all doing the same as there were before, but the following changes have happened:

  • The right hand bank fresh air return line has now reversed flow and is being used to suck additional gases out and into the right hand air intake just in front of the throttle body (as noted above) where they are then drawn into the engine through the throttle body and consumed.
  • The left hand bank fresh air line is now supplying ALL the make up air being sucked out by all the other pathways.

So, in full throttle conditions this just sucks more air to keep up.

I could see this design leading to more oil build up in the right hand plastic air intake pipes ahead of the throttle body.  The same entrained mist effect could be happening here during full throttle events.

So now what?

OK, so I am amazed you’ve read this far, you must be motivated to try and do something about the oil build up in your DB9.

PCV Valves can wear out.  They spend a fair amount of time with their internals ‘fluttering’ as the air flow moves through them.  To test them is simple, blow in one end and the air should flow through, blow in the other end and it should be immediately blocked.  It’s normal in most any car to have to periodically replace them.

In the next few posts I will show you which parts you’ll need (and how you can get the PCV valves for cheaper since they are really Ford parts), and how to install them.

The Result

No Oil Pooling after about 20,000 miles

[Updated Dec 2020]  It’s been over 3 years now since I replaced my PCV Valves following the process above.  I just completed my annual service, and when I had the left hand throttle body removed I was able to snap this picture of a completely oil free inlet manifold plenum.   The change of the PCV valves seems to have done the trick!


Here is a video where I show you around the PCV system and describe the information above:


8 thoughts on “The Positive Crankcase Ventilation (PCV) System in an Aston Martin DB9 – and what Sucks about it!

  1. Donny Crayne

    I bought a 2005 DB9 this week and was looking for some information and found your website. I am a DIY guy really appreciate the quantity and quality of your site. Just knowing your help is at hand makes me sleep better at night. Thank you for this great how to site.


      1. Donny Crayne

        I got a black coupe with 23,000 miles. I do need some insight on removing the 3rd brake light panel. It rattles and I have read I need to remove the subwoofer grille to access the screws holding it in. Does the panel below the sub grille come out? I assume it snaps out but I don’t want to force it in the wrong direction. Any hints would be appreciated.
        Also if this question should be in a better place on your site please feel free to move it.


  2. H Steve, I have my Db9 2005 Volante since 2008. It is Pentland green, Sandstorm interior with Bamboo. I really enjoy the car. I made long journeys with it without major problems. My big issue has been with services and repairs. Like you wrote bills can run high. Until I found your website I though my car was the only one with troubles. I am just pleased to find out we have similar issues. Your website reassured me to keep doing repairs myself. I am in the process of replacing my Transmission cooler, since I found out the same problem with my PCV valves, I am looking forward to read your solution. You have a great website


    1. Nice, love that color. Thanks for the kudos. The next PCV article is nearly done, just finishing up a larger article on Costs. My car had the transmission cooler oil leak right at the connections to the cooler, and it showed as drips and blowback on the undertray you could see from above. $2K to have serviced (all pipes replaced) by a local Maserati dealer. Check out My Car page for my service history.

      Stay tuned!


  3. Chris Piazza

    Hi Steve, just curious if anyone has tried re-routing where the PCV line(s) return to the intake system? I noticed on my intake mainfolds there are ports on the sides of the manifolds that would make a better location then the throttle body itself. The drivers side manifold is connected to a vacum hose. The passanger side has a black plug with a “push lock” style fitting not hooked to anything. Not sure what this is for but seems like it would make a better return location. What do you think?


    1. Hi Chris. Those openings are for the Brake Servo vacuum. I’d be reluctant to make any change to the design though without first knowing for sure what is causing the entrainment issue. I’m not sure if the vacuum level would be the same at that location vs right behind the throttle body (It should be the same in theory) The last thing I’d want to do is to find a way to accidentally suck even more oil out and ingest it. Thanks for thinking about improving it!


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