Positive Ventilation

Turbocharged Buick V6 engines are famous for many things. One of them is puking oil out of every possible spot when the engine is under boost. They blow the dipstick tubes out. They force oil past the rear main seal. They spray oil out the valve cover breathers. ¬†Mine even was forcing oil out of the PCV valve grommet under the intake plenum. They can be a huge mess. Even my newly rebuilt engine is doing this. It’s blowby. Gas getting past the rings when the engine is under boost. My first few autocross events, I was coming into the grid smoking after my third run from oil escaping the valve covers and pouring onto the exhaust. It was embarassing, it made a mess of the lots we race in (BAD), and made a mess of the engine compartment. I have been determined to fix it. After several attempts, I think I’ve nailed it. I’ve come up with a system using two catch cans and an industrial strength check valve. It goes a little something like this:

In this first picture, you see the passenger side of the intake manifold. Down underneath the plenum is an OEM PCV valve. The Goodyear hose runs to a catch can bolted to the side of the intercooler, then back up and through that brass check valve. The check valve is rated at 400psi, and is there to prevent manifold pressure from getting into the crankcase when under boost. Without that check valve, positive manifold pressure would easily overpower the OE PCV valve and pressurize the crankcase, which forces the oil out and makes a mess.

Now, the stock set up simply had a hose running from the PCV valve to the PCV inlet tube you saw the check valve attached to. The catch can keeps oil from making it to the check valve and gumming it up, as well as keeping the oil out of the intake tract.

But, there has to be another part. The stock PCV system had a vent in the passenger side valve cover that was connected to the turbocharger inlet. That set up mostly worked, but once you turn up the boost, that single vent simply isn’t enough. I’ve added a second vent.

Instead of one vent line, I ran two. Each valve cover has a Mr. Gasket breather cap on it and a 5/8″ line coming off of it. The lines go into a tee just behind the alternator, then run to another catch can.

From this catch can, we run out to a fitting that’s been screwed into the inlet pipe ahead of the turbo but behind the mass airflow sensor. This is important and I’ll explain.

PCV systems are basically a tuned leak. A port on the intake manifold provides a vacuum source, and a vent in the valve cover/intake tract provides a source of fresh air. Engine vacuum draws air into the vent, through the crankcase, and into the intake manifold where the crankcase vapors are burned in the cylinders.

In a computer controlled vehicle, this poses a problem. If you vent to atmosphere, say as if you’d used an open breather element on the valve covers instead of the closed ones I used, you would get extra oxygen in the cylinders that hadn’t been metered by the MAF. On a Buick using the stock computer, this extra oxygen is detected by the O2 sensor, and the computer adds fuel. In the Buick’s case, it adds WAY too much. So much that it washes out the rings, contaminates the oil, and eventually ruins the bearings. I didn’t want this to happen.

So, the vent is plumbed into the intake tract after the MAF. This ensures the air entering the crankcase through the vents has been metered, so when it shows up in the intake manifold via the PCV valve, the computer has already taken the air into account. It keeps the mixture correct, and doesn’t kill itself.

Under boost, the check valve on the manifold side closes, and the turbocharger inlet should draw out the crankcase gases via the breathers. In all cases, pressure should not build up inside the crankcase. It shouldn’t leak, and any atomized oil will condense in the catch cans and not foul up the turbocharger. All the air in the system goes through the MAF, and all should be happy. So is it?

Yes. In the screen capture below, you’ll see a grid on the right side. That is the Block Learn Multiplier (BLM) table. It’s basically a fuel trim table. If everything is absolutely perfect (70 degrees F, no leaks, perfect engine), all the numbers would be 128. ¬†They’ll vary with conditions (temperature, whether the gasoline is RFG or not, etc.). If you have a vacuum leak to atmosphere, like you would with a PCV system vented on the valve cover to the air, you’ll see BLM numbers above 150, and that’s bad. These are all in the low 130s, which is pretty good.

The PCV system isn’t leaking in air from the atmosphere. I’ve already found a very slight amount of water/oil mix in the smaller catch can after a 20 mile drive, and I’ve got no leaking oil running down the valve covers or collecting under the intake plenum. Preliminary indications are positive, and I’ll report further after my next autocross event to see if this system stands up to competition.

Leave a Reply