Sale ends todayGet 30% off any course (excluding packages)

Ends in --- --- ---

Testing Crankcase ventilation efficiency

Practical Engine Building

Forum Posts



Tech Articles

Discussion and questions related to the course Practical Engine Building

= Resolved threads


Hi all,

I have been stumped diagnosing this excess crankcase pressure in my engine. This is a 2.3L Turbo MZR from a Mazdaspeed3. It uses a similar design to the Ford PCV plates behind the IM. OE turbo pushing slightly more than designed.

WOT through the gears will eventually result in my dip stick popping out and spewing oil, even worse when it is hot outside. Although rust proofing the engine bay isn't terrible, I am tired of lubricating my accessory belt. But the real concern is oil drain from the turbo is fighting pressure and RMS.

The engine was freshly assembled (aftermarket Mahle pistons/rings) about 40k km, block was hot tanked.

I have upgraded my PCV system to include a catch can and have replaced the PCV valve more than once. I even pulled off the PCV plate and fed air through the orifices to the crank and can feel a steady stream of air out of the oil cap.

Crankcase is being pressurized faster than it can be evacuated and that can be due to:

1. Blow-by: due to rings

2. Inefficient/clogged PCV system: pinched hose, clogged passages, etc.

Compression numbers are good, but I take it that is not a good test, I suspect a leak down is in order to confirm the rings are okay?

Any assistance with isolating this problem would be greatly appreciated!


I attached a nice diagram off a similar system to the one used on the Mazda.

Attached Files

Might be worth running a separator tank on the turbo drain with a separate breather line regardless of leak down figures.

NOTE, not a 'turbo' guy', so comments only.

It appears the PCV is plumbed to the intake manifold, this is normal practice and is fine for N/A engines.

However, with forced induction the manifold is going to be subject to whatever boost level the engine uses when the throttle is opened, this in turn means the crankcase pressure is going to be at least that much, even without ring leakage potentially increasing the pressure.

For that reason, some will direct the breather to the inlet side of the turbo' as that's not pressurised, possibly using a diversion/bypass arrangement for manifold vacuum under light running and to the inlet side under boost.

This may be of some help understanding this - https://www.knowyourparts.com/technical-resources/engine/pcv-system-pressure-managment-in-turbocharged-applictions/

Using an external catch tank may be the simplest option, with separators used at the cam' cover and block (if used) and large diameter hoses to help promote the movement of air rather than picking up oil with the blowby.

Thank you for the replies guys!

As these engines are Turbocharged from factory, I am confident in saying that with the OEM setup, the engine shouldn't struggle with the PCV system - even with a slight threshold beyond the regular demand.

I forgot to mention that these cars have a breather in the valve-cover going into the intake, downstream of the turbo. This is used when the PCV valve is shut closed due to crankcase pressure, as you noted Gord.

To try to assist oil separation I introduced a catch can with a check valve to prevent crankcase pressure when the manifold is pressurized. Even with this in place, under constant WOT, I am occasionally able to push oil out the dipstick. This tells me that the pressure is coming from blowby.

With that said, I have a rebuild engine with aftermarket (Mahle Powerpak 4032) pistons and rings that are 0.5mm larger than OEM bore (1/4 cylinders were out of round). And I am wondering if that may have something to do with it.

What I am wondering is any method I can use to diagnose this and narrow it down? I am going to do a leak down test next.

We usually reply within 12hrs (often sooner)

Need Help?

Need help choosing a course?

Experiencing website difficulties?

Or need to contact us for any other reason?