Hi Guys! New member, recently purchased a used 2008 Mazda3 2.0L with 152k miles. It idles very smooth at 750rpm, no misfires, no stumble, doesnt even feel like the engine is running at idle. MPG is normal at 28. But I did notice oil was low well before the next scheduled oil change. After lots of searching on these forums, I think its probably a failed PCV, some signs:

-valve cover is covered in oil residue, especially on the top and back side seal, im thinking excess pressure is comprising gaskets, no leaks on the front or sides of the cover seal

-black soot on exhaust tip, im thinking oil is getting into intake and being burned. I've cleaned it off since purchasing. Does soot build up fairly quickly with a failed PCV? i'll keep an eye on it.

-at idle, when oil filler cap is taken off, there is positive pressure out of the filler hole, I think it should be negative pressure but not 100% sure as others have posted their Mazdas with a good PCV also have positive pressure. Small drops of oil also splashes out of the filler hole.

-this video shows slight negative pressure

You Tube

-no oil leak/residue on the underside of the engine, just on valve cover

Just changed to full synthetic 5w20 (was conventional previously) so I will now be able to get an accurate idea of how much oil its consuming per mile.

With those signs, it could just be a failed valve cover gasket but black soot on the exhaust and positive pressure from oil filler cap had me thinking it was a bad PCV as well. Any thoughts?

If it is a bad PCV, will a oil filler breather cap help reduce oil getting into the intake, reduce pressure on valve cover gaskets (ie leaks) and reduce consumption? My thinking is it will reduce crankcase pressure by aiding a failed PCV, but im not very familiar with using one and never had to troubleshoot a PCV other than shaking one (not easy on the these engines, haha).

I understand this is a band-aid fix. Its a winter vehicle for me so not wanting to spend lots of money on it since it's still running really good but would like to do preventative maintenance and keep it for a long time as these Mazdas seem very reliable and can run up to 300-500k miles so it may be worth spending $500 for a shop to replace the PCV and valve cover gasket.

Thank you for reading, any thoughts are appreciated. I'll update on oil consumption and soot build-up after a few hundred miles. Look forward to participating on this forum!

 

DO NOT REPLACE A FAILED PCV VALVE WITH BREATHER(S) ONLY ON THIS ENGINE.

The only way you could get away with eliminating a PCV valve is to install a crankcase evacuation system.

 

Thank you for advice, much appreciated.

Did some more digging and found this article regarding PCV Inspection
Mazda 3 Service Manual - Positive Crankcase Ventilation (PCV) Valve Inspection [Mzr 2.0, Mzr 2.5] - Emissions

At idle, when i disconnected the vent hose from the air intake there was no negative or positive pressure but there was quite a bit of oil in the fitting.
Does this sound like a partially clogged PCV?

Also, I spoke with a very experienced mechanic when getting some quotes for PCV replacement and he said on most newer japanese cars, it is normal to have positive pressure at the oil filler. Most german cars have negative pressure. Take that with a grain of salt, but it did ease my mind a little.

 

LOL.... really so that mechanic is redefining the meaning and how a PCV "system" works? LOL

PCV= (positive crankcase ventilation) What does it do well.... recycles crankcase gases back into the engine to burn. If you are having anything less then this there is a problem and it needs to at least be determined. In your case aside form changing or completely cleaning the PCV system I would also recommend (stop talking to mechanic's and rather an ASE certified Automotive Technician or a MAZDA certified technician in the future) doing a properly performed cylinder compression test followed by likely needing a cylinder leak down test. I think you will then find the cause why you are getting your results.