How to test what caused pressure to build in crankcase?

Question

After an oil and filter change I took my Toyota Avensis 2006 Estate with 1ZZ-FE 1.8 liter petrol engine for a long distance ride. All of the sudden when riding at around 60-70mph I heard some bang and oil started leaking very fast. I lost about 2 liters of oil (half of all oil) and then it stopped leaking. Mechanic was unable to find the reason but he said it is probably PCV valve was stuck and overpressure squeezed oil through gasket or somewhere else. Bottom part of engine on accessories side is quite oily but it is hard to tell exact spot for oil leak because it is not leaking anymore. I checked PCV and valve moves freely (shake test). I suspect that some of tubes/channels that come from crankcase to valve cover for ventilation are clogged, but I am not sure how to test it. Will it work if I disconnect fresh air inlet in valve cover ventilation system, start engine and expect to have some suction as vacuum should build if pcv valves works correctly and sucks crankcase gases? Also I am a bit concerned about loud bang right before oil loss. What could it be? I also get check engine error with P0420 code which is related to catalytic converter work and not sure if it is related.

Bonus question: how crankcase and pcv and fresh air inlets are connected? I was unable to find any channels/routes undrer valve cover and I assumed that valve cover is not ventilated.

Answer 1

You may have excessive pressure in the crankcase coming from excessive "blowby". Blowby refers to combustion gases that leak past the piston rings, either due to worn rings, mis-gapped rings or mis-aligned rings.

The PCV system is intended to deal with blowby, which is a pollutant if allowed to escape to the atmosphere.

The bang that you heard may have been the dipstick shooting out of the dipstick hole and hitting the hood (bonnet?) as oil spewed out of the crankcase through the dipstick hole. This could have happened if the oil was overfilled at the just-completed oil change. When you checked the oil during your roadside "adventure", do you recall of the dipstick was already loose instead of snapped home as it should be?

If this is what happened, either the PCV valve was stuck or the blowby was so excessive that it was too much for the PCV valve to handle.

Every model of engine has a blowby volume per minute specification. You might find it in a dealer's reference materials. It's possible to measure blowby by connecting a tube to the PCV port with the engine running at a specific RPM and routing the tube to an inverted, graduated container filled with water and measuring the volume of water displaced per unit of time. If blowby is much greater than spec, a ring job may be the remedy.

It also may be possible to diagnose excessive blowby by performing a cylinder leakdown test on each of the cylinders and referring to dealer reference materials to see how much leakdown is too much, suggesting bad rings.