I've read articles, watched video's and personally witnessed the prius’ infamous cold start engine knock. I owned a 2005 prius (sold) and recently acquired a 2011. My girlfriend owns a 2010 prius. I noticed a lack of oil in the engine while doing a 10k oil change on the 2010 with 80k miles on the clock. After inspecting the pcv system, I discovered a trail of oil in the hose along with two tablespoons of motor oil in the bottom of the throttle body. THIS IS NOT NORMAL!!! . People have said this is normal - This is absolute craziness - This is not a 2stroke engine where oil is mixed with gas to lubricate and cool pistons. Oil, Fuel(excessive), Water are incompressible solids, and when in the combustion chamber, will make the motor knock running on 87 octane. Incompressible solid = increased combustion. Ever put 87 octane in a turbo charged or high compression/performance engine and driven it??? Loss of power and knocking is the result of pre ignition, this happens before the cpu can tell the spark plug to fire. 92 octane stops this because it needs higher heat to ignite. I used to have a 300zxTT and when my ex put 87 octane in it, I ended up having to put octane booster in the tank to fix the pre ignition. The prius runs on 87 octane, but higher grade octane or boosters will not fix the knock. The prius needs an Oil Catch Can (OCC). The pcv hose that toyota put on the block it too low and too short. Engine oil is pushed into the intake plenum, where it builds up, and then sucked into the cylinders upon start up. This is bad! Crankcase pressure is the engine’s hot air being produced by the friction during the combustion cycle. Higher mileage prius engines experience blowby because when rings wear out, they cause the combustion gases(fuel) to flow around the rings and increase’s that crankcase pressure x2. This blowby pushes oil, water(condensation) and fuel into the intake plenum from the pcv. If anyone has taken apart the throttle body and seen the oil, it is because of the blowby. During the cold/humid months you will notice that the intake plenum has white/milky oil, this is caused by condensation contamination. Oil and water in the combustion chamber on colder/humid month start ups is were people are experiencing this engine knock/pre ignition. An Oil Catch Can will filter out the majority of oil/water and prevent the cylinder from knocking. I change/check the OCC every oil change on the 2010 prius and find about two table spoons+ in 10k miles. During the cold/humid seasons, I would highly encourage you to check the OCC weekly due to more moisture buildup in the crankcase. In the winter, water in the OCC will freeze and stop the OCC from doing it's job. Our 2010 prius has not had a cold/humid knock start since the implementation of this OCC. It has been on the prius since 80k and now has 107k miles (1year1/2). This investment can cost as little as 15-150$, some are way over priced obviously. The 2010 is using a ball jar, because it was an experiment. Other car manufacturer's have thier pcv valve’s on top of the valve cover. Like my Ridgeline. These engines do not push as much engine oil into the intake/cylinder's because it is so high on the motor. The 2010 prius has two crankcase vent's, one on the valve cover(no pcv) and one in the middle of the engine block(with pcv). The naturally aspirated motor (NA) runs @ 14:1 compression ratio with computer making micro adjustments for smooth operation. 14 parts air and 1 part fuel. When fuel/oil/water are introduced to the air ratio, ignition is not stoichiometric. When I rebuilt the 2011 prius' engine I did not see any oil/condensation build up on the valve cover crankcase vent. I did however found oil sludge buildup (not carbon) on the pcv hose and bottom of the intake plenum. I will show you the two OCC on the prius’s we have. I know that the toyota has done nothing to solve this problem. The tsb was a temp fix because the intake plenum was clean/new. These are my findings to why the prius has a rough idle problem. If your car has a blown head gasket and or clogged egr or any other problems, well that is in other threads. Million dollar question is, Could this engine knocking problem lead to a BLOWN HEAD GASKET??? HELL TO THE YES So could a lot of other things. 2011 Prius Oil Catch Can OCC found on amazon/eBay. The 3/8 hose is from Home Depot/Lowes. The mounts are 3d printed by me.