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This video suggests removing the oil cap when the engine is running and checking for the amount of air that is coming from the oil fill hole. (They call this air "blow-by".) He says that if the air is strong, that might mean that the engine is bad.

However, I did this same test on a 2020 Corolla and there was a really strong wind coming, stronger than any of the cars that I have checked out. Most of the cars that I have checked out were 2000 to 2010. Granted, the 2020 Corolla was a rental car hence, it might not have been taken good care of, but I don't think that a Corolla can go bad in less than a year, even if it wasn't taken good care of. This made me question the validity of this oil cap wind test.

So what is it? Does it make any sense to check under the oil cap for air/wind coming when the engine is running, or is this a completely useless test?

  • @Criggie Actually I'm looking at used cars 2000 - 2010. – Utku Oct 28 '19 at 11:03
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It's just an indicator. All vehicles have some amount of blow-by. Since the vehicle you were looking at is probably a brand new car with very few miles on it, it might be the engine is not completely broken in yet, which might allow for more blow-by. There are a lot of reasons why this may be happening. It might be something in the design and continue to happen for the life of the car. It might be the engine is bad. You don't know. There are other ways to know if the engine is bad, such as oil usage, doing tests like leak down and compression (wet/dry), oil pressure, etc. Don't rely on just one very subjective test to tell you if the engine is bad or not.

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I'd also wonder if this rather traditional/old fashioned test is entirely applicable to newer vehicles. There's a lot of changes that have happened in the cylinder head (where the oil filler is usually, though not always located) down the decades which might make noticeable air movement more likely than it used to be... depending how old the advice is, that may even extend to simply having an overhead cam vs pushrods, but also stuff like VVT and EGR could have an influence.

(The theory behind it, of course, is that either your valves, and particularly valve stem seals/collets, or your headgasket or piston rings, may not be sealing quite as well as they should, and are allowing gases to escape from the combustion chamber into the head either directly (valve side) or via the crankcase/sump or other parts of the oil system (rings, gasket). However, I'm still a bit wary of the validity even of that, given that I've had several vehicles - mainly older ones, at least where it was obvious, probably down to emissions regs? - which had specific "breather pipes" connected to certain parts of the engine intended to equalise pressures between e.g. the crankcase and head, and that could cause issues if they were clogged with dirty oil residue... those would seem to be something that could cause the mentioned "blow-by" symptoms just by operating as the manufacturer intended)

Anyway, in terms of whether it's a useful test ... I don't think I've ever been recommended to carry it out before. I would expect it's something you'd only ever be driven to do if you already suspected there was something wrong with your engine or were having trouble with it, like very hard starting/poor idling, losing power/misfiring, bad fuel economy, funky smells/fumes in the cabin, excessive oil use, funny noises etc. And for the most part, unless you were absolutely stuck without either the proper tools to do better and more direct tests yourself, or access to a garage/mechanic/friend or family member who may have them, it's a bit of a waste of time and nothing more than a bit of minor contributory evidence towards trying to decide between one of several different possible causes. For the sort of issues this test may suggest, I'd much rather cut the crap and just hook up a proper cylinder compression tester and see if all the engine cylinders are reaching a good, consistent (within themselves and between each other) internal pressure, both when warm and cold. If they manage that without needing too many turns of the crank, then you're unlikely to be suffering much blow-by regardless of cause, and need to look for some other reason for the apparent problems. If you do have a leaky cylinder, a compression test should point it out within a matter of moments.

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As you are aware, since you tagged your question with it, most (if not all) modern engines have positive crankcase ventilation, which under normal conditions, maintains a lower pressure inside the crankcase relative to the atmosphere. If anything, you should feel a slight suction when you remove the oil filler cap.

If you feel pressure instead, the first thing to check is to see whether the PCV valve is clogged (which is an easy fix). Only after that is verified do you need to start worrying about excessive blow-by.

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  • Never felt any suction on oil fill hole in any of the cars that I've checked out honestly. Just the wind blowing. Some more strong, some less strong. The blow (instead of suction) was evident by getting oil on my hand. – Utku Oct 28 '19 at 17:25
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    You're probably getting oil on your hand flipped up by the rocker arms, @Utku, not by so much air coming out of the valve cover that it's lifting oil out the fill hole. That kind of pressure would be blowing valve cover gaskets, oil pan gaskets, and various seals out all the time. Wind felt underhood could just as easily be coming from a fan as inside the engine; sometimes it's hard to tell with the air currents going around so many shapes, especially when the engine and radiator air is also hot. – dannysauer Oct 28 '19 at 19:43
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    Either way, engine vacuum isn't all that high (and is moving relatively minimal volume) at idle, so crankcase pressure is probably higher due to the pressure not being evacuated. So if there's a time to feel some pressure, idle would be a good candidate. – dannysauer Oct 28 '19 at 19:47

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