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Too much oil in the crankcase not the cause of this blow-by

Too much oil in the crankcase not the cause of this blow-by

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Dear Car Talk:

I have a 2015 Nissan Frontier. It only has 57,000 miles and does not leak or burn oil, to my knowledge. I take it to a reputable national oil change place every 3,000 miles for oil and filter. I have never checked the dipstick right after the oil change, as I assume that is being done professionally.

They now tell me I have “blow-by” on my air filter. I looked in my truck’s owner’s manual, and it said my vehicle takes 5.3 quarts. The past two repair slips say they put in 6.1 quarts and 5.8 quarts, respectively.

When I told them this, they drained out a bit of oil for me and gave me a new air filter. But could this “blow-by” be a result of them overfilling the oil, or is it something else? — Bob

For those who aren’t familiar with the term, “blow-by” doesn’t mean what happens when you’re waiting for the bus on a winter morning, and the driver doesn’t see you and blows right by the stop.

Blow-by is when exhaust gasses from inside the cylinders sneak past the piston rings, and get into your crankcase.

A little bit of blow-by takes place in every engine — even a new one. And normally, it’s not a problem. There’s a system called the PCV — positive crank-case ventilation — that regularly removes exhaust gasses from your crankcase by sending them back into cylinders to be burned. But if your PCV system isn’t working properly, those gasses in the crankcase can build up.

And from your experience over a lifetime of eating beans, Bob, I’m sure you know what eventually happens when gasses build up in your combustion chamber.

That’s right, they find their way out! And in the case of blow-by, they push back through the air intake, and contaminate your air filter (which, I guess, is kind of like your car’s BVD system).

So if you’re seeing extensive oil on your air filter, the most likely cause is a stuck PCV valve — especially on a reasonably young engine like yours.

Overfilling the crankcase by half a quart or so is nowhere near enough to cause this problem.

On older engines, excessive blow-by can be caused by piston rings that are all worn out and let too much stuff get by them. And that’s a much more serious, and expensive, problem. But I think that’s unlikely in your case.

A new PCV valve for your truck costs about $10 online. And it’s really easy to change — I think it just sticks into one of the valve covers.

If you’re so inclined, you can buy one, take it out of the box, and then spend a Saturday afternoon trying to find a part that looks just like it on the top of your engine, and swap it out.

Or just let your mechanic replace it for you. Then, since you have a new air filter, at your next oil change, you’ll know whether that solved your blow-by problem. I’ll bet it does, Bob.

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