Why do car engines smoke?

If you are seeing smoke coming from your exhaust, what color is the smoke?

If it’s blue, then it’s oil
If it’s black, it means you are running rich (too much fuel).
If it’s white, the car may be burning antifreeze or (quite rare) auto-trans fluid.
Since it’s blue smoke you are seeing, you can know that oil is the problem. What can cause oil smoke?


If the smoke only comes at start-up and quickly goes away, it can be valve seals and/or valve guides. This is because while the car sits for extended periods of time, the oil has time to seep past the valve seal and collect on top of the valve (or if the valve is in the open position, it could run past and on top of the piston). When you go to start your car, the oil is then burned, giving the tell tale puff of blue smoke.


If you see smcar smokeoke as you are decelerating, this too is a possible sign of valve seals and/or valve guides. This is because as you decelerate, there is a large vacuum build up within the intake tract. There is enough vacuum to pull oil past the valve seal if it’s worn.


If you see smoke while accelerating, this is a sign your oil control rings are worn. These are the bottom rings used in the ring pack. When they are worn (or the cylinders are worn past tolerance), oil can flow past the rings as the piston travels down the cylinder. The oil control rings normally will scrape the cylinder of the oil, pushing it back down into the crank case.


If a PCV is bad, you normally won’t see burning oil. What you’ll see is seals and gaskets failing. This can cause a loss of oil (and a very dirty engine bay). It’s one of those things which can sneak up on you if you aren’t paying attention. While a bad PCV is not a good thing, don’t look here for a reason you’re seeing oil smoke coming out the tail pipe.


If you are seeing blue smoke which goes away after an oil change, but slowly comes back as you get closer to your scheduled maintenance, this could be a sign you have an internal fuel leak which is thinning the oil. As the oil gets thinner it passes by the oil control rings easier, causing your vehicle to smoke. An easy way to check this is by pulling the dipstick and smelling the oil. If you smell fuel, this may be the issue. This can be caused by an injector which is stuck open, or possibly an internal failure of a fuel pressure valve which would allow the gas to escape to where it doesn’t belong.


If you are not seeing smoke, but you’re seeing the oil drop lower and lower as time moves on, this may be a “sort of” normal oil usage in your vehicle.  As these engines get up in age, it is common for them to use more oil. You don’t see it from the exhaust, but it uses it up, none the less. The engine should run great and the gas mileage has stayed about the same.

Your vehicle could also be losing oil via leakage (or seepage). As engines get up in mileage, this is a very common thing. If you see spots on the driveway where you park the car, this could very well be the issue. Be mindful that vehicles will loose more oil during operation than they will just sitting in your driveway.



Source: Automedics

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