What causes an engine to burn oil?

I have always wondered.....plus my g/f's oil is always really black at regular oil change intervals. Her engine has to be burning oil somewhere, but how and why?

Is it actually losing oil? Just because it's black, that doesn't mean it's being burned. There's a bunch of dirt and stuff that gets into the oil and makes it black. Mine is always black and I know it's not burning any oil.

yep, old seals also tend to stain the oil black as they desintegrate and high-rpm driving can cook it too

One thing that really causes black oil is a rich fuel mixture. If you notice your gas mileage isn't what it used to be, there's your answer to the black oil.

I've got about 5000 miles on my oil and and it's just starting to turn darkish brown -- but still quite clean, so if it's turning black after 3000 then something is probably wrong. Worn piston rings, bad valve seals, or burnt valves are a couple of things that will cause burning oil

:)

Most of what turns the oil black is soot. Check the oil on a diesel engine sometime. It gets past the piston rings in the blow-by. The particles are too small to be caught by a normal oil filter. (There are dual filter setups that can actually catch this)

It is pretty much harmless. You cannot really gauge an oil's condition by it's color. (Remember my diesel comment)

Most cars burn oil because it slips past the piston rings, or leaks past valve stem seals. ALL cars burn some oil, just usually in small enough amounts that you don't really notice. My exhaust is totally clean and spark plugs do not foul, but due to the fact that I run 10,000 miles between oil changes I will usually have to add a half quart or so every 3,000 miles.

black oil doesn't mean bad oil ! :D

MIKE

Sorry I have to bring this thread back to life, I just have a question regarding to what I was searching on and bare with me with such a simple question:

What is the difference between burning oil and if ones valve cover gasket is causing a slight leak that runs downwards on the block causing some actual burnage of the leaking oil in that area and causing a bit of smoke from the engine bay due to the burning? Is the term "burning oil" the same as what I had just described?

Thanks :)

DBMaster is right on about burning oil. It is almost entirely blow by that causes blackening of oil. It happens on new engines as well. Oil also slips past the rings and gets burned. If it didn't, your pistons / rings would seize. The thin film that's left in the combustion chamber gets burned on each power stroke of the engine. Blow by is burned gas being forced into the crank case during detonation. That contaminates the oil as well as the burned oil left on the cylinder walls. To answer hkplayser's question what was described above is oil burning from inside the engine. Leaking oil that drips on your exhaust manifold is external oil burning caused by a leak in a seal or valve cover gasket.

Thanks blkblurr. So if there is a leak of oil that is drippnig onto the exhaust manifold causing oil to be burned by a leak in a seal or valve cover gasket, is it anything serious to take care of immediately? I guess that is what I am experiencing at the moment and smoke occassionally rises out of the hood due to the burning of the oil externally. The smell is tolerable when I go to the hood to smell but of course its burnt oil smell (yey).

(Funny thing is, the vcg was changed to stop the leak. It stopped most of the leak and just the burning externally now rather than the whole drip to the ground in puddles type dealie) :rolleyes:

The burning externally could be coming from leakes in your oil pan gasket. Mine did that before I changed it. It's not serious. Just annoying.