Off topic question - How to tell why car is burning oil?

767Jockey · #1 (**permalink**) 02-10-2009, 04:48 PM

OK, off topic I know, but there's lots of auto talent on here so I figured I'd give it a shot. Car is a '94 Nissan Sentra, 114, 000 miles, 5 speed manual transmission. Until last week, no smoke at all - never. All of a sudden the engine is sometimes smoking badly through the tailpipe, sometimes not. It's definitely oil, it's blue/grey in color and smells like burned oil. I can't find any pattern to it other than when it does smoke it smokes worse on power application that when you lift off the gas. Could be rings, could be valve seals, what else am I missing? If it's an oil ring I doubt that a compression test would tell, no? What surprised me was the rapid onset, it sort of happened overnight - from no smoke to lots in the snap of a finger. If it's rings the car is junk, it doesn't pay to do a rebuild on it. Any ideas, and way to tell what it is? Anything else you guys can think of other than rings or valve seas and an easy way to check? Thanks!
(Sorry for the non-Cobra topic, but there is so many talented guys on here I though it would be worthwhile to give it a shot.)

Silverback51 · #2 (**permalink**) 02-10-2009, 05:49 PM

It could be the PCV valve.

767Jockey · #3 (**permalink**) 02-10-2009, 06:01 PM

Quote:

Originally Posted by Silverback51

It could be the PCV valve.

Geez, would I love it to be that simple. How would that cause the oil burning? Would that malfunction in a way that it sucked up oil from the crankcase and burned it? That would sure be a quick easy fix.

DAVID GAGNARD · #4 (**permalink**) 02-10-2009, 08:30 PM

Quote:

Originally Posted by 767Jockey

Geez, would I love it to be that simple. How would that cause the oil burning? Would that malfunction in a way that it sucked up oil from the crankcase and burned it? That would sure be a quick easy fix.

Yep, could be that, easy to check.....if it is not that, pull all the plugs and run a compression check, then one at a time, add a couple of squirts of engine oil into each cylinder and run compression check again..... if the compression goes up, it's the rings, if the compression is low and stays low, it's the valves...

David

Ronbo · #5 (**permalink**) 02-10-2009, 08:47 PM

Look those plugs over when you pull them... If only one's fouled you've probably got a stuck ring. (I'd shut it down right after it stops smoking and pull the plugs)

If the PCV valve is pulling oil look in the vacuum line right where it connects to the manifold. If there's more than a slight film the valves bad.

Here's two more possibilities: Cracked block or blown head gasket, these will also cause low compression on the effected cylinder(s).