| |
Jim Grant's Tech Tips
95
Plymouth Neon, Suspicious Oil Disappearance
Q: I have a ‘95 Plymouth Neon which is eating
2.5 qts of oil in 900 miles. I am the original owner of the car and have
had no crashes or accidents. There is no white smoke or smell of burning
oil. No suspicious puddles on the ground. The Chrysler techs did an engine
compression test which was ok and they placed dye in the oil twice to
find the leak, but no one can find it. The Chrysler techs suggested tearing
the engine down and rebuilding it, but didn't hold much hope of finding
the problem. I called Chrysler Corp. and they said that a car over 70,000
miles normally uses up to this amount of oil !!! And finally, why would
a car that almost replaces the full amount of oil twice in 2700 miles
need an oil change? Help please or send me a pair of fabulous walking
shoes...Diana
A: I
can tell you about brake shoes but not much about walking shoes. Why change
the oil in an engine that seems to be doing just that on its own? When
an engine is consuming oil in the manner that you describe, it's using
the good part of the oil and leaving the junk behind. The oil filter can
only deal with so much junk before it will not be a filter any more. The
result will be increased engine wear and shortened engine life.
As for the
oil consumption, it's most likely a sticking or weak scraper and/or oil
control ring on the piston. Why doesn't a bad scraper or oil ring show
up during a compression test? Because these parts don't have much involvement
in sealing the cylinder. That's the job of the top compression ring. Oil
control is performed by the two rings located below the compression ring.
If the compression ring is doing its job compression will test great but
the engine can still burn oil. Why don't you see blue smoke bellowing
out the exhaust? Well there are a couple of reasons. The scraper and oil
ring really don't have to work until the engine RPM is up, like at highway
speed. At highway speed the wind inside the engine, caused by the spinning
parts, is like a hurricane. This hurricane sends oil flying as well. That
is when the scraper and oil rings have to really get down to business.
If they don't, oil moves past and into the cylinder where it is burned.
The compression ring has no oil control ability so it's no help at all.
Then there's the catalytic converter. At highway speed the converter is
cooking and it has no problem toasting the smoke made by the oil. The
end result is little to no blue smoke out the exhaust to be seen. One
other note, it is not common for an engine in today's vehicles to use
that much oil when the engine has 70k miles on it. Someone just doesn't
want to deal with your problem. They're feeding you a bone, hoping/thinking
you'll go walking.
|
|
