Easy test for worn rings

When I spent time working with a certified mechanic, I was taught
that the easiest way to determine if an engine had worn rings was
to do a compression check.

While reading a Tom and Ray column in the local newspaper, I learned
an even easier method of checking for worn rings.

I now quote from the last portion of the column:

Tom: However, what you CAN tell by removing the oil
cap [while the engine is running] is whether the
car has excessive blowby. Blowby is created when
the rings wear out, and lots of combustion gasses
sneak by the rings and into the crackcase, where
the oil is stored.

Ray: If there is more combustion gasses in the
crankcase than the ventilation system can expel,
pressure buildsup in there. When you remove the
oil cap, you might see smoke coming out.

Tom: That'd be a sure sign that you'd want to
avoid buying that car.

My comment: A compression check is far superior to
the easy test mentioned above. It's my guess that if
the rings were only slighltly worn--that you would
not see any smoke when you removed the oil cap.

 

the blowby test is not quantitative, but it is a very good qualitiative
test. compression reveals rings and/or gasket and/or valve problems.
only blowby indicates rings alone.

 

Owning an '80s Volvo leads me to add a caveat to the Car Talk method: that
vintage has a valveless crankcase ventilation system. Regardless of how
little or how much blow-by the engine has, the crankcase pressure check only
reveals whether the crankcase ventilation is working right.

As you say, a compression test is far better.

Mike

 

On Fri, 19 Aug 2005 20:27:07 -0700, "Michael Pardee"

If the engine fails the compression test, there is a simple (shade
tree mechanic) way to distinguish between worn rings (BIG expense) and
possible worn or burned valves (much simpler and less costly rebuild
head). Remove the spark plugs on the bad cylinder(s) and squirt some
heavy weight oil right into each bad cylinder through the spark plug
hole. Replace the plug, then measure the compression again (by
turning the engine over with the starter). The heavy weight oil will
provide a temporary seal between the piston and the cylinder walls. If
the compression goes up, the rings are shot and the engine needs a
rebuild. if the compression does not go up, there is a CHANCE only the
valves are bad and a head rebuild may be all it needs.

In modern engines, the valves are the most likely source of low
compression, assuming the engine has not been run dry at some point.


Elliot Richmond
Freelance Science Writer and Editor