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.
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Jason wrote:

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