Compression tests

It is like any other test. Like an EKG. It takes somebody that knows what
they are doing in order to interpret the results correctly. Depending on
whether overall compression is low or just an isolated cylinder or two would
determine the fix.

Pertaining to the other poster I have never heard of a running compression
test. In as much as connecting a psi gauge to a cylinder and running the
engine minus that spark plug. If indeed this is the case that makes no
sense because you would have a misfire with that cylinder and it would set
off codes in the computer that would need to be reset. Mostly compression
tests are just done while cranking the engine over and disabling the engine
so that it temporarily doesn't fire.

CaptainKrunch.

 

What is the point of a compression test? I know what it measures, but if you
get a poor compression ratio, how do you fix it?

 

A cranking compression test is a measure of whether the cylinder is
operating normally. If the cylinder can't compress air, it won't
produce power. A running compression test (idle and 2000 RPM)
sometimes helps point to camshaft and valve problems. All this
combined with a cylinder leakdown test allow you to identify or rule
out engine problems.

The fix for low compression can be valves, rings, engine replacement..


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N38.6 W121.4

 

May be when you want to buy a used car?

 

I always check compression when buying a new car. If a 4 cyl, I'll check all
of them. A 6 or 8, I would do enough to satisfy me that there is not a high
"range".

 

Compression 1 2 3
4
Cranking 178 175 170
175
Idle 60 60 40
60
2000 RPM 40 40 20
20

May indicate valve or flat camshaft problem on #3.

In conjunction with a cylinder leakdown test, is very helpful for
identifying problems.

You may not be able to peform running compression tests on all
vehicles. Especially OBD-II which will catch the misfire, unless you
can disable the individual cylinder.

Only on OBD-II Lots of pre-OBD-II cars out there which won't bat an
eyelash on a running compression test.

And disconnecting power to the fuel injectors.

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N38.6 W121.4

 

Excellent post--I was going to say some of these same things--but you done
it better and included more details than I would have included. Keep up
the great work.