Firing on All Cylinders? A Running Compression Test is a Great Tool for Diagnosing Density Misfires
BY ALLDATA STAFF
A running compression test or dynamic compression test is an
important, but little-used and often overlooked engine diagnostic
tool. Why is this test so significant? It can be used to pinpoint
the cause of a misfire when all the usual tests return normal
results. It’s also used by performance tuners to verify that each
cylinder is contributing equally to the engine’s overall power
When you perform a static compression or cylinder leak-down test,
you’re checking the sealing capability of each cylinder. A vacuum
test is a great way to determine an engine’s overall breathing
capabilities but does not provide results for individual
cylinders. The advantage of a running compression test is that it
gives you an idea of the volumetric efficiency of each cylinder.
In other words, how efficiently each cylinder is pulling air in,
retaining it for the correct amount of time, then releasing it
into the exhaust. If a cylinder cannot perform these functions
properly, the result can be a loss of volumetric efficiency or a
How to Perform the Test
Create a chart similar to this example to record your test
Begin with a normal (“static”) compression test on a warm
engine to eliminate obvious problems such as badly worn
rings, burnt or bent valves or holes in pistons. If you have
a diagnostic trouble code, you may know which cylinder is
misfiring, but it’s a good idea to test the compression of
all the cylinders in that bank to get a good comparison.
Record your static compression readings.
Install all of the spark plugs except the one for the
WARNING: GROUND THAT PLUG WIRE TO PREVENT MODULE
DAMAGE, OR WHEN EQUIPPED WITH A COIL-ON-PLUG IGNITION, SIMPLY
DISCONNECT THE COIL HARNESS PLUG.
Disconnect the injector for that cylinder on a port fuel
Install your compression tester in the suspected cylinder.
The test can be done with the Schrader valve removed, but
most techs recommend leaving the valve in the gauge and
“burping” the gauge every 5-6 “puffs”.
Start the engine and take an “Idle” reading. Be sure the idle
speed is a consistent 1200 rpm. Record the results.
Now, from 1200 rpm, snap the throttle to 2500 rpm and release
quickly. Reading should rise. Record the results.
NOTE: Don’t use the gas pedal for this snap acceleration. The
idea is to manually open then close throttle as fast as
possible, forcing the engine to take a “gulp” of air.
Sample Test Readings
Comparing measurements between cylinders is important.
Running compression at idle should be 50-75 PSI (about half
cranking compression). Snap throttle compression should be
about 80% of cranking compression. Let’s analyze the results
from the following sample test readings.
In Example 1, cylinder #1 has a snap test
reading that is much lower than the other cylinders. If a snap
reading is low (much less than 80% cranking compression), look
for air intake problems such as severe carbon deposits on intake
valves, worn cam lobe,worn valve guides and springs, rocker or
push rod problems, or “shutter valve” mis-positioned in the
runners of a variable runner intake system.
In Example 2, cylinder #4 has a higher than
normal snap test reading. If a Snap measurement is significantly
higher (over 80% of cranking compression), it means the air is
not leaving the cylinder efficiently. Look for problems on the
exhaust side of that cylinder such as worn cam lobe, bent push
rod or collapsed lifter. If the snap readings are all high, look
for exhaust restrictions such as a clogged catalytic converter or
In Example 3, cylinder #2 has a low Idle and
snap test reading. These types of numbers indicate that the
cylinder is not holding compression efficiently. Look for issues
such as slightly bent or burned valves, excessive carbon build-up
on valves or seats, worn valve guides and springs, scored
cylinder wall, or a leaking head gasket.
As you can see, information gathered from a running compression
test can be very helpful when diagnosing misfires and tuning for
total engine performance. It’s a great test to add to your