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Old 05-03-2021, 01:32 PM   #4
142 guy
Board Member
Join Date: May 2014
Location: Saskatchewan, Canada

The difference in the test results in a car with a conventional throttle will likely be in how quickly the cylinder reaches its final test pressure. The throttle plate creates a restriction in the intake which drops the intake manifold pressure. However, on older EFI cars with conventional throttles, they typically have an external idle air control valve and some of those valves are open when they are unpowered (Bosch 2 wire valves) and will admit enough air to permit a compression test. DBW (E throttle) throttles typically do not have an external idle air valve and rely on the DBW system to hold the throttle open to control idle speed. If the DBW throttle is completely closed during the test you will end up with significantly lower pressures in the intake manifold during the test which will screw up the results - like doing a compression test at the top of Pikes Peak. I have an Acura with a DBW throttle and the service manual is explicit that the throttle must be propped open for the compression test.

The theory of back flow through the other cylinders will not work. In order to get back flow through the other cylinders you would need less than atmospheric pressure in the intake manifold which corrupts your test results. Compression test results are always based upon having atmospheric pressure in the intake manifold. Factory test specs are based upon sea level so you also need to correct for test results at higher altitude.

Check your service manual to find out the recommendations for the compression test procedure. The system may have an auxiliary air by-pass that is sufficient to allow the compression test or it may have a special procedure for electrically holding the throttle plate open for the test - something more special than my Acura's pull the rubber intake boot off and jam a stick in there.
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