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Old 12-29-2019, 09:37 PM
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Join Date: May 2006
Location: Ann Arbor, MI
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strange star diagnostics compression test

My GL350 is misfiring and shaking badly. I can't get it to move when I put it in gear. I took it to the dealer and they ran the Star diagnostics Compression test. I just want to confirm these are results that make sense, or if there was some sort of potential issue with how they ran it (i.e. without a charger on the battery). They indicated to me that this test shows that cylinders 2 and 3 have very low compression. My GL350 has 88k miles.

I have codes of excessive egr gases, and misfires on cylinders 4, 5 and 6.

Here are the results:

Results:
Cylinder 1 0 min-1
Cylinder 2 368 min-1
Cylinder 3 496 min-1
Cylinder 4 0 min-1
Cylinder 5 0 min-1
Cylinder 6 0 min-1

Questions:
1. Shouldn't there be an above zero value for all cylinders?
2. Which sensor tracks the rpms when this test is done? I want to make sure that sensor is actually good and wouldn't be the reason the test failed and potentially is the cause for the misfiring and lack of power.

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Old 12-30-2019, 12:01 PM
ILUVMILS's Avatar
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Join Date: Sep 2002
Posts: 3,067
The next step should be to verify the results by performing a mechanical compression test.....
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  #3  
Old 12-30-2019, 07:59 PM
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The test results are mean deviation from the average. The two cylinders with the large deviation indicate that the speed of the engine changes significantly during their respective compression stroke. This is determined through the crank position sensor. Were it bad the engine wouldn't run at all.

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