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Mechanical Compression Gauge Vs Relative Compression

  • John Curtis
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04 Mar 2020 15:59 #38066 by John Curtis
I had intermittent cylinder-specific misfire caused by intermittent low compression due to a bent rod.
Relative compression waveform looked perfect for a random amount of crank revolutions and intermittently would appear low on my misfiring cylinder.

A compression test never showed the low compression and a leak down test did not reveal the problem. I had to crank up the pressure during the leak down which actually gave me direction.

Have you guys seen this before? Without the ability of a scope I’m not sure I would’ve found the problem.

Thinking out loud always helps me in the process.

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23 Mar 2020 15:12 #38569 by JarheadDiagnostics
Replied by JarheadDiagnostics on topic Mechanical Compression Gauge Vs Relative Compression
I ran in to this one a Kia Soul with a 1.6L GDI. The cylinder walls were scratched just below the compression line. So it passed leak down test perfectly.
The following user(s) said Thank You: Noah

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  • Tyler
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24 Mar 2020 21:01 #38600 by Tyler
Been there with Chrysler Pentastar (3.6L) engines. If you did the relative compression test long enough, you'd catch one cylinder acting up.

The newest series of SD Premium talks about this phenomenon, and how a vacuum gauge + compression gauge missed a leaking intake valve. :silly:

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