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I thought they were the same with potential difference being the fittings, but in my Haynes manual they are listed and shown as two different tools.

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I guess it really depends on how you look at it, but yes, they are two different specific tools.

They are the same in the respect they measure pressures. They are different in that one measures compressed air and the other measures liquid pressure. The ranges they each measure is different, too. The oil pressure gauge range will have a top end of (probably) a maximum of 100psi. The compression tester will max out somewhere around 180psi for gas engines and could be up over 300psi for diesel engines (thanks for the add Solar Mike).

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  • compression gauges for diesel engines need to go to above 300psi and some above 550psi (some VW 1.9 TDI engines). Good answer though.
    – Solar Mike
    Jun 4, 2017 at 12:25
  • @SolarMike - Good point! I was thinking more on the lines of gas engines, but you're right about the need for higher compression ranges for diesel engines.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jun 4, 2017 at 12:30
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A compression gage has a one way valve to hold the pressure between strokes. When you use it you will see the pressure rise incrementally over several engine revs, and after the check there is button to push to release the pressure from the gage.

Note the one way valve is actually in the fitting so you probably could remove it if you wanted to re-purpose the gage.

Note also that check valve is essential, if you try to build your own compression gage without one you will not get a proper reading.

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  • To check running compression you need to remove the schraeder valve.
    – Ben
    Jun 4, 2017 at 12:54
  • @ben whats that have to do with the question? running compression is such an odd thing to do i wouldnt even give it a footnote. Surely gearheads doing that are not learning about it from stack exchange.
    – agentp
    Jun 4, 2017 at 13:18
  • to clarify "Note also that check valve is essential, if you try to build your own compression gage without one you will not get a proper reading."
    – Ben
    Jun 4, 2017 at 13:19
  • imo running compression is more important. easier with a dso and a pressure transducer, but it tells you more about what's happening on the compression stroke. max compression with the valve in place only tells you that. not whether the cylinder can maintain a pressure over a number of revolutions.
    – Ben
    Jun 7, 2017 at 11:03
  • @Ben Thanks for naming the schraeder valve, I didn't know what it was and it removing it gave a better result on the compression gauge.
    – OrigamiEye
    Jun 13, 2017 at 14:09

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