Depends on which generation of 5.9, but basically you want to remove the fuel injectors, insert a gauge adapter fitting into the injector bore, hook up compression gauge that is capable of >500 psi at least, and finally bump the starter over for about 10 seconds or so, about 4 or 5 compression strokes and record the highest reading. For the 12 valve 6bt motors that are indirect injected, snap on makes a gauge adapter it's number 3584. For the ISB 24 valve, it is EEPV313A they use standard shraeder valve air fittings.. Other adapters are available. For example, Miller Tools number 9007 works for both the ISB 24 valve with the vp44 pump and the common rail but is designed for the ISB with the common rail. To use it on a vp44 isb you just need to remove the exhaust rocker, remove the 8mm hex head shoulder bolt for the injector hold down, install miller tools 9007 into bore, replace 8mm hex head should bolt with 8mm full thread hold down from another cylinder in addition to the full thread bolt for the cylinder you are checking compression and reinstall the exhaust rocker arm, being sure to not fiddle with the adjustment nut for the lash. Pretty foolproof.
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