We quit doing compression tests for prepurchase inspections many years ago unless specifically asked for.

Making a good judgement on diesel compression is real hard. MB has numbers that are a specification. Something like 320-340psi or so. Almost all gauges I have used have read closer to 400psi on good motors.

Compression gauge readings are used for comparative judgements. Like this year to last year or this cylinder to the next. Absolute readings are based upon a single techs knowledge of his gauge in that motor over many tests. No tech can get everything with a brand new gauge even if he has tested that motor group multiple times with other equipment.

The worse problem for diesels is that they probably have relatively the same compression cyl to cyl till they fail. I have seen huge oil burners with good even compression.

The best judgement of a diesels condition is to know how much oil it is using and how much blow-by it currently has.... another very experience laden judgement call.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician