I've been reading (I know, that can get dangerous....), that El Cheapo compression testers can give false/bad/error readings on diesel engines. Seems a Snap-On grade compression tester is required to guarantee the results. Thoughts?

The Harbor Freight ones have occasional failures but have several attachments that if bought individually could be $20-$30 each.


The tester depends heavily on Schrader valves; the little valves that go into valve stems on tires. They can be messed up to begin with or the heat of compression can fry them.


The 90 degree Glow Plug attachment on the Harbor Freight ones are fragile and known to break.


Occasionally a Gauge is wonky. In either case you can return them to harbor freight for an exchange or replace the valves and or the Gauge (with a better quality one).


You can look on eBay and get an idea on price as to say a Glow Plug adapter to hook the compression gauge to and the other used good quality ones and other cheapie ones to help make a decision.

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84 300D, 82 Volvo 244Gl Diesel

The 90 deg glow plug adapter can be replaced with a 3/8" JIC 90 deg fitting. I think it was 3/8", would need to verify. Much more skookum fitting and costs five bucks.

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617 swapped Toyota Pickup, 22-24 MPG, 50k miles on swap

The main thing you are looking for is consistency between cylinders. Accuracy of the gauge is handy but if you have a significant difference between cylinders you have a problem.

Compression testing on a diesel is done DRY with fuel lines disconnected. Adding oil to the cylinders to get a wet reading is useless and will give you false readings.

A quick, rough check of your gauge's accuracy can be done using shop air.

ALL gauges, including Snap-On, OTC or any brand, will suffer from some inconsistencies. It is just the nature of the beast. Barometric pressure, humidity, etc. create variances.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
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