So, is a compression test the best way to be certain of the head gasket?

A compression test is a good test, if you have the right tools for this engine.

Another way is to pressure test the cooling system.
Hold it at 16 psi for at least 10 min.
Then, remove all 4 glow plugs and unplug all 4 injectors.
Crank the engine.
If water comes shooting out the glow plugs holes, you have a head gasket leak (or a cracked head).

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U.S. Army Retired

I have an OME used turbo that was functioning fine and used GDE Hot Tune for several years till I replaced with the GDE Turbo. I would be willing to sell it for a reasonable offer. Vehicle had 150,000 miles.

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05 Blu LIM, OME GDE Tbo, wk II 245/75/17, KC Lights, bull bar, 195K H TC
05 Blu Lim, Dayton, GDE HT, 255/75/18 , 210K , H TC
06 Bla Lim, GDE Eco, Destination AT 245/75/17, 151K, H TC
06 D Khaki Lim 126K wkII Eur TC
05 D khaki Lim 145k refurbishing
All CRDS: Fumoto, Lift , Fan Shroud mod, fuel head Gen II, SAMCOS,self TB, 2 Mic filt, Hayden