So I tried to do a compression test on my 6.9 for the first time and found I need some clarification. When people say cranks, do they mean compressions or turns of the crankshaft? On other words, would four cranks be four compressions or would it be two compressions?

Also, how many compressions do you all do when compression testing an engine?

As for the results I got, I only did half the cylinders (1,2,7,8), at four compressions most of them went to 450psi (#2 was only 400), at 5 most were at 470-480 (#2 was at 420)-430 and at six or seven most were at 490 or 500 while #2 was at 450. This was with a warm engine but only the glow plug being tested removed.

 

i go by compression strokes. 400 psi in 4 compression strokes is a healthy engine. the fact that the others were 450 is not a real big deal, the general rule of thumb is + or - 10%.
since it went up to 450 while the others were at 490-500, that puts you in the solid 10% rating.

 

While like tom says thats within spec, there is something different going on in that cylinder. Was this warm or cold? On a warm engine i would think you should see peak compression after 3-4 revolutions.

IE i compression tested my datsun, got like 75 , then 90 then 95 without going higher with more cranking.

Did you pull the glow plugs or injectors of all cylinders?

 

All cylinders I tested were increasing up through seven or eight strokes. This was on a warm engine but with all glow plugs still in except the one I was testing (obviously it couldn't have still been in on that one ).

 

Ok, that may be part of it, obviously going to crank over slower when compressing all 8 cylinders vs just the one.

For future reference, compression testing should be done with all the glow plugs removed.