how many hours are on the motor could be leaky injector or dried valves, wore rings. i would try the balloon trick first and see if its an injector its pretty easy to do remove the left side engine cover get some small elastics and a couple balloons. remove your injector lines at your injector then put a ballooon over the end of each injector then use the elastic to wrap around a couple times to keep it on there can use small hose clamps to then roll your motor over a couple times. if any of the balloons fill up at all the injector needs to be rebuilt it not sealing properly hope this helps. if that doesnt work you can pull a glow plug out take it to lowe's, home depot and find your thread size and and make a custom fitting to go in where the glow plugs do and test each cylinder and see what your compression is. also well machine is running look in your rad once you start it and rev it a couple times and see if you see any bubbleing in your rad check when hot to give it a couple revs, if there is bubbleing follow through with the compression test you have a bad head gasket or you dont even wanna think of this one possible a slight hair line crack in your block i could imagine it being a crack it usually a lot worse hope all this helped you