Pull the glow plug and inspect it.
1) If bubbles are due to a fluid outside the engine (Leaky injector line, turbo, etc.), I would pull the GP, test the resistance to confirm the GP is good, cover the threads with anti-seize , and put it back in. Should seal it up.
2) If the bubbling is from diesel coming up from the pre-cup, I would also pull the injector and test it. Also test the compression. Diesel could be coming up the threads from the compression stroke but I would want to verify the cylinder, valves, and injector are good.
3) If engine oil do a leak down test for a bad set of rings or head gasket. If not caused by a head gasket or valve guide seal, not much you can do but rebuild the engine.
4) If coolant, pull the head and inspect for cause. Head gasket, cracked head, cracked block, or cavitation pin holes in the cylinder. But there should not be coolant in the cylinder or pre-cup.
Add some pics and let us know what you find.