bursts of air from engine

[merc007]

Hi, I am uneducated diesel head. Need help with my 2000 ml270 cdi. Was working on the engine today cleaning and servicing. Put all back together, apart from engine cover and started it. Then I heard a pop noise from the top of the engine. When I went to investigate, there was tiny bursts of air coming from the side of the first glow plug (spark plug) where it meets the engine. There is a black thick silcone like substance across the base of all 4 plugs. Question 1, whats is wrong? Question 2, was that some sort of damage and someone tried to seal it with something and put oil over it to disquise the damage and patch up job????
Car is running fine.

cheers.
ps, sorry about my terminology, as i said uneducated.

 

[Dieselman]

Are you sure these aren't the injectors, if so the sealing rings are U/s and there is compression blow-by, hence the black crud.

 

[renault12ts]

As above, an injector seal has gone. The black stuff is the product of this...a tar like substance, which is rock hard when the engine is cold. The injector will have to be removed and a new seal inserted.

But the black stuff (referred to as black death) will have to be removed first. Best done with a flat blade screwdriver...on a cold engine.