He's asking what the reason for them being covered in oil is, not how to test them
. Generally speaking your glow plugs should be BONE DRY, and at most, have a slight coating of dark black soot (carbon) around the very tip, which is the sign of a healthy engine. They should not, by any means be wet, and definitely not oily. The only liquid that should (in theory) be entering the cylinders, is diesel and only via the injectors. Oily glow plugs mean that there is oil getting into the cylinders, which is not what you want for a few reasons; 1) the engine will not perform optimally, 2) it could possibly lead to engine failure. Causes of this problem can be a few things, ranging from worn piston rings, leaking valve seals or a blown head gasket, even split piston crowns in some cases.
Before you start to worry yourself however, I'd suggest that you start with the simpler things. First of all, disconnect the hose between the intercooler and the air intake manifold and peep inside the hole
o). Is the inside of the air intake manifold blathered in oil? Also check the air filter. If it's blathered in oil too, that wont be helping things, so swap it. I'd also suggest that you remove and clean the cyclone breather (here's a guide -
http://www.landyzone.co.uk/lz/f41/cyclone-breather-76543.html) to ensure that the pipe between the cyclone and the air intake isn't drawing in pure oil, should the cyclone breather be gunked up.
If there is no significant sign of oil being 'sucked' into the engine, or forced into the intake manifold via the turbo, then you need to take it a step further and remove the head to inspect a couple of things, but we'll get to that if you need it.
One last thing that you can check is the following. Start the engine and get it up to its usual running temperature. Then, have a friend sit in the cab ready to accelerate when you need them to. Start by removing the oil filler cap. Place your hand over the opening and then ask said friend to rev the engine hard. Can you hear or feel any chuffing? Replace the filler cap and then move around the passenger side of the engine. Remove the oil level dipstick and press your thumb firmly over the opening so that it creates a seal. Have your friend hard rev the engine for five seconds, and then quickly release your thumb. Does the dipstick tube release any pressure (creating a hissing sound) or launch oil at you? If either of the above symptoms are apparent, you may well have a blown head gasket (which would explain the oil in the cylinders, oily glow plugs, blue smoke and struggle to start the engine). It could also be a couple of other things (as mentioned earlier) like worn piston rings, split piston crowns, worn oil seals or in the worst case scenario, a cracked cylinder block or cylinder head.
-Pos