I've got the same problem with my 87 300D. How do I check the ohm reading for each glow plug at the relay? I've found the relay and I have a good volt meter. . . While the intake manifold is off, is there anything that I should replace in addition to the glow plugs? Also, I see there are two filters for the intake manifold. Do I need both of them and how do I make sure they are OEM?