First thing to do is to check the glowplugs individually. Check the resistance first, but that is not a slam-dunk confirmation of goodness. It is a slam-dunk confirmation of badness, if they are "open" - (i.e. show no resistance)

Then check them with an electrical load. If they show a good resistance and will take a load, then they are good.

You can check voltage from the relay at the connector plug. This should be pretty easy to confirm. Turn the ignition switch to the run position, and check each circuit with a voltmeter.

If you have to change the glow plugs, yes, you should remove the intake.

SteveM

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'93 190E/D 2.5 Turbodiesel 5-speed (daily driver)
'87 190D 2.5 Turbo rustbucket - parts car
'84 Dodge Rampage diesel - Land Speed Record Holder
'13 Ram 2500 Diesel
'05 Toyota 4Runner