You should be able to remove glow plugs easily. They should only be tightened to somewhere around 7 to 10 Nm for an M8 glow plug. But, if there’s corrosion, carbon build up, or they have been overtightened, you may have issues?
Apply some penetrating oil to the glow plugs. Leaving it to soak in a few days before you intend to remove them. Just before you do attempt to remove them, try and drip some oil onto the glow plug threads. This is to help lubricate the threads when you unscrew them.
Try removing the glow plugs when the engine is warm. Heat is an excellent aid to removing tight or stuck objects. Take precautions to ensure you don’t burn yourself though…
Using a torque wrench to undo glow plugs, may be an idea… You can set the torque wrench to the ‘shear value’ of the glow plugs. Unfortunately, the shear values don’t appear to be readily available on most of the manufacturers websites..?
So what can we do? (this method is at your own risk!)
The torque values for tightening are readily available. So the shear value should only be slightly more, (other wise glow plugs would be breaking off all the time on installation).
For example: If the installation value of the glow plug is 10 Nm.
There’s a bit of trial and error here, but you can probably see the logic. Whatever you do, don’t go mad… and be careful. Don’t go way past the original torque installation value, otherwise they will break off!
When replacing the glow plugs, make sure the threads are clean and there is no damage. Bear in mind the contact of the thread with the head of the vehicle, may form part of the glow plugs electrical circuit. You could use some copper grease to aid insertion and removal (for next time).
When inserting, hand start the thread to ensure there are no issues, and to prevent cross threading. Always use a torque wrench when tightening the glow plugs. Follow the suppliers recommendations, (which is often written on the side of the packaging).
More detailed information can be found on this page: Diesel Won’t Start When Cold – How to Fix Glow Plugs: (Opens in a new window / tab)