I have a 1970 220D that is chewing through glowplugs, all manufacturers of loop and pencil style, like crazy. I've tried reaming out the glow plug holes of any carbon, replacing the prechambers, installed new injectors but nothing has fixed it.
At first it was just the loop filament breaking off (1st picture) at the tip so i figured it was probably just a bad spray pattern and replaced the injectors. Now it's escalated to the point where the entire half of the glow plug that sits in the prechamber will be broken off (2nd picture). I know there's no foreign objects in the prechambers to smash the plugs apart so I'm at a loss here.
Any help is appreciated.

 

What ForcedInductions said. Also if the Glow Plugs remain on all of the time while the Engine is running.

One pic is part of the Mercedes Manual the other is a pic of an Injector with a bad Nozzle on it. If the Injector Spray gets too close to the Glow Plug Tip it will burn it off eventually.

 

Thanks for the replies. I've checked the emergency shut off valve and it's wide open, the muffler is fine and the throttle linkage bushings are nice and tight. I'll check the timing of the injection and cam. I have yet to find a way to get my injectors testsed, I'll keep searching around town.

 

The Injector Opening/Pop Pressure new is 115-123 Bar. Minimun suervicable is 100 bar.

The differeance beween the highest and lowest Injector Openin/Pop Pressure in the set of Injectors is 5 Bar.
If you want to know what that is in psi (pounds per square inch) you can use this conversion site; use the pressure conversion calculator.
Worldwide Wholesale Distribution of Valves, Fittings, Tubing & Flanges: World Wide Metric

If you are in a Rural Area Tractor and Farm Impliment Repair Shops may have the Injcctor Tester and someone that knows how to use it.
Fuel Injection Shops would also have one.

Some of the other Mercedes forums have people that will rebuild your Injectors with New Nozzles and set the opening pressures.