Apparently stuff happens even to E320 CDIs.
Stuck Glow Plug Removal - Mercedes-Benz Forum

Google search =Mercedes OM648 with broken glow plugs

http://www.dailymotion.com/video/x27y6wq_mercedes-cdi-e320-diesel-seized-or-stripped-glow-plug-problem_auto
"Cylinder 5 scared me....."
http://mbworld.org/forums/diesel-forum/466006-om648-glow-plugs.html

http://mbworld.org/forums/diesel-forum/466006-om648-glow-plugs.html

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84 300D, 82 Volvo 244Gl Diesel

That is why MB specs. torque to loosen plugs at. To minimize trouble, don't exceed the torque values, use pentrating oil, and work with warm engine.

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-- Chris

'95 E300, 216k miles, Silver Surfer
'05 E320 CDI, 138k miles
'07 S550 4matic, 69k miles

Gone but not forgotten:

'76 300D, 350k miles?, SOLD in 1995
'75 240D, 300k miles, SOLD in 1991

The 648 motor is much better than 606 for changing glow plugs. I've broken off a glow plug on my previous 606, then successfully changed others on the 606, as well as successfully changed GPs on my 648.

The key to success is three things...lots of PB Blaster, warm engine, torque wrench. You don't need to soak it weeks ahead, hours is OK. Engine doesn't need to be hot...warm is OK. Torque wrench is important...don't exceed about 40 Nm of off-torque. If it doesn't break free at 40, stop, spray more PB blaster, wait. repeat. It just takes the discipline to be patient.

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Mark in MA
05 MB E320CDI 402k Granite Grey Metallic
05 MB E320CDI 267k Black
05 MB E320CDI 232k White
05 MB E320CDI 209k Tectite Grey
99 Dodge 2500 Cummins 5sp 148k
62 Jeep CJ-6 120k

Yes, there is a group of methods that reduce the chance of breakage but there is no group of methods that guarantees 100% success.
However, the fact that the Prior Owner changed them in the past is likely to increase the odds of easy removal.

There has even been 2 instances I have read of where people have broke of the Glow Plugs in an OM617.952 and about as many have gotten the Glow Plug body our only to find the Tip/Heater Element was still stuck in the head.
The frequency of Glow Plug breakage during removal of the longer Glow Plugs in Aluminum Cylinder heads on Mercedes is rather common.

I think you have a choice to be pessimistic (I am nearly always pessimistic about things) and plan for the worst by having another ride available or being optimistic and suffering that extreme disappointment and having to scramble to get a ride to work or school during the time it takes to resolve the issue.

Anyway best wishes.

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84 300D, 82 Volvo 244Gl Diesel

What if all the above is done and you did not break the Glow Plug off but still cannot get it out?

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84 300D, 82 Volvo 244Gl Diesel

Be optimistic....most plugs will come out....unless a gorilla over torqued them before. Remember to torque the new ones in to spec. as well.

__________________
-- Chris

'95 E300, 216k miles, Silver Surfer
'05 E320 CDI, 138k miles
'07 S550 4matic, 69k miles

Gone but not forgotten:

'76 300D, 350k miles?, SOLD in 1995
'75 240D, 300k miles, SOLD in 1991

Got a glow plug symbol illumination with check engine light. Icarsoft in OBD II setting showed bad #6 glow plug.

I used Mercedes procedure previously referenced.

Ran car till temp came up about half way. Off came engine cover. Unplugged glow plug using curved pliers from Harbor Feight. Hit plug with some pb blaster and let sit a few minutes. Used 3/8 inch Torque wrench set for 25Nm with 3-inch extension and 10mm deep socket to unscrew glow plug. It came loose under the Toque setting.

Used some of my father's pipe cleaners to clean hole. End of cleaners captured some gunked carbon.

Installed new beru glow plug with torque wrench set to 19 Nm.

Reinstalled cover. Cleared code.

Pleased it was straightforward. Dealer or another mechanic will recommend you do all plugs at once which makes sense if you are paying some one else by the hour.

Given OBD II will show which specific plug has failed as well as the short amount of time necessary to do the job, I suggest carrying a few glow plugs in your trunk and change them out as needed.

__________________
-- Chris

'95 E300, 216k miles, Silver Surfer
'05 E320 CDI, 138k miles
'07 S550 4matic, 69k miles

Gone but not forgotten:

'76 300D, 350k miles?, SOLD in 1995
'75 240D, 300k miles, SOLD in 1991