Had a couple of glow plugs finally fail at 103K on the OM642 in my Jeep. These used the earlier 4.4V system so I bought the same Beru plugs which were installed at the factory for around $19 each.

Due to the age, I decided to replace all 6. I brought the engine to full operating temperature and very carefully tightened (just a nudge), then loosened each plug and all came out with no issues. It appears that using power service every tank may have helped with carbon buildup, but I'll find out more on that when I have to pull the intakes down the road for a walnut shell blast, oil cooler seal upgrade, and swirl flap/egr delete project... hopefully those seals hold for another 100k at least.

I did note that there was an excess amount of oil on cylinder two's glow plug, which has me concerned. I haven't noted any oil loss, and there did not appear to be oil on the top of the engine. Has anybody out there had a similar experience?

This was one of the higher resistance plugs, so it could just be residual unburnt fuel, but it sure felt like liqui-moly. I'm going to run it for a couple thousand miles and take another look down there for similar residue. If it is oil there could be issues with that piston, valve, head gasket, or ???, though it sure seems to run smoothly.

Here is a picture of all of them in their respective positions:

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1983 Mercedes 300SD
1987 Mercedes 300SDL
2001 VW Passat 2.8 AWD
2007 OM642 Jeep WK 4x4

If it is Oil getting up into the Pre-chamber (if your engine has pre-chamber) when you remove the Injector and heat shield and look down inside you will see a shiny carbon build up.

Normally the pre-chamber will look dry and flat black colored.

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84 300D, 82 Volvo 244Gl Diesel

I can't answer most of your questions but do want to tell you the experience with my '98 e300 diesel, with a 606 engine. The previous owner had the glow plugs changed by a dealer when my car had about 80,000 miles. By about 125,000 miles two of the glow plugs failed, and I replaced all of them. Five of them came out very easily; I had trouble removing the other one which was the #1 plug. Every time I took the socket off the #1 plug, I shot a lot of penetrating oil (can't remember the brand) on the plug, and it had a lot of oil on it when I finally was able to remove it.

You got 103,000 miles from your glow plugs. I and the PO got 80,000 from the first set and 40,000 from the second set. You got a lot more miles from your glow plugs that I have gotten. I don't see how there can be much wrong with your jeep as it relates to glow plugs.

Common, every 642 (3 to date) I have removed the glow plugs on has one oily glow plug it seems. I think it is due to the crankcase design one cylinder gets most of the blow by (a guess).

Another explanation is that this engine fires the glow plugs not only for starting but for the DPF regen cycle. If you have one plug not firing it will get oily/sooty due to the extra fuel being injected but not burnt.

Go figure, burn more fuel (hurts the fuel economy) to clean out the filter (to reduce emissions). The same genius thinking that thought re-breathing sooty exhaust (EGR) is a good thing. I better stop before I begin ranting...

Saw this with our 642 in the ML. It started running crappy when the GP light would not turn off.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

I'd glow them and compare one to another to see if there's a variance in temperatures at different locations on the tips. It could be a simple as that.

Perhaps it's just the nature of an old glow plug.

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84 300SD
85 380SE
83 528e
95 318ic