NCV3 Glow Plug Notes


These are notes from the perspective of a 2008 Sprinter 2500 - 906 NCV3 3.0L V6 OM642 CDI owner who was very nervous about changing out their glow plugs, as the previous owner had the unfortunate experience of a DIY job gone bad that ended with one of the cylinder heads being removed and a huge repair bill. So I had no idea what shape the rest of the plugs would be in, and I really wanted to avoid the same fate. There is some speculation in here! If you see anything wrong, or that you could add to, please do, and I'll correct it. I'm an amateur self trained mechanic/electrician, so take with a grain of salt. There are some references in here to Dennis's recommendations, which refer of course to Dennis Williams (@lindenengineering), who graciously shares some of his hard earned knowledge with us.

Unlike gasoline engines which use timed sparks for ignition, diesel engines use detonation caused by compression of air/fuel, triggered with existing heat sources in the cylinder head. When starting a cold engine, glow plugs are used to create a heat source during startup and shortly afterwards. Like spark plugs, glow plugs have a finite lifespan, and periodically need to be replaced. Unlike spark plugs, they are very long, thin, and delicate - and incorrect removal can easily lead to expensive problems.

The glow plug module is what is responsible for providing appropriate voltage and timing to the glow plugs. While early modules were simply relays, NCV3 Sprinters use microcontroller-based “Glow Plug Control Units” which interact with the ECU, regulate voltage and duration, and can provide feedback diagnostics back to the ECU regarding plug condition. Some modules use fusible links, which can blow when the corresponding glow plug goes bad, requiring the entire module be replaced. Newer modules use self-resetting circuit breakers. The voltage regulation is likely switched (PWM). Unsure if they are effectively current regulated (ie: voltage adjusted to meet current thresholds), follow feedback from ECU parameters, preprogrammed with voltage profiles, etc..

Mercedes states the glow plugs each use 8-25a. When tested cold, a functioning glow plug has a resistance around 0.4 ohm. As the glow plug heats up, the resistance decreases to the order of a few hundred milliohm. Considering that fusible links blow in some modules as plugs go bad, the means of plug failure is likely reduced resistance (a “short”), resulting in increased current draw that causes the heating elements to burn out, ultimately resulting in an open circuit (which will show up as high resistance when measured in vehicle). Because of how the resistance changes with temperature, a DC clamp meter on a running glow plug may provide a more accurate assessment for testing glow plugs than simply measuring resistance, but is probably not necessary.

Failed plugs and/or module will cause the check-engine light to go on, and P0671-P0676 (the last digit corresponds to the cylinder with the bad plug) error codes will be available via an OBD-II diagnostic tool. The left (passenger) side cylinders are numbered 1, 2, 3 (from front to back), and the right (driver) side are 4, 5, 6 (from front to back). Plugs can also be tested by removing the wiring harness from the module, and measuring resistance. Harness pins 1-4 correspond to glow pins 1-4; harness pins 7 & 8 correspond to glow plugs 5 & 6. Harness pin 6 is ground.

Glow plugs are also used during DPF regeneration, in order to raise the temperature of the exhaust gases.

Glow Plugs
NCV3 sprinters use 7v "ceramic" or 4.4v "steel" glow plugs with M8 x 1.0 mm threads and an 8mm hex “nut”. 2008 up to mid-2012 (possibly 6/14/2012) used steel glow plugs, while mid-2012 and later use ceramic. Make sure your glow plugs are compatible with your module. Both "ceramic" and "steel" refer to the composition of the heating element at the tip of the plug.
  • Mercedes OEM 0011597101 is a Beru GE105, “High Tech” GE series plug that Beru specs for 06/2006 to present Sprinters.
  • Mopar OEM 68102087AA is a Bosch 0250403008, “Duraterm” series steel 4.4v plug that Bosch specs for 2007-2013 Sprinters.
Dennis recommends Beru. Do not use cheap plugs - they can cause very expensive problems. Unsure to the extent these are interchangeable. The Bosch plugs are available online for about $15, and come with an anti-counterfeit code that can be validated on Bosch's website to ensure they are legit.

Mercedes glow plug modules are OEM'd Beru GSE114 (4.4v / steel) or GSE116 (7v / ceramic). Unsure if these units use fusible links, or circuit breakers, and unsure to what extent they are interchangeable (if at all). Unsure if the ECU requires the original type of module and plugs to be used. Both modules are available in the $110-125 range online.

As per Dennis, Beru is the only smart choice. Dorman 904-310 modules are cheaper, but the quality is shit - they will not last, and have been reported burning out 4.4v plugs by putting out excessive voltage. 5 out of the 6 plugs on my NCV3 were burnt out (high resistance, or open-circuit when measured out of the vehicle), and the previous owner used a 904-310, which had visible corrosion on the circuit board. There was only about 30k miles and 3-4 years on the plugs and module.

Plug Lubricant
While many people recommend using a nickel based anti-seize when installing new plugs, Mercedes uses 001989425110 ceramic grease, which Dennis uses and recommends. In his words, “apply to threads and very light smear coating on shaft”. Bosch states their Duratherm plugs are nickel plated and do not need to be coated for installation. Mercedes 001989425110 appears to be OEM'd Febi Bilstein 26712, which is available for about $8.

Penetrating Oil
Dennis seems to have settled on Kroil; soaking up to 1 week prior if viable, possibly cycling the engine temperature to help work it in. Kroil, along with some other penetrating oil compounds, will degrade the carbon buildup that can cause plugs to stay stuck into heads. Some penetrating oil compounds like PB Blaster and Sea Foam Deep Creep may work well as thread lubricants, but lack corrosion/carbon buildup reduction properties. Kroil is not widely available for consumers, but is available on Amazon. You may want to use a precision pen oiler or syringe to apply it in a focused manner to the base of the glow plug nut, where it touches the head.

It is recommended to remove glow plugs with engine warm. After the engine has been heated to operating temperature, wait 15-45 minutes to avoid burns. The air filter housing, and top heat shield need to be removed. With appropriate tools and technique, for a typical situation, nothing else needs to be removed. On the driver side, there is a thick wiring harness directly above the glow plugs, but it can be moved to make space for the socket.

Using a torque wrench, gently unthread the glowplug. Bosch rates the breaking force at less than 15 ft-lbs of torque - this is literally the amount of force that the factory specs state will cause them to shear off, with the body remaining in the head. While unthreading, it is recommended to alternate threading 1/4 turn, and unthreading 1/2 turn - glow plugs apparently respond well to these counterrotating motions. Once the glow plug is unthreaded, if it still does not easily slide out, there is likely carbon buildup on the tip. Do not force it out, if you break the tip off in the head, you are in for an expensive repair. I tested with 5 of the Bosch plugs I removed from my engine, and they all sheared right at 16-17 ft/lbs. I would be cautious exceeding 10 ft/lbs.

Torque Specs
Bosch states on page 39 of this data sheet that their M8 steel or ceramic glowplugs have the following specs:
  • Breaking torque - 20 Nm (177 in-lbs, or 14.8 ft-lbs)
  • Tightening torque range - 6-10 Nm (53-89 in-lbs, or 4.4-7.4 ft-lbs)
Breaking torque is the shear force at which the plug will break (shear in half). When using a lubricant, reduce tightening torque specs by 30-55% (depending on lubricant). The low end of the Bosch torque range represents a 40% reduction from the high end of the range, which may correlate to the recommend lubricated torque spec.

Tools & Misc Parts
  • Special pliers like the PMD Products Angled Glow-Plug Connector pliers are often recommended to avoid damaging the plastic glow plug ends, and are inexpensive (about $15). Mercedes and Dodge sell OEMd Hazet 4760-5 pliers, which are also available for 5-10 times the price. I bought the PMD pliers, and for occasional use they seem well made, and worked great.
  • While 1/4” drive 8mm deep-well socket with a universal joint is often used, specialty tools exist like the Mercedes 001-589-80-09-00 gow plug socket, which has an optimal depth for plug with an internal rubber retainer, thin sidewalls, and an ideally length articulated extension. It appears to be an OEM'd Hazet 4760-2 which is available for about half the price ($65). Not all deep-well sockets have clearance for the glow plug terminal. The 3/8 depth deep well sockets I tried were too thick to fit in some spots, and a deep well socket will not have an internal rubber retainer. There are cheap 8mm articulated glow plug sockets on the market, but for the price of the Hazet, I'm very pleased with the quality and how well it worked.
  • Glow Plug Reamers like the Hazet 4760-89 are available which can be used to remove carbon deposits once the plug is out.
  • OM642 Glow Plug Connectors and Repair Kits are available to repair broken connectors, if necessary. I have one on order, and will update the notes when I get a chance to use it.
  • To repair damaged threads, once a problematic glow plug has already been removed, Dennis uses Time-Sert threaded inserts“90% of the time”. The exterior dimensions of the thread insert are M10 (diameter) x 1.0 (pitch) x 12mm (length) Time-Sert inserts. If you get to a point where this is necessary, you really may want to consider whether it is time to turn to a professional. But if you DIY:
Misc Additional Links
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Looks like I lost the ability to edit the first post, but I wanted to post an update on the reamer, which I'm still trying to source. My Hazet order eventually was cancelled, so I'm back to square one. The reamer is a tool I'm not willing to skimp on, because this is a critical job that can have expensive consequences (re: this Glow Plug Reamer Disaster thread). Right now, I have plugs 1-5 out, but plug 6 has been sitting with Kroil for another week because it's being really stubbon. I think I'm going to have to clean up the threads if I can get the plug out, if I can find an M8x1.0 thread chaser tap.

  • Laser 5854 is an inexpensive 8mm glow plug socket that looks like it might fit, but would probably require a 3/8 extension

  • Unsure what the MB part is for the OM642 8mm reamer, whatever it is, I'm sure it's expensive, but it's probably very nice
  • It looks like the Hazet 4760-89 8mm/9mm reamer has been discontinued. Hazet does still make a couple tool sets for removing broken glow plugs (4760/6), and repairing threads (4760-M8X1/9).
  • Baum B642-0053 is available online for $75 shipped. Not sure about the quality, it looks like it may have an aluminum body, which is not what I want I'd want to thread into my head after fighting with a stubborn glow plug, leaving the threads in questionable shape
  • Laser Tools 5547 Aperture Cleaner 8 & 9mm appears to be a steel body with a free floating steel reamer and an adjustable depth stopper. Looks a bit more universal than the Baum tool, might require comparing to a glow plug to set the depth of the reamer, but seems like it might be a better made tool.
  • Laser Tools 5210 Aperture Reamer Set looks like a nice set, but is expensive and for M8, M10 and M12.

  • Mercedes 642-589-01-99-00 - can't find much info other than it's really expensive, probably an extractor/re-thread set?
  • Baum B900-0199 - M8x1.0 broken glow plug removal tool
  • Win Tools W900-1099 - appears to be a cheaper/lower quality version of the Baum kit
  • Laser 5205 Damaged Glow Plug Removal Set
  • Laser 6777 Glow Plug Threaded Insert Kit M8x1.0
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Well this turned out to be a real barrel of fun. So I ended up getting 5 out of the 6 plugs swapped - no problem after pre-soaking them in Kroil for a couple weeks, and doing the 1/2 turn off, 1/4 turn on method with a torque wrench, trying not to exceed 10 ft-lbs at any point. Afterwards, I put the old plugs in some vice grips and took a torque wrench to them, and they all sheared right at 16 ft-lbs. So they (barely) exceeded the mfg spec on shear strength. I would not wrench on these without a torque wrench unless I had developed a really good feel for them, and probably not even then. There's just not much of a margin of error, and the cost of shearing one could be pretty damn high compared to a quality torque wrench and some patience.

So cylinder #6 was a different beast. It was harder to wrench, and took a while to get to where I thought it was unthreading. Then it kept going...and going...and going. It never loosened up, I was getting nervous that it might be stripped, so I cut my losses, and decided I'd save it for the next time I'm driving through Colorado and could visit @lindenengineering to let someone I trust do it. So, I thought I tightened it back up, though it never felt like it seated. I was pretty sure the threads were stripping and I didn't want to push my luck. Not a great feeling.

We took a road trip down to Bend (about 170 miles each way), had a really nice weekend and headed back. On the way home, around 5pm, about half mile from Government Camp (~50 miles from home) the van suddenly sounded like a train (choo-choo-choo...) so we pulled over. First thought was a blown tire, but after a brief inspection and firing it back up at idle it was pretty obvious we were hearing compression escaping. Pulled off all the plastic fixins and saw the glow plug had BLOWN OUT. So I guess the Kroil and about 300 miles of driving had loosened up enough carbon to shoot the glow plug right out. The glow plug harness plastic connector was smashed up a bit but still works. Took a couple busses and an Uber back home, came back with some tools and a glow plug and we were back home before midnight. Cleaned out the threads with the (hardened?) steel reamer, they seemed fine - I was amazed. Surprisingly, running the reamer through the shaft, there was almost no resistance in the hole. New glow plug threaded right in, and it's been working better than ever since. Judging by the threads on the old glow plug, the carbon/Kroil was melted and eventually pushed out enough to free the plug. The threads on the plug were saturated with the (re)solidified gunk. So I must have unthreaded it but it never threaded back in. The carbon buildup on the shaft had enough resistance to feel like some stubborn threads.

Moral of the story, these things are not like any spark plugs I've ever wrenched on. I wouldn't even think about working on them without some research, patience and the right tools (torque wrench and the Hazet 4760-2 works like a champ to reach them all). When in doubt, step back and revisit it later, or bring it to a competent pro. I think I got lucky.


PS - Since I couldn't get my hands on the Hazet reamer, I bought a Laser Tools 5547 Glow Plug Aperture Cleaner on eBay for $76 shipped. Construction quality seems nice, but when I use it I don't feel any resistance, so it may be too small to be reaming it out (or it's possibly, but doubtful, I don't have enough carbon buildup to be reamed). So I can't really recommend it, but I don't know of a better option.


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New member
Holy cats. Original post is incredibly helpful. Just did all 6 plugs on my 2007 and it went like butter (they did soak in kroil for a month so that probably helped too). Only thing I didn't do was ream the holes but I'm not sure they needed it as all the new plugs slid right in.

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