I had the classic signs of a bad glow plug, no pre-glow light, rough starts. I ohmed out the connector leading to the plugs, and saw 2.6 MΩ on #3. With the cross pipe off, and #3 disconnected, I measured the plug directly and see 2.6 MΩ to ground. So definitely a bad plug. Luckily, I can reach it without taking the intake manifold off.

My question is, what's the chances of the plug breaking after being in the car just one year? Are they weak from the get go, or does it take time? The top end was rebuilt and clean as a whistle when the new plugs were installed; I don't know if that makes a difference. Also, should I try to take it out cold, or with the engine warmed up?

Thanks.

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Jay Kickliter
Chasing 'trons (my thoughts and tutorials on electronics and stuff)


1995 E300D-Black on Black

It's pretty rare to break a glow plug. If you're worried about it get the engine warm and spray some penetrating oil on the threads of the plug, then let is sit for a hour or more. Then try to take it out. If it's tight don't use hammer on the wrench to break it loose. They aren't supposed to be screwed in really tight but people tend to ever do it and/or they get tight from the heat.

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Just do a search on "broken glow plugs", and you will find out that it is not a "rare" occasion. With that being said, I think you have an excellent chance of getting yours out without a problem. Buru recommends that you stay within the 45-50 N/M range when trying to loosen the plugs. Some forum members recommend warming up the engine, and using generous amounts of PB Blaster. When I did mine, 4 plugs came out just fine, but the other two took over 2 days of spraying and moving them back and forth 1/4 turn at a time before the were able to be removed. I believe that the major problem is that the design of the plug allows them to become carbon bound, and prior to installation, the plug hole should be reamed with the appropriate tool. Good luck....

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96 E300d

I dont understand why the system won't glow at all if there's one open plug. Seems to me that starting on 5 plugs is a lot better for the engine than starting on none.

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Jay Kickliter
Chasing 'trons (my thoughts and tutorials on electronics and stuff)


1995 E300D-Black on Black

I can't say for sure cause I don't know too much about your model, but in the earlier diesels, the GPs are still on with on bad one, it's just that the light doesn't come on to tell you that you have a problem. And depending on your compression it may be quite hard to start with one GP burned out. In my 123 wagon it was quite difficult to start with one GP out. And it will idle rough for the first bit cause the cylinder with the dead GP is much colder and not running well at first until it warms up.

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The system still operates with a burned out glow plug. Just because the light doesn't come on doesn't mean the relay isn't cycling.

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92 300D 2.5L OM602 OBK #59

Are you saying that the plug is burnt out or that it is mechanicaly broken somehow. Entirely different problems. Glowplug life is guite variable sometimes short as weeks or months sometimes years and years. Cheers Dan

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It's always something simple
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I think OP needs to answer this question before we get any farther. It sounds burned out to me, others have assumed it is physically broken. Both are entirely possible.

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TC
Current stable:
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Sorry for the confusion. It's burnt out. I decided to just take it to my mechanic that did the top end job last year. He told me that he used anti-sieze when he put them in. Thanks for the info everyone.

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Jay Kickliter
Chasing 'trons (my thoughts and tutorials on electronics and stuff)


1995 E300D-Black on Black

Glow plugs are like light bulbs. They either work or they don't, and there's no set mileage or age when they go bad.

If this plug is only 1 yo, I assume it was installed with Never Seize on the threads so it should come out easily.

Regardless... warm engine, lots of PB blaster, remove with a torque wrench, never exceeding 50Nm. If it won't budge at 50Nm, respray with PB Blaster, go inside and have a beverage, try again in an hour.

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Mark in MA
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This isn't totally true. As GPs get old the resistance starts to increase and you get a less than optimal heat from the plug. They do burn out like light bulbs but they don't necessarily burn at the optimum for their whole life. If you bench test and old GP you'll often find that it burns bright red to bright orange. A new one will be extremely bright orange to almost white. New ones will often leave sunspots in your eyes, whereas the older ones won't. Don't touch them though, they're HOT, obviously, something like 1600F.

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I figured my statement would trigger this response, but I still stand by it. Perhaps heat areas do not stay optimum throughout the life of the GP, but 99% of GP-related questions on the forum are a go/no go issue. If you're reading megohms, your GP is bad, if you're reading an ohm, there's 144 amps going through that plug on a 12v system. Ohm's law holds...If there's 144 amps going through your GP, it's turning into heat. No other option.

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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

The Fuse is 80 amps. I would expect the Fuse to burn long before it reaches 144 amps.

Also the Glow Plug in my picture that is heating mainly in the middle could case a miss on that particular Cylinder because the Heat would be trying to heat up the Cylinder Head and the Coolant (because that is what surrounds the Glow Plug in that area of the Plug) instead of heating the Air in the Prechamber.
Even though the Glow Plug has only partially failed it may functionally mimic a dead Glow Plug.

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84 300D, 82 Volvo 244Gl Diesel

It was a math exercise. If the fuse is 80a, then the resistance of the glow plug has to be more than 2.178 ohms (if the battery is at 13.2v) to not blow the fuse. I'm not makin' this stuff up...Ohm's law holds everywhere.

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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

Do you have a weak battery in your calculator perhaps because those numbers are way off!

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