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2003 Mercedes S430
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806 Posts
Discussion Starter #1
I have an 85 300D that starts hard when cold. When you turn the key, no glow plug. Warm the engine up a bit, then shut it off, turn back on and you have a brief glow plug light.

What's up?
 

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1981 240D
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185 Posts
It sounds like you may have an electrical continuity issue. You should try to diagnose the glow plug circuit. I would start with the 80A strap fuse found on the glow plug relay. Check it for continuity. They are less than 50 cents each, so try replacing it. Your 1985 will have the new "pencil" type glow plugs, check each plug for continuity. Try this pictorial guide:

http://www.dieselgiant.com/glowplugrepair.htm
 

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1984 300D 375,000 miles 1996 e300d 266,000 miles 1983 300SD 174,000 miles Ascot Grey
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453 Posts
longshot, but just had the same problem. car was also difficult to start in cold (so probably not the same situation), then one morning (yesterday), it just would not start. Glow plug light on and off-made no sense. Turns out i had a broken glow plug (the center would just spin when i tried to remove the 8mm nut). Replaced all 5 plugs and all is well.
 

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2003 Mercedes S430
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806 Posts
Discussion Starter #4
I checked today and the strip fuse is fine. The relay actually clicks in and out like it is supposed to. I am ready to replace all of the glow plugs before doing any more diagnosing. That's the most logical culprit, right?

Looks like they can be bought for less than $50 all five, Bosch.
 

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2013 MINI Copper S Clubman, '84 300CD-weekend car
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10,152 Posts
^Yes, but if you have a multimeter you can be sure by using the above procedure.
 

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1981 240D
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185 Posts
It can never hurt to replace the glow-plugs. I just commented on another pre-glow related thread that I once replaced all 5 GPs on my 1983 300SD because I was too lazy to take the time and troubleshoot. In the end it turned out to be my 80A strap fuse. But you said yours is fine. Another thing, when you replace your GPs make sure you ream the holes. If they are caked with soot, the GPs will just heat up that "blanket" of soot, which in turn acts as an insulator and fails to pre-heat the cylinders. If you are going to spend the money to replace the GPs, give them a fighting chance. Also, it wouldn't hurt to use the multimeter to check the resistance of each GP to be sure. If they all check out fine, you might be wasting your time and money replacing them only to have the same problem when the job is done.
 

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1981 240D
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185 Posts
longshot, but just had the same problem. car was also difficult to start in cold (so probably not the same situation), then one morning (yesterday), it just would not start. Glow plug light on and off-made no sense. Turns out i had a broken glow plug (the center would just spin when i tried to remove the 8mm nut). Replaced all 5 plugs and all is well.
I had a similar experience. When I first purchased my 240D I noticed the glow plug never came on when I did a pre-glow, but it would go off and on while driving. I pulled the GPs and #2 was cracked. Someone torqued the thing down like it was a lower control arm bolt.
 

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2003 Mercedes S430
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806 Posts
Discussion Starter #9
I am a dealer and its been a while since I had a 300D. I always replace glow plugs with Bosch units if any question. While I could get by with an $8.00 plug, I'll spend the money to make the car right. Proactive verusus letting a customer replace a glow plug in a few months, etc.
 

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1984 300D 375,000 miles 1996 e300d 266,000 miles 1983 300SD 174,000 miles Ascot Grey
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453 Posts
I had a similar experience. When I first purchased my 240D I noticed the glow plug never came on when I did a pre-glow, but it would go off and on while driving. I pulled the GPs and #2 was cracked. Someone torqued the thing down like it was a lower control arm bolt.
should have mentioned that all 5 of my plugs were non-functional (all 5 had been checked last winter). Would the broken plug have shorted out the other 4?... could all of the over-glowing that i was doing to compensate have caused the issue, espcecially in light of potential carbon buildup (regretting not having reamed them before installation)? Either way, i'm pulling them out and reaming this weekend.
 

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1981 240D
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185 Posts
should have mentioned that all 5 of my plugs were non-functional (all 5 had been checked last winter). Would the broken plug have shorted out the other 4?... could all of the over-glowing that i was doing to compensate have caused the issue, espcecially in light of potential carbon buildup (regretting not having reamed them before installation)? Either way, i'm pulling them out and reaming this weekend.
It is pretty uncommon for all 5 to be non-functioning considering you just checked them last year. It is possible that the additional pre-glow load you put on it shortened their lifespan, but I don't think so. Unless they were cheap plugs. If you have the new "pencil" type plugs and they are in parallel circuit, it would not have shorted the other 4 out. I am curious how you came to the conclusion that all 5 were bad? Did you follow the dieselgiant wiki? The reason I ask is that you mentioned you will be pulling them this weekend, which leads me to believe the GPs are still in the engine and I am not familiar with any procedure that allows you to check the GP's with them still in the engine (I could be wrong, that is why I am curious to know how you made the determination that they were all bad).
 

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1984 300D 375,000 miles 1996 e300d 266,000 miles 1983 300SD 174,000 miles Ascot Grey
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no, i replaced them this weekend. i just meant that i was going to pull out the new ones and ream, since i was too impatient/unprepared before. I pulled each one out, and tested them with a battery and jumper cables, and tested the new ones before i put them in. None of the old 5 were functional.
 

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1981 240D
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no, i replaced them this weekend. i just meant that i was going to pull out the new ones and ream, since i was too impatient/unprepared before. I pulled each one out, and tested them with a battery and jumper cables, and tested the new ones before i put them in. None of the old 5 were functional.
I see. It will be worth your time. Then you won't have to worry about it for a few years. Let us know how it goes.
 

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2003 Mercedes S430
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806 Posts
Discussion Starter #14
I got to the bottom of this tonight. There was only one working glow plug and the rest were history. Looked like originals.

Replaced all five with Bosch and the light works properly and it cranks right up.

As far as those reamers, it is easier to pull the precombustion chambers and have them cleaned at the machine shop then to try and fish the reamer in the side of the head.

I have the tools to pull prechambers and that's the best way to clean out the carbon.
 
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