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1984 300sd
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Hi Folks! New 1984 300SD owner with 234000 miles. I am hoping that I can get a little help from this group in restoring my car. My first question has to do with hard starting. Just got the car, but this morning it took 3 cycles of the glow plugs to get it started.



Question: Why do glow plugs go bad? If they have the correct resistance and voltage, they should heat up properly, right? I suspect that a build up of junk on the tip insulates the heat into the glow plug. So I am guessing that if the tip is cleaned properly, it would restore the glow plugs operation? The timing of the Relay appeared to be ok.



Just a note on my restoration – I am mechanically inclined, have owned and worked on GM diesels in the past. I purchased the shop manual and am in the process of looking through it. I need to keep costs down, so I am always looking to rejuvenate an old part before I start buying new things. I plan on doing all of the work on this car. That what makes it fun.
 

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1991 300 SE
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18,534 Posts
Although contamination can effect the resistance of the plugs due to the nature of the contamination and the heat and pressure involved cleaning is not effective. If you have no records as to when the plugs were last changed a replacement is in order. It’s inexpensive and easy to do.

But I respect your desire to not throw parts at the car; in that case test the plugs with an ohmmeter to determine resistance. Resistance should be under one ohm – a plug over one ohm should be replaced.

Good luck, keep us posted, and thanks for keeping a classic alive – it will pay off.
 

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2013 MINI Copper S Clubman, '84 300CD-weekend car
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10,152 Posts
Although contamination can effect the resistance of the plugs due to the nature of the contamination and the heat and pressure involved cleaning is not effective. If you have no records as to when the plugs were last changed a replacement is in order. It’s inexpensive and easy to do.

But I respect your desire to not throw parts at the car; in that case test the plugs with an ohmmeter to determine resistance. Resistance should be under one ohm – a plug over one ohm should be replaced.

Good luck, keep us posted, and thanks for keeping a classic alive – it will pay off.
Second.
 

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CH4S Admin , Outstanding Contributor
1985 500SEC, 1991 190E 2.6.
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40,755 Posts
Hi Folks! New 1984 300SD owner with 234000 miles. I am hoping that I can get a little help from this group in restoring my car. My first question has to do with hard starting. Just got the car, but this morning it took 3 cycles of the glow plugs to get it started.



Question: Why do glow plugs go bad? If they have the correct resistance and voltage, they should heat up properly, right? I suspect that a build up of junk on the tip insulates the heat into the glow plug. So I am guessing that if the tip is cleaned properly, it would restore the glow plugs operation? The timing of the Relay appeared to be ok.




Just a note on my restoration – I am mechanically inclined, have owned and worked on GM diesels in the past. I purchased the shop manual and am in the process of looking through it. I need to keep costs down, so I am always looking to rejuvenate an old part before I start buying new things. I plan on doing all of the work on this car. That what makes it fun.


Have you tried a forum search? The archive has a wealth of information.

Please click 'User CP' on the top tool bar, and complete your profile with a location and model of your car.
It makes it easier for members to offer advice. MB produced different, and region / Country specific versions of our cars.

Welcome, to take advantage of all the forum features, please re visit the 'Welcome to the Forum' tutorial sticky on top of the W126 page, as well as the two DIY stickies.
It will show how to search the forum (3 words min.), and has a link to the Russian MB site where you can run a VIN#.
Cheers
 

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MBZ 300SD
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Personally I have taken the glow plug and connected it to the battery admiring the red glow on the good ones. The bad ones will stay cold. You could start the engine with only 2 good glow plugs.
To check them just turn the key until the light goes out but do not start.
Touch each plug to feel the temp.
 

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I purchased an 83 SD last year and it was fairly difficult to start. I tried something and it work - instead of starting it when the indicator light went out I waited until the seat belt light went out and voila it starts really quick. It may be that you just need to wait a little longer for the GP's to heat up.
 

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2013 MINI Copper S Clubman, '84 300CD-weekend car
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10,152 Posts
I purchased an 83 SD last year and it was fairly difficult to start. I tried something and it work - instead of starting it when the indicator light went out I waited until the seat belt light went out and voila it starts really quick. It may be that you just need to wait a little longer for the GP's to heat up.
There is a timer on the GP',s just listen for a click at the relay. Depending on the outside temp, you shouldn't have to wait for an entire cycle.
I'll suggest checking the plugs with an ohm meter, the smaller the value, the better.
 
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