So, I have been chasing a poor start on my 300SD. It has been taking a second or two to fire recently rather than on the first or second cylinder. And then running rough for a minute or so. Valves are set. Recent glow plugs.

I checked again and found #2 glow plug had gone bad again. That cylinder seems to be eating plugs. Bad plugs or some other problem I don't know.

I checked for voltage at the glow plug relay and found battery voltage of 12.5. When I checked at the glow plugs it only had 10.5v. Is it normal to lose 2 volts in the wires? As well I coated the glow plug threads with anti sieze. Could this be part of the voltage drop?

Any ideas?

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1984 300 SD, 2 tank WVO
1996 Ram Cummins, 2 tank WVO,
2004.5 Ram Cummins, 2 tank WVO

With the amount of current flowing a two volt drop is not extreme in my opinion. If there was a negative with the anti seize it would tend to lessen the voltage drop. It has suspended conductive metals in the paste usually. I would tend to discount it as an issue in your case myself.

If another glow plug meets it's demise in the same cylinder I might tend to at least have the injectors checked for spray pattern or certainly that one. It may not also hurt to decarbonise the glow plug holes with a reamer. This has been shown to cause some symptoms by ignoring the proccedure. Maybe you did? Carbon build up can be a problem and thats why specific reamers are manufactured for these cars.

Thanks for that. I guess I forgot that it was a complete circuit when I tested it; not just the potential circuit.

I have never reamed them. It has never been a problem pulling the plugs and they normally just have a dusting of carbon. Should it still be reamed?

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1984 300 SD, 2 tank WVO
1996 Ram Cummins, 2 tank WVO,
2004.5 Ram Cummins, 2 tank WVO

The Glow Plugs do not have to be stuck in head from the Carbon to cause a problem.

From what I have read on this forum the though seems to be that the Carbon build up allows some of the heat that is supposed to go to the Pre-combustion chamber to instead get conducted into the Cylinder Head it self.

I tested a Glow Plug ounce by connecting it to a Ammeter and connecting it to a battery. What I found was the Amperage was high (it pegged the 60 amp gauge out) while the plug was cold. As it heated up the Apmerage fell and settled down to around 16 amps.

One of the other member said that the cold plug had more resistance and that as the plug was heated the Amperage/Resistance dropped.
So I am speculating that the heat being conducted into the Cylinder Head is keeping the Glow Plug colder longer and keeping the resistance higher and some how this is effecting the life of the Glow Plugs.

Buy the way you do not have to buy a Reamer to clean out the holes if you do not want to; there is at least 1 thread that has some alternative methods.

As the other member said the other thing that kills Glow Plugs is an Injector not spraying where it is supposed to.

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84 300D, 82 Volvo 244Gl Diesel

A factually incorrect statement.

When cold, the plug has less resistance and flows more current. When hot, the opposite situation is applicable.

A slip of the pen. I knew what he meant.

You also agree that reaming is required?

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1984 300 SD, 2 tank WVO
1996 Ram Cummins, 2 tank WVO,
2004.5 Ram Cummins, 2 tank WVO

Looks like I am haveing trouble unlearning the wrong information.
(I started an electronics class ounce when I was out of work but about 2 weeks into the class I got a job an I never finished the class. I guess I am gettin payback for not finishing it now.)

http://www.peachparts.com/shopforum/member.php?u=44791
My post#11
So when the GP was cold it pulled high amps=high resistance, hot GP= Low amps and Low resistance.
A Glow Plug that is conducting the heat into the Cylinder Head due to Carbon Build up would remain cooler and pull higher amps longer and maintain a higher resistance for a longer period. My guess is this is what kills them early.


Post #16 by funola
You probably did not mean what you wrote but it is incorrect. Should be:
High amps = low resistance,
hot GP= low amps and high resistance.

My post #17
Actually, I could just be wrong; I am not so swift on the finer points of Electrical stuff.
If the plug pulls higher amps until it is hot a Plug encrusted in Carbon is conducting heat through the Carbon into the Cylinder head would maintain a Higher Amperage due to the Plug tip being cooler over a longer period. Maybe the extended high Amperage is what damages them.

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84 300D, 82 Volvo 244Gl Diesel

If the anti-seize were a problem, you'd see a higher voltage at the glow-plug. The voltage you're seeing indicates about a one-tenth ohm resistance between battery and glow plug. (GPs, IIRC, are about .6 ohm). You might try re-seating your GP relay to get a fresh bite on the contacts, cleaning the fuse contacts, making sure bolts are tight. Make sure all connector pins/contacts are clean. Checking the voltage to ground along this path (Battery, GP relay contacts, Fuse, Connector, GP) will give you clues - voltage drops equate to unwanted resistance. Replace relay if you suspect that the internal relay contacts are burnt. (you mignt be able to disassemble the can to look at the contacts.)

(Note: this is all general electrical troubleshooting. I've never had an issue with my GP relay, just plugs themselves, but MB relays in general are "cans", the GP fuse is bolted in, connectors come apart, etc. So that is the basis for my post here.)

Good luck!

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)

You could also check the voltage between the engine block and the negative side of the battery. Any voltage during glow cycle would point to the ground strap. Clean/tighten as required. Not really suspect here unless you have trouble with your starter as well.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)

Thanks guys.

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1984 300 SD, 2 tank WVO
1996 Ram Cummins, 2 tank WVO,
2004.5 Ram Cummins, 2 tank WVO

"I checked for voltage at the glow plug relay and found battery voltage of 12.5. When I checked at the glow plugs it only had 10.5v. Is it normal to lose 2 volts in the wires? As well I coated the glow plug threads with anti sieze. Could this be part of the voltage drop?"


The answer is "yes" Volts= pressure amps= volume, what I mean is when the glow
cycle starts amperage are high "volume", volts are less "pressure".
anti-seize applied to glow plug negative side will not effect the voltage.
and it is a good idea to ream out glow plugs holes when installing new ones.