This problem is driving me nuts. Last week the fuse holder on the firewall of the 77 240d had melted and blown the fuse. Put in a new one and all was fine for a day then the #4 plug burned up (one closest to firewall) Went and replaced all four plugs with new bosh (old style loop types) and the #4 plug has burned up at the rate of one a day since. Just plain melted.
What I have done....

Used a GP reamer to break out any old carbon none was found.
Used a very small magnet inside each GP hole to look for old GP parts (none found)
Turned motor over several times before install of new plugs to blow out anything
Checked relay with a test light across plugs and it goes off as soon as car starts or as soon as dash light goes off.
Checked for short in system and power is going across all four plugs

Each time it took out the #4 plug I use a magnet and get all the melted chunks out of the pre chamber. Then crank it over for several seconds to blow anything else out. Never see anything except a mist of fuel.

Replace plug and car is fine, can start and run several times and drive around the block then park for an hour or two, go back out and all is fine. Give it to my Daughter to drive to work and to friends houses and by the time she comes home at night she tells me the GP light is out but the car starts fine, just takes longer.

Sure enough go out and the #4 plug is dead.
She knows the difference between acc and on so I know it's not her letting the car sit and cycle the plugs for extended times. Fuse has not blown since I replaced it, and while I have been driving around I never have noticed the GP relay light comiing on so I don't think the relay is faulty.

Is there anyway the pre-chamber could be bad on this pot and cooking the plug? Car runs fine, no miss, does not use more than a 1/2 qt. of oil in 3000 miles etc.
Should I scrap the whole series plug thing and go with the fast glow upgrade?
Anybody have any ideas?

Rebootit,

It might be an electrical problem. Connect a voltmeter across each GP and measure the voltage drop across each GP. It should be about 1.5 volts. You shouldconnect the VM as close to the GP terminals as you can.

Be careful near the resistance wires between the GP. they get very hot.

Let me know what you find.

P E H

Ok found my digital meter and here is what I have.

Battery 12.5v

Across GP fuse to ground 10.46v

Across #4 plug 8.74v

Across #3 plug 7.21v

Across #2 plug 5.71v

Across #1 plug 4.17v

So it seems like the 1.5 drop is pretty much right on the money. Unless the start voltage at the fuse past the relay should be higher than the 10.46 I was getting.

Battery is also brand new series 49-1 like what I use in my 300d and the engine start speed is much faster than it used to be.

rebootit:

How are you measuring the voltage across the GPs?

I don't have the same type system as yours, but I thought that if you measure from the GP tip to ground you should read the 1.5V.
I don't really understand what those voltages are that you measured.

[edit] ahh...I see. Series arrangement. :p

In this case, since the GP is only dropping the expected 1.5V, that pretty much rules out the GPs being the problem. Considered glowing the plug OUTSIDE of the engine?
This really does look like something inside the prechamber is causing the GP to fry.

In the meantime, if I were blowing GPs like that, I'd temp. hookup a circuit breaker to the #4 GP. Much easier to reset a breaker than going to all the work you seem to be doing every time #4 burns up.

__________________
Chris
1985 300SD - 'Grace' (198K mi.)
2018 Honda Civic Sport
2018 Honda CRV LX
2010 Honda Fit Sport (RIP)
2013 Honda Accord Sport (Sold)
1996 Lexus LS400 (Retired)
1995 Ford Contour SE (Retired)
1976 Porsche 914 (Sold)
1972 Datsun 240Z (RIP)

I'm thinking the same thing. This problem only started last week. Relay "seems" to be ok. Using a test light it does kill power to the GP's as soon as you turn the key. I'm not sure what the clearance is from the loop to the inside of the chamber but I did catch some tiny little chunks of metal out of the #4 today that I had not seen before. These may have been causing a short and thus a burn of #4 in the last week, I don't know. I have started the car and left it running for 2 hours over the weekend and then let it sit till cool and the plug was still fine. Kind of rules out prechamber problems and relay problems. Only unknown factor is a 16 year old female in the car when it blows but I have been over and over the start procedure and make-out with boyfriend procedure of where the key to belongs to turn on the radio and she does know the difference I have put several thousand miles on this car since I bought it with no problems. I am at the point of saying it's a female driver problem right now but still in 25 years of use the car HAD to have had some female use problems and the plugs I replaced looks original.

Rebootit,

I meant to read the GP voltages across the GP , not GP to ground. But by subtracting the voltages, you are getting the correct voltage across each GP.

Here's another thought: Inside the prechamber is a ball that the injector sprays the fuel onto to improve combustion. This ball is mounted on a shaft that mounts into 2 holes in the side of the prechamber. Sometimes the shaft breaks. So is possible that this ball or shaft is shorting the GP and causing it to burn out.

You would have to remove the prechamber to check if the ballshaft was broken. If the ballshaft is broken, it can be replaced in the prechamber.

I don't agree with the female theory causing the GP to burn out.

P E H

PE,
I think you may be on target here. If this plug is blown again after fishing out some small chunks of metal today which looked like GP elements then I am going to have the prechamber pulled and looked at. If this part can be replaced do you know the part number?

A short will kill the GP, they don't like too much current. So will not having the total resistance right, you did put the "wires" between the plugs back the same way they were, right? If you leave one of the resistor (zig-zag) links out, you will burn the GPs off, they get too hot.

If the fuse holder melted, I'd bet you have something intermittantly shorting out the GP system along the way, probably right after #4.

A bad injector can also burn off the loop by shooting out a solid stream of fuel rather than the proper cone of fuel mist -- they can spray almost sideways when really worn.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Peter,

My "first MB" was the 200d which as far as GP wires go is exactly the same as the 240. I have looked for any dead short or bent wires or even cracked insulators and all is fine. I get full current across all 4 plugs. The way MB designed the loops in the wires it is almost impossible to get it wrong anyway. Simple series plugs are more or less exactly like a neon sign is wired so after doing 1000's of them over the last 20 years I know the jump between plugs is good.
When this first happened the wire to the #4 plug was old and almost worn in half. Figured this and loose screws caused to much heat for the old fuse holder to handle so it melted the plastic and the little brass blocks fell out. After replacing and putting in a new 80 amp fuse no problems. Never have blown the fuse but something is causing a short that is cooking the #4 plug that much I am sure of. I suspected either something in the prechamber was doing it but my indy said there was nothing besides carbon that would do that. There is no carbon build up but P.E. has said there is a little ball in the chamber that could be broken causing a short.
Now if there was a bad injector shooting a stream and not the cone of fuel would you feel this is the way the engine runs? It runs great, no miss even when cold at startup, no excessive smoke for a 240, etc.
If the car eats another plug after today I am going to pull the chamber and check it. Also will have injector checked if all checks out in the PC. I know from experience that a bad injector can blow a hole right through the chamber and destroy a motor. Almost did that to my 200d. PC tip fell off after shutting down the engine which caused it to lock up when I went to start it back up.
This is the first problem I have had that I have not been able to figure out either by looking for the obvious or searching the boards and asking everyone else here. Feel like beating my head on the block at this point

Rebootit,

Very recently burned up (melted the insulation right off) both battery cables, starter etc. on a 78 240D due to believe it or not, an improperly cut ignition key that hung up slightly and did not fully overcome the spring return built into the ignition switch.

Do you and your daughter have different keys or do you use the same one? I ask this because just a couple of weeks after the starter and harness repair, and a new key, the glow plug light did not work and we found a melted #4 glow plug. Replaced the plug and it is working.

Also check that the plastic tube insulator is on the vertical linkage rod to the injection pump that gets very to the connector between #3 & #4 glow plugs. This could cause a short.

The idea of a circuit breaker by another member is an excellent idea until you isolate the problem.

Try putting a very light coat of grease on the new plug loop, install and tighten it, remove it and see if it contacted anything that could be causing a short. Hope it works out for you.......

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Those holes in the precombustion chamber are tiny,,, if you pulled ANY pieces out with the magnet you sure have the possibility that some of those ( 7?) holes have junk forced into them sufficient that a magnet would not pull it out.. particularly since the holes are radial to the center of the chamber where your magnet would be. Very safe recommendation to pull the injector/chamber and have BOTH checked carefully.... this is apart from whether the Cause of the meltdown was electrical... this would be cleanup from the meltdown at the very least.

keeping my fingers crossed but she had the car all day yesterday and as a kid will was all over town plus to her two jobs and all is fine. "knock on wood" but the ideas here have at least helped with possible problems. If it blows another plug the prechamber is going to come out. Where can I find the wrench to pull it? And once the hold down ring is off does the chamber just lift out of the hole or do you need a special tool to pull it out? Fastlane has the chambers and seals so If I end up pulling it I may just replace it with all new anyway unless the old one has no pitting or heavy wear. The car only has 177k and was cared for by MB only it's whole life. I have all the service records and from what I can tell the plugs I replaced were factory. They all had a little star on them and brass parts where the new Bosh plugs looked like polished steel.
That may be the problem, rejection of non MB parts

Rebootit,

If the #4 GP is the first GP connected to the controller, it is possible that a short somewhere between #4 and ground is causing #4 to burn out. Thus, the problem could be somewhere else than #4GP. But if the former is true, I can't understand why the fuse doesn't burn out instead of the GP.

P E H.

According to your voltage measurements, GP # 4 is the series glow plug that is closest to the B+ 12 volts and this means it has the highest voltage on it.

If there is something metallic in the pre-chamber that shorts out this glow plug to GROUND, then this one glow plug sees ALL of the B+ voltage instead of a normal 1.5 V or so drop.

I should think a glow plug designed to operate with a 1.5 volt drop (is this right, because that only adds up to 6 volts for four glow plugs) or something in that range would QUICKLY burn up if it saw 10 to 12 volts.

My guess is that you are either getting a short of GP # 4 inside the prechamber, or the wire/connection to GP # 3 is shorting to ground. Since you have mostly ruled out the external wiring, it is very like to be something metallic inside the prechamber.

Ken300D

__________________
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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles

Since using a small magnet yesterday and pulling or I should say catching some small metal fragments of the blown former plugs the problem seems to be solved.
However I wondering about the way I was reading voltage across the system. As I said before the voltage out of the relay is around 10.5v. This is what hits #4 first and then it drops 1.5 from there to each plug. This means that #4 gets hot faster than the others and thus you would expect it to be the plug to always burn up first in the series under normal use.
What I was thinking about last night was reconfiguration of the GP wiring to use a midpoint harness. Say I made a custom harness to feed #2 and #3 and run a ground wire from #4 like what is run from #1. This would the way I see it split the amount of current to the system and each plug should receive close to the same voltage. Anyone ever seen this or tried it? Am I wrong in my thinking?