New Development on Glow Plug Problem

After days of grappling with a glow plug issue on my 1979 W123 300D, I called the dealership to verify the part number of my glow plugs, and was told that, with all the cranking I've done in the course of
working the problem, my engine would have started regardless of the glow plug type or specification, just on compression alone. Evidently, glow plugs are not the problem.
I found bubbles in the in-line fuel filter, so purged the fuel system. Still no luck. The engine will not even fire, even after 60- seconds of cranking--not even a cough.
I am completely stumped at this point.
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wrote:

Uhm... how's oil looking like, on the measurement stick? Black and with normal density? Or white and foamy? Or viscosity is low?
Than: try to touch cooling fluid in the radiator: no foam? No oily feeling on the fingers?
I'd think that the dealership is thinking in terms of new (not today's...) diesel engines, not old ones... Engines of the late '80s could start up simply cranking enough. Without glow plugs running fine your diesel engine will NEVER start up, imho. Not even after days of cranking! In the older days truck drivers on the alps used to put embers under the engine, to help glow plugs... but it is quite dangerous and I wouldn't suggest you to make such experience. If you can send me the 6 digits that identify the engine, I can help with some WIS information related to p.n. of glow plugs. Or, check: http://ecat-online.bosch.de/toc/en/00008D201286BibXyADHNyJIVNe:-1.srv?view=VIndexFramesetJsp ;-) Put in the right country, language and chassis number.
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Thank you very much for the advice. I agree that the newer engines would be much more prone to starting without the glow plugs.
Oil looks normal--black and normal viscosity. No foaming, discoloration, etc. I also purged the fuel system of air, and there was air, for some reason. I could see bubbles in the in-line filter, and there were bubbles when I released the pressure on the main fuel filter and purged. I do not know why there was air, but in any event, purging didn't seem to help any. It was at that point that I sent the car to the shop by tow truck. I had simply exhausted all of the theories.
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I had a similar problem with my 1982 300 TDT. It was the EGR valve which was stuck open. I ended up spending $6500 on having my engine rebuilt and now it starts right up. At first the new engine would not start properly because of the stuck EGR valve, which I had reinstalled after the rebuild.
I did some research and learned that the EGR valve can be set to closed and just left that way with no ill effect to the engine, so that's what I did. I removed the valve and replaced the gasket with one I custom made that does not have an opening in it. It just seals off the EGR valve permanently.
Paul Fretheim
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But as I recall, the big problem in this case is the wires to the glow plugs are glowing red. And that ain't right and should be easily solved with a volt/amp meter and some simple tracing of the wiring.
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wrote:

Yes, they do glow red, indeed. I have given up on the diagnostics and am awaiting word from the Benz garage as to what the problem is. I don't mind serial troubleshooting, but after five unproductive days and no solution, it was off to the shop.
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I seem to recall that some early diesels had the glow plugs connected in series and later ones in parallel.
Is it possible that you have the wrong type? Presumably they would have quite different resistances and that could account for the glowing - You may have resistors designed for 2.4v instead of 12v - sort of like putting 120v on a 12v light bulb.
The other thing that could be wrong, is the glow plug relay.
I would go to a MB dealer and get them to check the proper part numbers. Or, post your VIN and I will check the parts for you.
Graham

Yes, they do glow red, indeed. I have given up on the diagnostics and am awaiting word from the Benz garage as to what the problem is. I don't mind serial troubleshooting, but after five unproductive days and no solution, it was off to the shop.
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Early model like Randall's are in series and if one died, it would take the others out. However, those system can be upgraded to the modern parallel system which I told him...
The new system is easier to install, alot less hassle and faster glowing.
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I've ordered the conversion kit, and will install it as I wish I had done originally.
However, the latest development in the situation is that the start failure was not due to the glow plugs after all.
Thanks to all who suggested valves, whom I ignored, because I had just had the valves adjusted by a reputable shop whom I've used for many years, a Mercedes-only shop with qualified Mercedes-trained technicians. Yet the current report is that all the valves were tight, and the timing chain completely worn out, albeit, thank God, not broken. Apparently it had jumped time, which accounts for the engine not firing.
So now, the shop is replacing the timing chain, which I had asked the previous shop to do and was told "Don't worry about it." Then the technician will adjust the valves properly, and I only hope that they weren't burnt. It has been only about 1,200 miles since the flawed adjustment, so I hope that the valves were not burnt. Time will tell.
Many thanks to all who contributed to my abortive attempts to deal with this situation myself, and my apologies to those whom I dismissed due to my preconceptions of the work that I thought had been properly done recently.
Live and learn.
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A worn out timing chain has nothing to do with the red hot wire. I still think there is other problem.
On the parallel glow plug system, the relay has 4/5/6 (i.e. # of cylinders) points and wires going to the plugs. Just measure the resistance between each point and engine (or ground). Each plug is about 1 ohm resistance. Simple physic and math tells each draws around 12 amps. That is quite big and why it grows fast.
I am not familiar with the serial system but I would expect each plug is no more than 0.1 ohm or there won't be enough current (translating to power/energy) to grow the plugs. Another interesting thing is, what is the rating of the fuse on the relay? Why doesn't it melt before the wire goes red hot?
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I always found changing to synthetic oil did more good than having the valves adjusted. In fact it was the biggest thing when it comes to starting in the winter.
The other thing is you have to spin the engine fast enough. As has been pointed out it WILL start even with no glo plugs if the engine turns over fast enough. For example, tow it to 40 mph even with an automatic, ity'll just start. It has to, its purely mechanical. Unless the engine is siezed this will start any 80s diesel benz.
--
Need Mercedes parts? http://parts.mbz.org
Richard Sexton | Mercedes stuff: http://mbz.org
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Fascinating... valve was the culprit... along with timing chain... which would include the timing chain tensioner. It is most likely the timing chain tensioner is weak and cannot handle the stretched chain.
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I can't envision glow plugs connected in series. How is this done when the engine block is the common ground?
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Chas Hurst ha scritto:

It's true. You have two insulators and two connectors on each glow plug. The one that is... entering... is larger, than the one that is... exiting... is smaller. There's no ground connection on the mechanical part of the plug (at least, not internally).
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This is very interesting. I'll definitely keep this story in mind when I hear from the Mercedes garage later today.
I'll expose my ignorance and ask: Where is this EGR valve located?
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The EGR valve on my 1982 300 TDT is to the passenger side of the valve cover. It is mounted on the exhaust manifold, toward the front of the engine. There are two vacuum lines connected to it. The vacuum actuator part of the valve is sort of shiny metal. It's shaped a bit like two wide funnels with the spout part removed welded together at the open fat end of the funnel. So it is a cylindrical shape that is much wider at the middle than at the ends. Easy to draw, but hard to put into words. What is does is, it controls whether or not exhaust gases are recycled through the engine. If the valve is open, then the gasses do recirculate, which helps the engine more completely burn the fuel, but at low RPMs makes the engine difficult, if not impossible to start.
I am not positive about where it mounts, it might be part of the air chamber. It's dark and below freezing outside right now or I would go take a look.
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