78 300d glow plug question - Mercedes-Benz Forum

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post #1 of 17 (permalink) Old 01-03-2012, 11:25 AM Thread Starter
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Post 78 300d glow plug question

I have a 78 300d that will not start. I replaced the glow plugs a month ago and it started up just fine, until this week. My car has the loop style plugs wired in series. When I turn the plugs on, the first 3 get really hot, the 4th gets barley warm, and the 5th is cold. All of the wire loops in between the plugs get hot. I ohmed out the 4th and 5th plug and they tested good.

I tested the voltage at each plug and here is what I got:
#1 10 (closest to the firewall)
#2 7.5
#3 3.5
#4 3.5
#5 0

Is it normal for the last glow plug to not feel hot?

Thanks
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post #2 of 17 (permalink) Old 01-04-2012, 12:38 AM
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No, it is not normal. I would think #4 or #5 is not working correctly or the contacts are dirty. Why did you replace the glow plugs?
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post #3 of 17 (permalink) Old 01-04-2012, 08:36 AM Thread Starter
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When I bought the car, it was starting slow, and I wasn't sure how old the plugs were. When I took them out and checked continuity, I noticed that a few were bad so I replaced all of them.

I did notice that there was some carbon build up in the GP holes. Could that cause them not to work?

Thanks for your help.
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post #4 of 17 (permalink) Old 01-04-2012, 08:59 AM
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If you replace the glow plugs, MB advises to ream out the glow plug holes with a glow plug reamer (basically a threader filled with grease). It might cause bad heating of the prechamber or it might cause a short circuit.

Where did you check the voltage of the plugs? There is a difference whether you check it at entry or at the exit.

Check each plug for continuity: remove the resistance wires between the plugs, so you can check the plug itself. You can leave the plugs in the engine.
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post #5 of 17 (permalink) Old 01-04-2012, 06:49 PM Thread Starter
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Thanks, I don't have that reamer, so I might try a round wire brush to clean the carbon out. I checked Voltage at the end of each glow plug.

I'm still confused on why the resistance wires are all getting hot, but not all the glow plugs. I would think that if I had a bad glow plug, or a short, the resistance wires downstream would not be hot.
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post #6 of 17 (permalink) Old 01-05-2012, 03:16 AM
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If the resistance wires are getting hot, the current is flowing. Perhaps it is (partly) shorting at #4 or #5. The 0V at the exit of #5 is good, what is the entry voltage? it should be around 1V. The FSM indicates that you should measure the voltage difference between entry and exit of the glow plug.

Do you have a current clamp to measure the current flow? it should be around 50–55A.

You could also use a standard threader and fill the slits with grease.
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post #7 of 17 (permalink) Old 01-05-2012, 07:49 PM Thread Starter
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Thanks so much for your help. I'm going to clean the GP holes tomorrow. I'll let you know if it works.
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post #8 of 17 (permalink) Old 01-08-2012, 08:17 PM Thread Starter
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ok, I cleaned out my glow plug holes, and they are all working now. However, my car still will not start.

I rebuilt the fuel injctors hoping that would help, but it didn't I removed the the fuel lines off the injectors and egaged the starter to check fuel flow. The fuel seems tojust drip out the lines, even during max throttle. Is this normal? Does suction from the engine pull the fuel out from the IP?

So far I have:

changed battery
installed GPs
changed fuel filters
instlled now nozzles on injectors
Checked compression (only getting mid 200s cold)

What would you do next?

thanks so much for your help
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post #9 of 17 (permalink) Old 01-09-2012, 04:05 AM
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During starting the IP should deliver the maximum amount of fuel irregardless of the position of the accelerator pedal. The engine does not suck out the fuel, the IP pumps the fuel. It should deliver more than a few drops. When you pump the hand pump, do you hear the distinctive hiss of the fuel-return valve opening? that means that the IP fuel chamber is pumped full of fuel and than the excess is pumped back to the tank. If the fuel chamber isn't full, the engine will not start.

Did you change the fuel strainer at the tank?

Did you check the fuel vent, it is under the car near the left rear wheel.

How did you rebuild the injectors?

Checking the compression cold isn't useful.
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post #10 of 17 (permalink) Old 01-09-2012, 06:47 AM
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Originally Posted by Govert70227 View Post
Checking the compression cold isn't useful.
Ehhh? If you don't have compression cold then how could you ever get the engine started?

The low 200's compression is your problem. The absolute bare minimum for those diesels is 275 p.s.i., cold or hot.

To check for MAXIMUM compression then you do indeed need to have the engine at operating temperature.

Do you have the ability to install a block heater? It just might do the trick for you until you get the chance to re-ring the engine.
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