Glow Plug Confusion - PeachParts Mercedes-Benz Forum

mjquillici · #1 10-10-2013, 01:52 PM

Hello,

Here's the story:

- Two days ago, I began having a very difficult time starting when cold. Before this, I had noticed that occassionally, my glow plug light would not come on when I turned the key to the run position, but had experienced no problems starting.
- I tested the relay, and I have full battery voltage at each of the pins and on both sides of the fuse. The battery is new.
- I tested the resistence at each plug and found that #2 is open. The remaining three show low resistence (~0.3 ohms)
- When I turn the key to the run position, the glow plug relay operates every time (although the light on the dash does not always come on). I can hear it click open after 30 seconds. I can also see the dome light brighten slightly after the relay opens.

My question is: Can one dead glow plug cause the very difficult starting condition that I am currently experiencing, as well as the sporadic operation of the dash light? Might something else be going on?

I am ready to order a new glow plug, but I want to know if I may also have another problem.

Thanks,

Mike

vstech · #2 10-10-2013, 01:59 PM

while a single glow plug being dead, can make a 240 difficult to start, especially, since it's not #1... I bet you have additional issues.the most common with the 240 is low fuel pressure, resulting in damage to cylinder #1, low compression from wear, and valves being out of adjustment is also common.
also note, that .3 ohms is NOT a good sign.
a good GP should be reading around .6 -1 ohm... .3 is nearly a dead short, and I'd bet they are all bad.

mjquillici · #3 10-10-2013, 02:06 PM

Got it. Three of the four actually read 0.6 ohms, but the meter yeilded 0.3 ohms, itself, so I was correcting to eliminate the resistence in the meter.

ichris93 · #4 10-10-2013, 02:12 PM

Quote:

Originally Posted by mjquillici

Got it. Three of the four actually read 0.6 ohms, but the meter yeilded 0.3 ohms, itself, so I was correcting to eliminate the resistence in the meter.

Replaced the bad glow plug with a Bosch glow plug, or replace all.

Happened to my 240. I had a glow plug, or two, that was bad and it would keep the light from coming on though I would hear the relay operate.

kerry · #5 10-10-2013, 02:19 PM

Yes. One failed glow plug can cause even an engine in good condition not to start, especially if it's cold.

mjquillici · #6 10-10-2013, 02:53 PM

Great. Thank you all for the feedback.

Next question: How important is it to ream the glow plug sockets when replacing? The reamer costs ~$90, so I'm running a cost-benefit analysis in my head. . .

vstech · #7 10-10-2013, 02:57 PM

a 7mm drill bit is about $4
you can use it by hand, or you can drill out the guts from an old glow plug, and use it the way the official tool does.

either way, if you have carbon, removing it will aid the glow plug in heating the AIR in the Prechamber to make starting the car easier.

mjquillici · #8 10-10-2013, 04:40 PM

Also, where do I look to figure out which engine I have, so I can order the proper glow plugs. Apparently they are specific for different years of the OM616.912.

cooljjay · #9 10-10-2013, 05:00 PM

Doesn't matter, that year all the diesels used the same glow plug Bosch 80006

mjquillici · #10 10-10-2013, 05:25 PM

Great! And it's available at my local auto parts store.

steamboatsykes · #11 10-10-2013, 08:12 PM

Quote:

Originally Posted by vstech

the most common with the 240 is low fuel pressure, resulting in damage to cylinder #1, low compression from wear

That is really interesting. What is the root cause for the low fuel pressure? Pump issues? Does this apply to the 617 also?

layback40 · #12 10-10-2013, 09:01 PM

Best take all GP's out & test with battery to make sure the glow properly. You are moving towards winter on your side of the pond.
Every year about now the cold start problem threads start about now.

Diesel911 · #13 10-11-2013, 01:00 PM

Quote:

Originally Posted by mjquillici

Great. Thank you all for the feedback.

Next question: How important is it to ream the glow plug sockets when replacing? The reamer costs ~$90, so I'm running a cost-benefit analysis in my head. . .

A factory made Glow Plug Reamer for your year and model can be had on eBay for about $37 from El Paso Tool or Zedmak Tools.

Or as mentioned you can use a alternative method.

Diesel911 · #14 10-11-2013, 01:05 PM

Quote:

Originally Posted by mjquillici

Also, where do I look to figure out which engine I have, so I can order the proper glow plugs. Apparently they are specific for different years of the OM616.912.

Any of the Diesel Engines in a W123 from 1980s on use the same Glow Plugs as mentioned; Bosch 80006.
Shop around on the Internet as they are about $9 each. Pad the order with Oil and Filters and Valve cover Gasket to go over $50 and get the free UPS Ground shipping.

Diesel911 · #15 10-11-2013, 01:20 PM

Quote:

Originally Posted by steamboatsykes

That is really interesting. What is the root cause for the low fuel pressure? Pump issues? Does this apply to the 617 also?

I can guess.
The Fuel inlet of the Fuel Injection Pump Housing is right by the #1 Element (that goes to Cylinder #1).

If the Fuel supply pressure is low He is saying that the #1 Element gets more Fuel than the other Elements do because they get less Fuel Supply Pressure than the #1 Element. Sort of like the one who starts Eating first gets more Food.

Because the #1 Cylinder is not getting more Fuel it carries more of the load and that increases the combustion pressure in the Cylinder compared to the others.
Increase combustion pressure pushes the Rings harder against the Cylinder Walls causing them to wear more.

Also the #1 Cylinder has a slightly larger bore then the rest right from the Factory because it is not as well cooled as the others due to the Timing Chan/Gear area; so Mercedes has that the bore should be slightly larger to begin with.

On the Big Rig Diesel that are always pulling a load and imbalance in the Fuel Delivery can actually do neat things like breaking a Crank Shaft.