Engine#617.912 and series glow plug wiring - PeachParts Mercedes-Benz Forum

Rudder · #1 10-24-2011, 03:09 AM

The other night I was at a evening gathering with my wife and daughter. My 1979 300TD failed to start because of a faulty glow plug, (# 5). (The glow plug lamp did not light as well) This I determined by testing the glow plugs with my ohmmeter that I luckily had in my crash kit that I carry in the boot of the wagon. Being quite late and the weather turning to rain I was motivated to to get my family home as quickly as I could rather than leaving the wagon in a parking lot overnight and hailing a cab. The 79 300TD glow plug system is wired in series and if one glow plug goes bad then the whole system is dysfunctional along with the glow panel light. In order to get the glow plugs working again, I decided to rewire the glow plugs wiring harness to by-pass the faulty #5 glow plug. I borrowed a steel coat hanger from the host of the party and fashioned it so as to go from #4 glow plug to the firewall relay/fuse-strip. This worked beautifully, the glow plug indicator lamp came on as normal and the engine started just fine although with a bit more smoke than usual.
So I figured the engine would start and run on four cylinders at first when the engine was cold but I thought that #five cylinder would start to combust and kick in as the engine warmed towards it's normal temp. As I drove home that night the engine ran smooth but a real reduction in power was quite noticeable. So I"m thinking here that #5 cylinder never got lit! (I drove over 20 miles to get home so normal running temp was achieved.) So my question to you all is, why didn't #5 cylinder fire off after the engine warmed up?

Low compression is my guess but the engine has run quite strong up and till this event. I have only 98,000 on the engine! I have yet to install a new plug and haven't driven the wagon since this event the other night!

Any insights or comments?

Thanks...

Mad_Matt · #2 10-24-2011, 05:54 AM

Good question. Is it possible that the Gp's stay hot all the time and ignite the fuel in the pre-chamber before the Intake valve opens and pulls the flaming ball into the cylinder?

I think these are IDI diesels and not DI but I'm not sure so I'm waiting for an answer to help fill in the blanks as well.

vstech · #3 10-24-2011, 07:11 AM

it's possible you have a valve out of adjustment, dumping pressure too fast, or perhaps the lack of start in that pot held you back long enough to put too much fuel in there and the wetness from additional fuel kept the chamber out of the sweet spot to fire.
it's also possible the all fired fine, but you have clogged fuel filter or tank vent or some other power robbing issue...

kerry · #4 10-24-2011, 09:27 AM

Not sure, but I'll hypothesize it was all in your mind?

If you have replaced the plug, do a 0-60 timed run with systems normal. Then let the car sit, get cold, rewire the plugs to exclude plug 5, start it, get the engine up to temperature and do a timed 0-60 run again and compare the results.

vstech · #5 10-24-2011, 09:31 AM

oh, Welcome to the forum!
update your UserCP and add in your location, and any info you have about your car etc.

kerry · #6 10-24-2011, 09:54 AM

Quote:

Originally Posted by vstech

oh, Welcome to the forum!
update your UserCP and add in your location, and any info you have about your car etc.

X2 on Welcome.

He has a boot in his wagon so I'm guessing he's not on the west side of the Atlantic.

barry123400 · #7 10-24-2011, 11:02 AM

Strange to say the least. I can understand that cylinder not contributing to the start. To not come on line though as soon as the engine started up is odd. Since from your description the engine always ran better before this episode.

I wonder if in the failure mode that particular glow plug developed a breech to the atmosphere. I have never seen this but strange things do occur. Personally I can think of no other sensible senario but you may find one.

Some people in the past just carried a short jumper when they own a model with the series plugs. Just a test light should identify the culprit to bridge. Actually this is important as many of these cars just will not start without at least some glow plugs operational.

Any failure should be checked also for no short to ground existing in any of the other glow plugs. This would apply too much voltage to the others depending on the one shorted and take it or them out. Usually just one as there is a time factor here where the weakest plug element will open up first. In your case there probably was not another shorted plug.

vstech · #8 10-24-2011, 12:22 PM

Quote:

Originally Posted by kerry

X2 on Welcome.

He has a boot in his wagon so I'm guessing he's not on the west side of the Atlantic.

heh, I skipped right over that explanation in his post. likely you are right.
or the guy uses a boot to store his emergency kit inside... steel toe perhaps?