I am having trouble getting my '81 300D(non-turbo) to start lately. It is VERY cold in the Texas Panhandle(9-14F).

My glow plug light and relay work fine. The light goes off just like normal and the relay kicks off at the same time as usual.

My question is whether or not it's possible for there to be a glow plug problem with the light still working the same.

The engine has been running very roughly lately so I do suspect that I have some injector issues as well. I also have noticed that it starts easily if the engine is warm but practically kills the battery/starter if it is cold and the engine has sat overnight.

I tested the number one(the front-most) glow plug and it glows cherry red when I run 12 volts through it. In the process I twisted the wire that goes to it because I couldn't get it off before hand. In order to get the little nut off I eventually noticed that I was screwing the whole post out of the plug. I tested the GP again once the post was back in and it still worked.

Now I have to splice some new wire to the number 1 GP wire. For all I know it's the only one that works.

I also had my multimeter stolen recently so I can't check them at the plug that goes to the relay.

The starter is three months old and the battery is brand new. I also have been charging the battery just in case the alternator(3 mos. old) is bad, still no cold start without some serious cranking and it has to be above 30F to pull that off.

I forgot to mention that the previous owner had upgraded to the pencil type glow plugs. They worked flawlessly last winter at temps as low as 12F. It would fire up immediately.

I have had to really crank on the starter lately so I hope I haven't weakened it. I was also running 15W40 in some very cold weather. I was low so I added about two quarts of Rotella Synthetic 5W-40 last night and ran it for about 10 minutes hoping it would thin out what was still in the engine.

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1993 W124 300D
-297K on the clock as I type this.

Have you set your valves lately ?

If you have someone else to help you test.... at its coldest try a two second spray of starter fluid into the intake tube before the air cleaner just after you engage the starter.
If that really helps then your injectors are certainly suspect. It could also be the condition of your precombustion chamber innards. That pintle helps disperse the fuel spray to the 7 tiny radial holes which then go to the bore... this is a much ignored but important part of a good running diesel engine... these types of things show up at the margin of the working envelope... IE... cold weather.
Without close inspection of the electrical diagrams of your specific model I do not know if the light and plugs can function fine without each other being affected.

You are really in the block heater zone of functioning right now...
Do you have one ?

Yes, I do, but it is still in the box.

I bought it last year when I was having similar problems but my problems went away when I replaced the starter.

This starter is only three months old.

I ran out of fuel about a month ago and whatever transpired magnified a rough running problem I was having. I spent a lot of time engaging the starter during that fiasco.

I thought that maybe my valves needed to be adjusted so I opened her up and found that only one valve was out of spec. I adjusted it and there was little to no improvement.

When I ran out of fuel I tried to get to the station with transmission fluid and motor oil. I only had about a third of a quart of TF and it actually ran on that but the motor oil was a no-go. It was 15W40 and it was about 50F outside.

It was running a little rougher than usual after that but I had already noticed that I was having some rough running issues.

I will be testing the other four GPs tomorrow so I can eliminate them as possible suspects.

I am pretty sure that I need new/rebuilt injectors and that is probably contributing to my cold starting issues but it fires right up when warm.

This leads me to suspect the GPs but the light and relay are working as normal.

The one GP I pulled tonight was practically spotless. No carbon whatsoever on it.

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1993 W124 300D
-297K on the clock as I type this.

Are you saying that the relay opens at the same time that the light extinguishes?

Depending on which relay you have, it is possible for the light to be unaffected when there is a single glow plug failure, provided it is not the #1 plug that has failed.

He's got retrofitted pencil plugs, so unless the relay was switched out, the original relay won't be comparing plug 1 against the others. So, if you have the original relay, yes you can have bad plugs and the relay will not be giving an indication.
If it were me, I'd switch out to synthetic oil. The difference in cranking speed between dino and synthetic at those temperatures is substantial.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

No, the light goes off at about 10-15 seconds but the relay stays on for about 30 seconds.


The relay was replaced apparently. It is on the left fender well and the old one is on the firewall.

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1993 W124 300D
-297K on the clock as I type this.

When I screwed up the wire going to #1 GP the light didn't function.

I spliced the line and now the GP light comes on.

I have checked three GP's now and they all glow cherry red when I give them twelve volts.

I checked 1, 4 and 5. Four and five have those "hand tighten nuts" on them. They look like a knurled wheel. They seemed very loose when I started taking them off. I don't know if this was causing a bad contact or not but I tightened them down as much as I could with some pliers.

All that I have left to test are 2 and 3. Three also has the knurled wheel thingamajig and 2 has a nut that is as stubborn as 1.

Either they are too tight or they are locking nuts because they won't come loose unless they bring the post out with them.

I am thinking that my oil situation needs to be sorted out and my injectors need to be replaced.

Unless the two GP's I didn't check are bad, my only problem could be the oil and injectors.

__________________
1993 W124 300D
-297K on the clock as I type this.

Given the fact that the original loop plugs have been replaced and we don't know how the new relay is wired into the glow light, I would not rely upon the light as an indicator of the condition of the plugs. I would ohm them out to get some sense of whether they are working correctly.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

My multimeter was recently stolen so I have been taking them out and connecting them to my Craftsman battery charger.

All three that I have tested glow red hot in a matter of seconds.

Would doing it this way give false results?

I figured that if they glowed they were good.

I guess there could be a problem with the wires.

I might just have to go buy a cheap Harbor Freight multimeter.

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1993 W124 300D
-297K on the clock as I type this.

If they glow, they are good. Whether they are getting 12 volts when you are glowing them in the engine is another story. My 85 had 1 working glow plug #4, and the light and relay functioned perfectly. Do not rely on that theory. I have a pic in another thread.

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79 MB 280 SEL Euro 133k
77 MB 450SL 154k
05 Mustang GT Vert (3) 104k
12 TSX Wagon Tech (66k) (192k)
06 Subaru Outback base (135k) 164k
16 Acura MDX (109k) 111k
18 Silverado 2500 LTZ Midnight (212) 56k
97 Ford Ranger 163k
11 RAV4 154k
01 Escape 173k
04 Honda Pilot 292k
1967 Mustang (Resto Project)
1968 Mustang (Parts Bin)
00 Ford Ranger 124k

I just replaced my GP in the 240D. Resistance was too high across all four, though they would still glow red hot. Resistance across the new ones was normal, so I figured that they were bad but not failed. The light worked fine and the car fired up pretty fast, even at temps as low as 35-40, but the engine would miss and smoke a bit until it got warmed up. New GP seem to have fixed that.

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Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT

OK, I will definitely have to test them before I can tell for sure if they are my problem.

Another question, what did your glow plugs look like when you removed them?

Mine are practically spotless. One did have a tiny bit of "scale" on it in a few places but even it was practically shiny when cold.

My rough running problem continues even after the car is warmed up.

My injectors for sure need to be replaced but I need to make sure if/how many of my GP's need to be replaced.

I will say that after I tightened up those fancy nuts on three GP's my startups are very smooth and quicker than before.

__________________
1993 W124 300D
-297K on the clock as I type this.

You can't judge a good plug by looks.

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Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)