So my glow plug light likes to illuminate sometimes after startup. As in, I'll be driving and suddenly there it is, shining back at me. Does it illuminate when I start it the first time? Thanks for asking, because no, no it does not. I don't generally have any issues with it starting, as A: I don't drive it when it's cold and; B: I still wait before I start it. I know that I should track down the latter issue, but I am (perhaps incorrectly) more concerned about it warming while running.



Car in question is a '91 350SDL with a 3.0 OM603.

I had, past tense, this same issue with my '91 350SD, 131K miles, so seldom driven. It was close to OK because it was still starting in the cold; at a diesel test the guy said I only need a new relay. He was probably wrong, it got worse (hard starting), and I likely needed new glow plugs and a relay right from the start. I am now looking for either a repairer in the Denver area (preferred) or a very good tutorial. I understand it is difficult on this car.

"On the 6cyl OM603 they are a royal pain. Most people remove the intake to get to them..." to quote this link https://dieselgiant.com/glowplugrepair.htm.

The glow plug light is telling you that you have at least one failing GP, simple as that. Replace them as a set and go another 150K miles before you have to think about it again.b

This. Mercedes engineered the car to tell you when it has a fault in the glow system. And what it does is not initially illuminate when you turn the key to the ON position, then it will illuminate when the car is running. So it's a tell tale.

Buy all new glow plugs. Replace them. They're usually the part that goes wrong. There's only 3 parts in the glow system; the glow plugs, the wiring, the GP relay.

Well, despite the posts to the contrary......when that happened to me several years ago, replacing the glow plugs did NOT fix it, however, a new relay did. Rather than guess and throw parts at it, test all the glow plugs and wiring with a meter. If the plugs are bad, replace them.....if they are not, replace the relay....

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2012 Mercedes ML350 Bluetec 102K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 238K (mine)
1998 Volvo S70 T5 Turbo 211K (kids)
1994 Ford F150 4WD 246K (firewood hauler)
1983 Mercedes 300D 384K (diesel commuter)

Those glow relays do go bad sometimes. I agree on testing the gp.

If you do change glows, use the bosch or ngk.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

IF you would like a good tutorial on troubleshooting the glow plugs......here is a link to what I use. Just my .02 Mercedes Diesel Glow Plug Repair

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2012 Mercedes ML350 Bluetec 102K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 238K (mine)
1998 Volvo S70 T5 Turbo 211K (kids)
1994 Ford F150 4WD 246K (firewood hauler)
1983 Mercedes 300D 384K (diesel commuter)