Hey guys, I just bought all new plugs for my 85 om617 in my 78 300d. I have a solenoid with a switch to activate the plugs that bypasses the relay. Long story short the glow plugs stayed on for an undetermined amount of time enough that the block was Luke warm. . .. . very pissed here. I am still battling either a bad solenoid, bad switch or wires touching or grounding somewhere. It had worked great for almost a year until I changed the glow plugs.

Your question is worded a little oddly so I'm not sure which car your referring to. The long and short of it however is that they can stay on until the burn out. Long glow session will also reduce the life of the plug. If they were left on for long enough to warm the block I would pull each one out and make sure that they are still functioning.
On a side note I believe the stock relay limits the plugs to 30 seconds of glow time before they switch off.

you basically answered my question. but sorry yes let me ask it more clearly. Did I cause any damage to them by leaving them on for so long? The car started right up a while later when the block was cold so I think I am still in good shape. Oh well lesson learned I guess

There is no way on earth that the battery would be able to keep the plugs hot long enough to warm the engine block. I bet the typical 100A battery would be lucky to keep them powered more than 5-7 minutes continually before quickly dying off. They draw several AMPS apiece!

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If your car was running and GP relay stuck on, then you'd have more to worry about because they would indeed continue to glow for as long as you drove. Like light bulbs, they burn out after X cycles or Y hours, or a combination of both. Metal fatigue.

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1987 300D

mmm, I bet you could get a fully charged good shape battery to hold the plugs in for more than 7 minutes before being discharged. when cold they pull nearly 80 amps, but after warming up to temp, it's more like 20... it'd be like leaving your headlights on high beams order of amp draw... should be good for an hour I'd bet... and still have reserve left to start the car... which would INSTANTLY start, because the head would be toasty for sure, not sure of the block temp, but there is a lot of coolant in the head, could certainly warm the block...

but as to the OP's question, yeah, likely the plugs life have been DRASTICALLY shortened!

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1987 300TD
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This would be factually correct.

If the relay sticks on, what do you think is going to happen when the drive is over?