In early stages of sorting out the 1989 190D my friend just bought. When I glow, the light comes on and then I start the car when it goes out. Starts perfectly with no rough idling whatsoever...so apparently all the plugs work. But then, after a couple of minutes, the glow light comes on. What could be the cause of this symptom?

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14 E250 Bluetec "Sinclair", Palladium Silver on Black, 153k miles
06 E320 CDI "Rutherford", Black on Tan, 171k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver, 142k mi, wastegate conversion

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Glow plug breaking down during after glow after it has been hot for a while?
Glow plug relay flaking out and turning on the light after start?

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

My dads 92 300D does that when the battery is low. After driving around all day and the battery is all charged up, it stops doing it.

It can also be indicative of a fault, maybe one plug is bad. It is spring you probably would have a hard time noticing a slow glow plug in a 5cyl car when the temps are high.

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68 280SL - 70 280SL - 70 300SEL 3.5 - 72 350SL - 72 280SEL 4.5 - 72 220 - 72 220D - 73 450SL - 84 230GE - 87 200TD - 90 190E 2.0 - 03 G500

Nissan GTR - Nissan Skyline GTS25T - Toyota GTFour - Rover Mini - Toyota Land Cruiser HJ60 - Cadillac Eldorado - BMW E30 - BMW 135i

More than likely you've got a plug that is borderline. I've seen that before, time to test them. It may also be a bad temperature sensor, I know that on later cars the GP relay can also indicate a fault in a sensor.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC