When I try to start my car the glow plug light does not come on anymore and starts real rough, Im guessing I have some bad glow plugs? Funny thing is, the glow plug light does not come on right away but does come on when the car is started and stays on for about one minute. I know there is a after glow on this car when the temp is cold, why would the plugs work after the car is started and not before the car is started?

just because the light comes on doesn't necessarily mean they are glowing. i just changed all plugs in my friends 90 300d with the same symptoms.. all plugs were bad.

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That symptom indicates that one or more glow plugs are bad....it doesn't indicate that the plugs are somehow working after the car has started.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 179k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 77k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Remember this system in the OM602 configuration (And Newer OM603s) is
the "Smart" system (ELECTRONIC).

1.It's got the I.C. light turned off to let you know you have at least one bad Glow Plug.
[the one minute or so WAIT period before the Light Illuminates is to give you time to recognize the Signal (No I.C. Bulb Illumination)
2.It's still delivering the required voltage to all the Glow Plugs.
3.The Bad Glow Plug(s) are not "Firing Up" which is why you get the "Shakes" and "Smoke".
4.The Bulb's Illumination after start lets you know the System (Afterglow) is working.

Hoo Boy,Sure sounds good?

Here Ya Go:

Attached Thumbnails

   

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'84 300SD sold
124.128

Thanks, new plugs will be here next friday. I replaced the plugs about 2 years ago, should they have lasted longer?

Depends on the brand of glow plugs. Bosch usually have a good life. Depends on glow cycles and miles I believe.

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92 300D 2.5L OM602 OBK #59

New glow plugs are in and she fires up just fine now.