Last week I my E300D started just fine, but the glow plug light stayed on for the entire fifty mile trip I took. After that and since it has been working just fine. I tested the plugs this AM and they all read at 0.7 to 0.9 ohms which is what they should read. My guess is that when I started the car that day, I wasn't paying attention and started it before the glow plug light went out and the glow plugs went through their cycle. Sound correct?

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Len
'59 220S Cabriolet-SOLD and living happily in Malta
'83 240D 351,500 miles original owner-SOLD
'88 560SL 41,000 miles - totaled and parted out
https://sites.google.com/site/mercedesstuff/home
'99 E300 turbo 227,500 miles
'03 SLK320 40,000 miles - gave to my daughter
'14 Smart electric coupe 28,500 miles
'14 Smart electric cabriolet 28,500 miles
'15 Smart electric coupe 28,000 miles

I have not read of that happening. If someone would post a site where you can look at the actual factory service manual it will have a description of what faults cause the light to act certain ways.

Some of the later models then my 1984 have an afterglow feature that allows the plugs to say on longer after starting. That is in conjunction with a Coolant Temp Sensor.

I don't know if the coolant temp sensor or a loose or corroded connection at the coolant temp sensor would cause your issue.

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84 300D, 82 Volvo 244Gl Diesel