1983 300SD - 245,000 miles. Until a week ago it was always: glow plug light for 10 seconds then start on the first crank and idle smoothly no matter the temperature. As of last week: glow plug light for 10 seconds, start on the first crank then a smoky, shaky idle for 10-15 seconds then a solid,smooth idle - no matter the temperature. I ran 2 cans of Purge thru Thursday and it seems a little bit better but still the same for the most part. Otherwise, hot start and overall performance seem the same. Dripping injector after shut down ? Is it possible that one glow plug is bad ? TIA.

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That sounds a lot like one bad GP.

How long does it take the glow plug relay to click off? I had a similar problem recently; the timer was clicking off after only 10 seconds.

My glow plug light illuminates for about 10 seconds below 40 degrees.

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Thanks Craig. Kinda what I was leaning towards. I may be wrong but I think all the glow plugs are original. Guess I know what I'll be doing Friday night - soaking them all with PBBlaster before trying to change them on Saturday..

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Try measuring the resistance on each one first, unless you want to change them all just so you know they are new.

Although the light goes off in 10 seconds, you should find that they are actually energized for about 40 seconds before the relay clicks off (you can hear it).

What should the resistance be ?

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About 0.8 ohms cold. Make sure you measure with the cable to the GPs unplugged from the relay. You can measure from the female connector at the relay.

Thanks. Is this kit worth a look ? 3 minutes vs 35 seconds... Interesting.

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2A21AKNT52A51BI7EM&year=1983&make=MB&model=300-SD-002&category=All&part=Glow+Plug+Kit

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I have one of those installed in my 300D, it helps cold starts and reduces smoke in cold weather. If you end up needing to replace a relay it's a good deal. I don't know if it's worthwhile to replace a good relay.

True. The 80A fuse in mine popped 6 months ago. Replaced it and haven't had an issue. I think it was simply a 25 year old fuse going bye bye...

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Yup, I would just start with checking the GPs. If the relay turns out to be a problem, I would consider putting in the post-glow relay.

The relay should remained closed for a long time after the light extinguishes. The length of the glow cycle is unrelated to the light, hence the original question.

Relay stays on for 33 seconds. Timed it today.

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That sounds about right.