Just purchased a used '99 E300, and am wondering what the expected warm-up time for the glow plugs should be. In mild weather here, first thing in the morning (60° F), they seem to take 8-10 seconds to warm up, and I was wondering if this was normal or a bit on the long side.

seems normal to me, mine usually takes the same 1st thing in the morning then becomes shorter the warmer the engine gets. hope that helps

Normal, Mercedes glow plugs work a lot longer than some other cars, VW TDI for example.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

They pretty much either work or they don't - no in between. But they are much shorter lived than previous MB engines. The average seems to be about 40,000 miles. The intake manifold has to come off to get to them, so you'll want to learn to do it yourself and change all 6 while you're in there. There's a post here by Lightman that explains the whole procedure and includes great pictures.

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References

I just dropped my 1999 E300 turbodiesel off at the independent shop I've happily been doing business with for a few years on routine maintenance jobs. I have a bad glow plug or two and I asked him to replace all six while he has the intake manifold off. Just wanted to share with the forum here that the car has 126k miles on it and all of the glow plugs worked fine until this weekend.
Ken

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2014 E250 Bluetec 4-Matic
2014 GL350 Bluetec 4-Matic
1996 Chevy Tahoe 4 x 4
2009 ML350 (sold)
2005 SLK350 (sold)
1999 E300 Turbodiesel (sold)
1998 ML320 (sold)
1991 300D 2.5 Turbodiesel (sold)