'97 E300 GP Light comes on and goes off like normal. Engine starts but is very sluggish for about 30 seconds then smooths out. What might be the cause?

My 1982 300SD did that till I found 2 bad Glow Plugs. On the SD the glow plugs stay energized even after the Glow Plug light goes off in the cab. About 30 more seconds. You can hear relay click after 30 seconds after the Glow Plug indication light extinguishes in the cabin. It may be the same as yours.

What I'm getting at is the glow plugs are there for starting the car of course, but also are there to stay on after then engine starts. If it starts ok once it is warmed up I'd look at the glow plugs. Easy to test with an Ohmeter.

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The glow plug relay light will indicate a bad glow plug if is #1 or if it is two or more other than #1. This is because the relay compares the current through #1 with the average current through the others and it usually takes more than one bad one to drag down the average enough to be noticed. If you have only one bad glow plug (and not #1) the system may not "notice." After 30 seconds or so, the bad cylinder starts firing on its own and thee engine smooths out. Your car also has afterglow (the glow plugs stay on for awhile after the engine starts) but that won't help if a glow plug is bad. There are other possible reasons for your problem but that's enough for post #1.

Jeremy

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Thinks for the help Found 1 bad glow plug.