1992 300d glow plug issues
 

I have a 1992 300d with a 5 cylinder diesel. The glow plug light no longer lights when the ignition is turned on, but the light comes on and stays on for a little while after the car starts. I have tested obvious things and determined the number 4 glow plug is bad. As it is now in the 40's the car will not start. Would one bad glow plug keep it from starting?

Also is it necessary to pull the manifold to replace all of the glow plugs?

You did not say how you determined the #4 Glow Plug was no good.
The Diesel Giant Site has a how to pictorial using an Ohm Meter.

Unfortunatley that is only part of the story. Every ounce in a while I have read of folks who had Glow Plugs that tested OK with the Ohm Meter but when the pulled the Plugs and tested them with a 12 volt Battery to see if the actually got hot they did not.

Another problem is that if you model has an Aluminum Cylinder head and uses long Glow Plugs; attempts to remove them often result in the Hex head portion snapping off.
This results in the need for the Glow Plug to be drilled out with the Cylinder Head on or the Cylinder Head needs to be removed and sent to a Machine shop.

The Glow Plugs get stuck do to Carbon buildup around the heater Element (when you remove the Glow Plugs ream out or otherwise clean the carbon out of the holes), corrosion between the threaded area or a combination of both problems.
On occasion the Glow Plugs do not seat well on the tapered area near the tip and carbon fills up the space between the Head and the Glow Plug.

I would take the time and read the Repair Links the pertain to your Engine or any of the Aluminum headed Engines and plan a time to do the job.

More bad news is that if you let a Mechanic do the Glow Plug removal job and one breaks off it is safer and makes more money for them to pull the Cylinder Head off and send it to a Machine Shop to have it drilled out.

What about why the car wont start?
 

I have the same problem with my 1992 300D not starting when in cool weather. Why does the glow plug light in the dash not come on for 30 seconds after engaging the key? Once it does, it stays on for about 60 seconds but the car will not start. Any suggestions?
Thanks
geosab