I was told this and sorta just ignored it: when your intake is off and it has been a few years since you did it, change the glow plugs. They are cheap.
I took off the intake to ream out all of the glow plug holes and clean the intake ports and have my injectors sent off and rebuilt (anyone who needs it done, contact Greazzer, he does a great job for a fair price). I tested all of my glow plugs when I reamed out the holes and they all tested correctly, glowing at the tip, even though they have not been changed in at least 2-3 years if not more, the PO's service records were incomplete, but they were the newer fast glow type. I had replaced two that were bad with some old used ones out of my dad's 240D that were still good. Needless to say, I got the intake back on about a week ago but then the glow plug light did not come on a couple of days ago so I tested it today and 1 bad glow plug and 2 more that were reading as if they are on their way out. Now this wouldn't be so bad if the bad one was the plug for cylinder 1,2,4, or 5 but it was 3 and as far as I can tell 3 is the only plug that you cannot get to with the intake on.
Hopefully I can get the intake off without tearing the new gasket but I am going to replace all 5 plugs for sure this time.

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Current: 1975 450SEL, 83 300D, 88 Yugo GVX, 90 300D OM603 swap, 91 F150 4.6 4v swap, 93 190E Sportline LE 3.0L M104 swap, 93 190E Sportline LE Megasquirt, 03 Sprinter, 06 E500 4Matic wagon.

You can get to #3 by removing the cruise actuator. It's a trick getting the linkage back in place with the manifold on, but less work than pulling the manifold and injector lines.

Sixto
87 300D