Alright, this morning I pulled each glowplug from the engine one by one to have a look at what I'm dealing with here.

Tips for other folks doing this job:

(1) I did not pull those injector hard lines, and I can vouch that the job is doable without pulling the lines, even with big hands, but I suggest you wear "Mechanix" gloves to protect your hands from getting scratched up in those tight spaces.

(2) A long 8-mm wrench for those little nuts is key. Mine was 6.3 inches long, and I wished it was longer, but it did the job.

(3) I used a 12-mm flex-head ratcheting box-end wrench to remove glowplugs, and this is a very nice wrench to have for this job, especially with the hard injector lines limiting wrench travel. Flex head really comes handy here, too.

(4) Loosen the clip holding injector hard lines #4 and #5 together and slide it toward the injection pump, out of the way, to avoid scratching your hands on it.

(5) Unhook the vacuum line (with the check valve) from the modulator (off-white plastic box) and shove the vacuum lines running above oil filter housing out of the way so you don't damage them in the process.

(6) For #4 and #5 glowplugs, you'll want to uncouple the thick throttle linkage rod that runs from the firewall and directly over the glowplug area. That rod connects to the manual "STOP" mechanism for the injection pump, and there's a spring pin which is easily removed, and you can free that throttle linkage rod. This way, you can maneuver it anywhere you need so you can get your hand/wrench in there.

(7) For #5 glowplug, there's that cooling hose in the way. Not to worry. Pull a little slack for the hose from the firewall and carefully flex the hose up and over the nut on top of the oil filter housing and let it rest on top of the oil filter housing as you work.

(8) On #5 glowplug, loosen the small nut holding the wiring to the glowplug but don't take it off the glowplug completely yet. There's no room to remove the nut by hand with the glowplug installed (especially if you dont want to lose the nut!). Instead, with the small nut loosened and the wiring free to move, use an open-end wrench to unbolt the glowplug from the hole and just pull it out with the wiring (unhook the oil pressure sending unit wiring to give you more slack). This way, you can have clear access to undo the little nut safely without losing it, and you can use the same procedure for re-assembly. Takes the pain out of #5 glowplug.

Observations:

Each plug was seated in carbon cake, which I could hear when I was pulling them out. Each plug was coated in soot around the tip area, although it was a thin coating and wiped off easily.

Reaming:

I reamed each hole using a 7-mm short-length drill bit, which I purchased at Mc-Master. Overall length of my drill bit was 74mm, and it fit even in the #5 hole, where access is obscured by oil filter housing. I improvised a little bit with my "reamer": I slid a piece of silicone hose on the handle of the drill bit. This served two purposes: (1) improve my handle on the drill bit and (2) limit the travel of the drill bit into the hole (hold the drill bit and the glow plug together, with the drill tip and the glowplug tip even and match the end of the hose to just past the bolt head on the glowplug). I put some dielectric grease into the flutes of the drill bit to collect the carbon cake.

I pulled out a LOT of carbon cake from each hole. I repeated 3 times for each hole. What I like about the drill bit solution is that you can sort of whirl it around gently in the hole after first reaming to scrape off maximum amount of carbon deposit. I pulled out quite a bit of carbon on both the first and the second reaming, while the third time produced just a bit of dusting.

Tests and measurements:

I tested each glowplug for glow, using the battery of my car to do it with the heavy-gauge cables that I made. Results: each plug begins glowing at the tip and produces orange-colored glow. Each plug took up to 3 seconds to develop that bright orange glow at the tip. Is that too long or normal?

After I reamed and re-assembled everything and tightened it all down, I re-measured the glowplugs...

Glowplug resistance (relay harness to battery ground):

#1: 1.7 Ohm
#2: 1.8 Ohm
#3: 1.7 Ohm
#4: 1.8 Ohm
#5: 1.7 Ohm

Voltage at each glowplug as soon as relay is switched on (glowplug to battery ground):

#5: 9.95V
#4: 10.00V
#3: 10.15V
#2: 10.18V
#1: 10.20V

This was the sequence in which I measured, so it's hard to say whether the creep up in the voltage was battery fatigue from glowing a bunch of times sequentially or diminishing efficiency of the plugs.

I should note also that while most of the glow rod is coated with some kind of an enamel, the tips have a different texture from the rest of the plug. I'm wondering if the coating burned off or corroded or whether I'm barking up the wrong tree with this observation?

So, based on my tests and observations, what would you guys advise me to do? Do I still need new plugs?

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84 300SD - bought in April '06 with 237,000 mi