Today I replaced all 5 glow plugs on my 1979 300SD. Normally I try not to work on my car on Sundays, but since it will get in the 30's this week, I thought this must be done today.
The glow plug light has been flashing after startup for several weeks now, and I know one or two cylinders is not firing right away from the sound and vibration.
The first 90 minutes was spent cleaning and repairing the relay, which was missing it's cover, and had been sprayed with oil, from an unknown oil leak, for years. So after I cleaned all that out, and snatched a relay cover from my parts car, I moved on to begin the real work.
I did a resistance test on the relay plug for each glow plug. It looked like 2 of them were out, do not remember which ones. I didn't really care since I was replacing all 5.
I ordered Monark plugs from Mercedessource, along with his tutorials, special tools, and reamer. I followed his directions to a tee. I removed all the fuel lines before I got started, for clearance.
Cylinder #4 was the worst, as my wrench had about 15 degrees of play to unscrew the plug. It took forever. I finally got fed up and removed all the throttle linkage in the way. That was the worst plug - hard to remove and rusty on the threads - probably because it was so hard to get to the previous owners ignored it as much as they could.
The most carbon was in the first two cylinders. Reaming was fine, I did not have any problems. I even vacuumed out the hole and used Kent's brush when I was done reaming. The hole for #4 scared me though, as the glow plug's threads looked a little damaged. There were a few sparkly metal shards in the hole, too, that I vacuumed out, and hopefully none fell into the cylinder. The new plug screwed in just fine, so the threads were not stripped in any significant manner.
When I was all done the sun had already set (I have no garage and work outdoors). The only fault I had was with the 8mm nuts that connect the wiring harness to the glow plugs. These did not have washers, and when tightened, they pulled the wiring harness connecter with it. I did my best to not let that happen, but unfortunately two of the 5 rubber (now brittle) wire housing tubes "cracked" when I tightened.
I lost one of the old 8mm nuts too, it fell behind the fuel pump. Oh, well. I made sure it was not jamming any linkage play.
Finally got everything back together and started the car. I did not make any special attempt to bleed air out of the fuel lines, I just let the starting process do that. After holding the starter for about 5 to 10 seconds, the engine started right up and ran beautifully - every piston firing. I killed the engine and cleaned up a little more, then went back out for a test drive on the highway. Started up, no flashing glow plug light.
Job done. And well, I might add.
I would have liked to take pictures and create a write-up as a tutorial, but it just takes extra time to do that. Since it was a Sunday afternoon, I said forget it. The entire job took 5 hours and 45 minutes. Most of the time was spent unscrewing and screwing the glow plugs and reamer. Because there was so little clearance, it took forever.