Just finished skimming over the other thread. Looks like this has been going on for a little while but I'll try and contribute if I can.
First off, yes: Too much timing advance can kill glow plugs. Individual experience seems to vary, which is why some are still skeptical of this possibility.
Overly advanced timing causes a more violent shockwave in the combustion chamber because the piston is still travelling up when the flame front hits the piston crown. This is not unlike a pre-ignition knock found in a gasser when the timing is too advanced, or fuel octane is too low. Timing needs to be at a certain point for a reason. This is true for gassers or diesels.
This abnormal shockwave causes more MECHANICAL stress on the glow plugs. Damage can occur without any electric current flowing through the glow plugs. I have personally lost glow plugs to this in the past (though not an entire set).
As one can imagine, this will also cause more mechanical damage on the other internal components of the engine. Rings, pistons, wrist pins, conrods and mains... they all take a hammering when timing is excessively advanced.
If the engine seems to run well where it is, don't be so easily fooled. Believe it or not, you can actually get an IDI to run with the IP turned 180 degrees off. Not well, but it will run. Getting it to seem about right with 1 tooth off is not impossible.
Ok, smoke analysis:
VERY subjective. However more advance will not necessarily mean less smoke. Too much advance and it will smoke plenty at idle. Same with too little. Mine had a slight gray haze at operating temperature when it was slightly too advanced. It's also entirely possible that a single bad injector could be dripping enough to completely overshadow any smoke effects from a timing error.
Engine power:
Also VERY subjective. My truck seems to "pull back" slightly when the high idle/cold advance shuts off while driving. That doesn't mean the truck should be running the higher advance all the time. There are other interactions happening in the fuel system that make it seems like you are losing power with less advance. Please don't get fooled into thinking more advance is always good. You must know where your timing is first before adding more or less.
Timing by ear:
Not generally recommended, but a shop with the right tools is not always available. The following assumes the cold advance/high idle are working correctly. Make the adjustment so that there is a noticeable (but not excessive) ignition knock when timing advance is on.
It should sound almost like a 7.3 powerstroke. When the timing advance turns off, the high pitched knock should go away. Also, the knock should not become a loud crackle when pulling hard under load and at higher RPMs. If you're still getting that powerstroke knock under load (with cold advance off), you are probably still a bit too advanced.
Again, I don't recommend timing by ear especially with an unknown engine history, but this should get you close enough until a proper timing can be arranged.
